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If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - - - -Title: Through the Telescope - -Author: James Baikie - -Release Date: March 18, 2017 [EBook #54378] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THROUGH THE TELESCOPE *** - - - - -Produced by Chris Curnow, Lesley Halamek and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - -THROUGH THE TELESCOPE - - - - -AGENTS - - - AMERICA THE MACMILLAN COMPANY - 64 & 66 FIFTH AVENUE, NEW YORK - - CANADA THE MACMILLAN COMPANY OF CANADA, LTD. - 27 RICHMOND STREET, TORONTO - - INDIA MACMILLAN & COMPANY, LTD. - 12 BANK STREET, BOMBAY - 7 NEW CHINA BAZAAR STREET, CALCUTTA - -[Illustration: - - PLATE I. - -The 40-inch Refractor of the Yerkes Observatory.] - - - - - THROUGH - THE TELESCOPE - - BY - - JAMES BAIKIE, F.R.A.S. - - WITH 32 FULL-PAGE ILLUSTRATIONS FROM PHOTOGRAPHS - AND 26 SMALLER FIGURES IN THE TEXT - - [Illustration] - - LONDON - ADAM AND CHARLES BLACK - 1906 - - - - -TO - -C. N. B. AND H. E. B. - - - - -PREFACE - - -The main object of the following chapters is to give a brief and -simple description of the most important and interesting facts -concerning the heavenly bodies, and to suggest to the general reader -how much of the ground thus covered lies open to his personal survey -on very easy conditions. Many people who are more or less interested -in astronomy are deterred from making practical acquaintance with the -wonders of the heavens by the idea that these are only disclosed to -the possessors of large and costly instruments. In reality there is -probably no science which offers to those whose opportunities and -means of observation are restricted greater stores of knowledge and -pleasure than astronomy; and the possibility of that quickening of -interest which can only be gained by practical study is, in these -days, denied to very few indeed. - -Accordingly, I have endeavoured, while recounting the great triumphs -of astronomical discovery, to give some practical help to those who -are inclined to the study of the heavens, but do not know how to -begin. My excuse for venturing on such a task must be that, in the -course of nearly twenty years of observation with telescopes of all -sorts and sizes, I have made most of the mistakes against which others -need to be warned. - -The book has no pretensions to being a complete manual; it is merely -descriptive of things seen and learned. Nor has it any claim to -originality. On the contrary, one of its chief purposes has been to -gather into short compass the results of the work of others. I have -therefore to acknowledge my indebtedness to other writers, and notably -to Miss Agnes Clerke, Professor Young, Professor Newcomb, the late -Rev. T. W. Webb, and Mr. W. F. Denning. I have also found much help in -the _Monthly Notices_ and _Memoirs_ of the Royal Astronomical -Society, and the _Journal_ and _Memoirs_ of the British Astronomical -Association. - -The illustrations have been mainly chosen with the view of -representing to the general reader some of the results of the best -modern observers and instruments; but I have ventured to reproduce a -few specimens of more commonplace work done with small telescopes. I -desire to offer my cordial thanks to those who have so kindly granted -me permission to reproduce illustrations from their published works, -or have lent photographs or drawings for reproduction--to Miss Agnes -Clerke for Plates XXV.-XXVIII. and XXX.-XXXII. inclusive; to -Mrs. Maunder for Plate VIII.; to M. Loewy, Director of the Paris -Observatory, for Plates XI.-XIV. and Plate XVII.; to Professor E. B. -Frost, Director of the Yerkes Observatory, for Plates I., VII., XV., -and XVI.; to M. Deslandres, of the Meudon Observatory, for Plate IX., -and the gift of several of his own solar memoirs; to the Astronomer -Royal for England, Sir W. Mahony Christie, for Plate V.; to Mr. H. -MacEwen for the drawings of Venus, Plate X.; to the Rev. T. E. R. -Phillips for those of Mars and Jupiter, Plates XX. and XXII.; to -Professor Barnard for that of Saturn, Plate XXIV., reproduced by -permission from the _Monthly Notices_ of the Royal Astronomical -Society; to Mr. W. E. Wilson for Plates XXIX. and XXXII.; to Mr. John -Murray for Plates XVIII. and XIX.; to the proprietors of _Knowledge_ -for Plate VI.; to Mr. Denning and Messrs. Taylor and Francis for Plate -III. and Figs. 6 and 20; to the British Astronomical Association for -the chart of Mars, Plate XXI., reproduced from the _Memoirs_; and to -Messrs. T. Cooke and Sons for Plate II. For those who wish to see for -themselves some of the wonders and beauties of the starry heavens -the two Appendices furnish a few specimens chosen from an innumerable -company; while readers who have no desire to engage in practical work -are invited to skip Chapters I. and II. - - - - -CONTENTS - - - CHAPTER PAGE - - I. THE TELESCOPE--HISTORICAL 1 - - II. THE TELESCOPE--PRACTICAL 14 - - III. THE SUN 47 - - IV. THE SUN'S SURROUNDINGS 68 - - V. MERCURY 81 - - VI. VENUS 89 - - VII. THE MOON 100 - - VIII. MARS 130 - - IX. THE ASTEROIDS 148 - - X. JUPITER 154 - - XI. SATURN 172 - - XII. URANUS AND NEPTUNE 190 - - XIII. COMETS AND METEORS 203 - - XIV. THE STARRY HEAVENS 230 - - XV. CLUSTERS AND NEBULÆ 256 - - APPENDIX I.: LIST OF LUNAR FORMATIONS 273 - - APPENDIX II.: LIST OF OBJECTS FOR THE TELESCOPE 278 - - INDEX 285 - - - - -LIST OF ILLUSTRATIONS - -PRINTED SEPARATELY FROM THE TEXT - - - PLATE _To face page_ - - I. The 40-inch Refractor of the Yerkes Observatory - [_Frontispiece_ - - II. Six-inch Photo-Visual Refractor, equatorially mounted 30 - - III. Twenty-inch Reflector, Stanmore Observatory 36 - - IV. Telescope House and 8-1/2-inch 'With' Reflector 38 - - V. The Sun, February 3, 1905. Royal Observatory, - Greenwich 48 - - VI. Photograph of Bridged Sunspot (Janssen). _Knowledge_, - February, 1890 50 - - VII. Solar Surface with Faculæ. Yerkes Observatory 60 - - VIII. Coronal Streamers: Eclipse of 1898. From Photographs - by Mrs. Maunder 70 - - IX. The Chromosphere and Prominences, April 11, 1894. - Photographed by M. H. Deslandres 74 - - X. Venus. H. MacEwen. Five-inch Refractor 94 - - XI. The Moon, April 5, 1900. Paris Observatory 102 - - XII. The Moon, November 13, 1902. Paris Observatory 108 - - XIII. The Moon, September 12, 1903. Paris Observatory 110 - - XIV. Region of Maginus: Overlapping Craters. Paris - Observatory 112 - - XV. Clavius, Tycho, and Mare Nubium. Yerkes Observatory 114 - - XVI. Region of Theophilus and Altai Mountains. Yerkes - Observatory 116 - - XVII. Apennines, Alps, and Caucasus. Paris Observatory 118 - - XVIII. Chart of the Moon. Nasmyth and Carpenter } - } 124 - XIX. Key to Chart of Moon. Nasmyth and Carpenter } - - XX. Mars: Drawing 1, January 30, 1899--12 hours. - Drawing 2, April 22, 1903--10 hours 134 - - XXI. Chart of Mars. Memoirs of the British Astronomical - Association, Vol. XI., Part III., Plate VI. 138 - - XXII. Jupiter, January 6, 1906--8 hours 20 minutes. - Instrument, 9-1/4-inch Reflector 158 - - XXIII. Jupiter, February 17, 1906. J. Baikie, - 18-inch Reflector 166 - - XXIV. Saturn, July 2, 1894. E. E. Barnard, - 36-inch Equatorial 172 - - XXV. Great Comet. Photographed May 5, 1901, with the - 13-inch Astrographic Refractor of the Royal - Observatory, Cape of Good Hope 210 - - XXVI. Photographs of Swift's Comet. By Professor E. E. - Barnard 220 - - XXVII. Region of the Milky Way in Sagittarius, showing - a Double Black Aperture. Photographed by Professor - E. E. Barnard 232 - - XXVIII. Irregular Star Clusters. Photographed by E. E. - Barnard 256 - - XXIX. Cluster M. 13 Herculis. Photographed by Mr. W. E. - Wilson 258 - - XXX. Photograph of the Orion Nebula (W. H. Pickering) 262 - - XXXI. Photographs of Spiral Nebulæ. By Dr. Max Wolf 264 - - XXXII. Photograph of Whirlpool Nebula (M. 51). Taken by - Mr. W. E. Wilson, March 6, 1897 266 - - - - -LIST OF ILLUSTRATIONS - -PRINTED IN THE TEXT - - - FIG. PAGE - - 1. Principle of Galilean Telescope 3 - 2. Principle of Common Refractor 3 - 3. Dorpat Refractor 7 - 4. Thirty-inch Refractor, Pulkowa Observatory 9 - 5. Principle of Newtonian Reflector 11 - 6. Lord Rosse's Telescope 12 - 7. Herschel's 4-foot Reflector 13 - 8. Star--Correct and Incorrect Adjustment 21 - 9. Small Telescope on Pillar and Claw Stand 26 - 10. Telescope on Tripod, with Finder and Slow Motions 27 - 11. Equatorial Mounting for Small Telescope 29 - 12. Eight-inch Refractor on Equatorial Mounting 32 - 13. Four-foot Reflector, equatorially mounted 36 - 14. Drawing of Sunspot 52 - 15. " " 53 - 16. " " 56 - 17. " " 57 - 18. " " 58 - 19. Eclipses of the Sun and Moon 69 - 20. Mercury as a Morning Star. W. F. Denning, 10-inch Reflector 84 - 21. The Tides 101 - 22. Lunar Craters 105 - 23. " " 118 - 24. Mars 146 - 25. Jupiter 157 - 26. Saturn 183 - - - - -THROUGH THE TELESCOPE - - - - -CHAPTER I - -THE TELESCOPE--HISTORICAL - - -The claim of priority in the invention of this wonderful instrument, -which has so enlarged our ideas of the scale and variety of -the universe, has been warmly asserted on behalf of a number of -individuals. Holland maintains the rights of Jansen, Lippershey, and -Metius; while our own country produces evidence that Roger Bacon -had, in the thirteenth century, 'arrived at theoretical proof of the -possibility of constructing a telescope and a microscope' and that -Leonard Digges 'had a method of discovering, by perspective glasses -set at due angles, all objects pretty far distant that the sun shone -on, which lay in the country round about.' - -All these claims, however, whether well or ill founded, are very -little to the point. The man to whom the human race owes a debt of -gratitude in connection with any great invention is not necessarily he -who, perhaps by mere accident, may stumble on the principle of it, but -he who takes up the raw material of the invention and shows the full -powers and possibilities which are latent in it. In the present -case there is one such man to whom, beyond all question, we owe the -telescope as a practical astronomical instrument, and that man is -Galileo Galilei. He himself admits that it was only after hearing, in -1609, that a Dutchman had succeeded in making such an instrument, -that he set himself to investigate the matter, and produced telescopes -ranging from one magnifying but three diameters up to the one with a -power of thirty-three with which he made his famous discoveries; but -this fact cannot deprive the great Italian of the credit which is -undoubtedly his due. Others may have anticipated him in theory, or -even to a small extent in practice, but Galileo first gave to the -world the telescope as an instrument of real value in research. - -The telescope with which he made his great discoveries was constructed -on a principle which, except in the case of binoculars, is now -discarded. It consisted of a double convex lens converging the rays -of light from a distant object, and of a double concave lens, -intercepting the convergent rays before they reach a focus, and -rendering them parallel again (Fig. 1). His largest instrument, as -already mentioned, had a power of only thirty-three diameters, and the -field of view was very small. A more powerful one can now be obtained -for a few shillings, or constructed, one might almost say, for a -few pence; yet, as Proctor has observed: 'If we regard the absolute -importance of the discoveries effected by different telescopes, -few, perhaps, will rank higher than the little tube now lying in the -Tribune of Galileo at Florence.' - -[Illustration: FIG. 1.--PRINCIPLE OF GALILEAN TELESCOPE.] - -Galileo's first discoveries with this instrument were made in 1610, -and it was not till nearly half a century later that any great -improvement in telescopic construction was effected. In the middle of -the seventeenth century Scheiner and Huygens made telescopes on the -principle, suggested by Kepler, of using two double convex lenses -instead of a convex and a concave, and the modern refracting telescope -is still constructed on essentially the same principle, though, of -course, with many minor modifications (Fig. 2). - -[Illustration: FIG. 2.--PRINCIPLE OF COMMON REFRACTOR.] - -The latter part of the seventeenth century witnessed the introduction -of telescopes on this principle of the most amazing length, the -increase in length being designed to minimize the imperfections which -a simple lens exhibits both in definition and in colour. Huygens -constructed one such telescope of 123 feet focal length, which he -presented to the Royal Society of London; Cassini, at Paris, used -instruments of 100 and 136 feet; while Bradley, in 1722, measured the -diameter of Venus with a glass whose focal length was 212-1/4 feet. -Auzout is said to have made glasses of lengths varying from 300 to -600 feet, but, as might have been expected, there is no record of any -useful observations having ever been made with these monstrosities. Of -course, these instruments differed widely from the compact and handy -telescopes with which we are now familiar. They were entirely without -tubes. The object-glass was fastened to a tall pole or to some high -building, and was painfully man[oe]uvred into line with the eye-piece, -which was placed on a support near the ground, by means of an -arrangement of cords. The difficulties of observation with these -unwieldy monsters must have been of the most exasperating type, while -their magnifying power did not exceed that of an ordinary modern -achromatic of, perhaps, 36 inches focal length. Cassini, for instance, -seems never to have gone beyond a power of 150 diameters, which might -be quite usefully employed on a good modern 3-inch refractor in -good air. Yet with such tools he was able to discover four of the -satellites of Saturn and that division in Saturn's ring which still -bears his name. Such facts speak volumes for the quality of the -observer. Those who are the most accustomed to use the almost perfect -products of modern optical skill will have the best conception of, -and the profoundest admiration for, the limitless patience and the -wonderful ability which enabled him to achieve such results with the -very imperfect means at his disposal. - -The clumsiness and unmanageableness of these aerial telescopes quickly -reached a point which made it evident that nothing more was to be -expected of them; and attempts were made to find a method of combining -lenses, which might result in an instrument capable of bearing equal -or greater magnifying powers on a much shorter length. The chief -hindrance to the efficiency of the refracting telescope lies in the -fact that the rays of different colours which collectively compose -white light cannot be brought to one focus by any single lens. The red -rays, for example, have a different focal length from the blue, and -so any lens which brings the one set to a focus leaves a fringe of the -other outstanding around any bright object. - -In 1729 Mr. Chester Moor Hall discovered a means of conquering this -difficulty, but his results were not followed up, and it was left for -the optician John Dollond to rediscover the principle some twenty-five -years later. By making the object-glass of the telescope double, the -one lens being of crown and the other of flint glass, he succeeded in -obtaining a telescope which gave a virtually colourless image. - -This great discovery of the achromatic form of construction at -once revolutionized the art of telescope-making. It was found that -instruments of not more than 5 feet focal length could be constructed, -which infinitely surpassed in efficiency, as well as in handiness, -the cumbrous tools which Cassini had used; and Dollond's 5-foot -achromatics, generally with object-glasses of 3-3/4 inches diameter, -represented for a considerable time the acme of optical excellence. -Since the time of Dollond, the record of the achromatic refractor has -been one of continual, and, latterly, of very rapid progress. For -a time much hindrance was experienced from the fact that it proved -exceedingly difficult to obtain glass discs of any size whose purity -and uniformity were sufficient to enable them to pass the stringent -test of optical performance. In the latter part of the eighteenth -century, a 6-inch glass was considered with feelings of admiration, -somewhat similar to those with which we regard the Yerkes 40-inch -to-day; and when, in 1823, the Dorpat refractor of 9-6/10 inches was -mounted (Fig. 3), the astronomical world seemed to have the idea that -something very like finality had been reached. The Dorpat telescope -proved, however, to be only a milestone on the path of progress. -Before very long it was surpassed by a glass of 12 inches diameter, -which Sir James South obtained from Cauchoix of Paris, and which is -now mounted in the Dunsink Observatory, Dublin. This, in its turn, -had to give place to the fine instruments of 14·9 inches which were -figured by Merz of Munich for the Pulkowa and Cambridge (U.S.A.) -Observatories; and then there came a pause of a few years, which was -broken by Alvan Clark's completion of an 18-1/2-inch, an instrument -which earned its diploma, before ever it left the workshop of its -constructor, by the discovery of the companion to Sirius. - -[Illustration: FIG. 3.--DORPAT REFRACTOR.] - -The next step was made on our side of the Atlantic, and proved to be -a long and notable one, in a sense definitely marking out the boundary -line of the modern era of giant refractors. This was the completion, -by Thomas Cooke, of York, of a 25-inch instrument for the late Mr. -Newall. It did not retain for long its pride of place. The palm was -speedily taken back to America by Alvan Clark's construction of the -26-inch of the Washington Naval Observatory, with which Professor -Asaph Hall discovered in 1877 the two satellites of Mars. Then came -Grubb's 27-inch for Vienna; the pair of 30-inch instruments, by Clark -and Henry respectively, for Pulkowa (Fig. 4) and Nice; and at last the -instrument which has for a number of years been regarded as the finest -example of optical skill in the world, the 36-inch Clark refractor of -the Lick Observatory, California. Placed at an elevation of over 4,000 -feet, and in a climate exceptionally well suited for astronomical -work, this fine instrument has had the advantage of being handled by a -very remarkable succession of brilliant observers, and has, since -its completion, been looked to as a sort of court of final appeal in -disputed questions. But America has not been satisfied even with -such an instrument, and the 40-inch Clark refractor of the Yerkes -Observatory is at present the last word of optical skill so far as -achromatics are concerned (Frontispiece). It is not improbable that it -may also be the last word so far as size goes, for the late Professor -Keeler's report upon its performance implies that in this splendid -telescope the limit of practicable size for object-glasses is being -approached. The star images formed by the great lens show indications -of slight flexure of the glass under its own weight as it is turned -from one part of the sky to another. It would be rash, however, to -say that even this difficulty will not be overcome. So many obstacles, -seemingly insuperable, have vanished before the astronomer's imperious -demand for 'more light,' and so many great telescopes, believed in -their day to represent the absolute culmination of the optical art, -are now mere commoners in the ranks where once they were supreme, that -it may quite conceivably prove that the great Yerkes refractor, like -so many of its predecessors, represents only a stage and not the end -of the journey. - -[Illustration: FIG. 4.--30-INCH REFRACTOR, PULKOWA OBSERVATORY.] - -Meanwhile, Sir Isaac Newton, considering, wrongly as the sequel -showed, that 'the case of the refractor was desperate,' set about -the attempt to find out whether the reflection of light by means -of suitably-shaped mirrors might not afford a substitute for the -refractor. In this attempt he was successful, and in 1671 presented to -the Royal Society the first specimen, constructed by his own hands, of -that form of reflecting telescope which has since borne his name. The -principle of the Newtonian reflector will be easily grasped from Fig. -5. The rays of light from the object under inspection enter the open -mouth of the instrument, and passing down the tube are converged by -the concave mirror AA towards a focus, before reaching which they -are intercepted by the small flat mirror BB, placed at an angle of -45 degrees to the axis of the tube, and are by it reflected into the -eye-piece E which is placed at the side of the instrument. In this -construction, therefore, the observer actually looks in a direction -at right angles to that of the object which he is viewing, a condition -which seems strange to the uninitiated, but which presents no -difficulties in practice, and is found to have several advantages, -chief among them the fact that there is no breaking of one's neck in -the attempt to observe objects near the zenith, the line of vision -being always horizontal, no matter what may be the altitude of the -object under inspection. Other forms of reflector have been devised, -and go by the names of the Gregorian, the Cassegrain, and the -Herschelian; but the Newtonian has proved itself the superior, and has -practically driven its rivals out of the field, though the Cassegrain -form has been revived in a few instances of late years, and is -particularly suited to certain forms of research. - -[Illustration: FIG. 5.--PRINCIPLE OF NEWTONIAN REFLECTOR.] - -[Illustration: FIG. 6.--LORD ROSSE'S TELESCOPE.] - -At first the mirrors of reflecting telescopes were made of an alloy -known as speculum metal, which consisted of practically 4 parts of -copper to 1 of tin; but during the last half-century this metal has -been entirely superseded by mirrors made of glass ground to the proper -figure, and then polished and silvered on the face by a chemical -process. To the reflecting form of construction belong some of the -largest telescopes in the world, such as the Rosse 6-foot (metal -mirrors), Fig. 6, the Common 5-foot (silver on glass), the Melbourne -4-foot (metal mirrors, Cassegrain form), and the 5-foot constructed by -Mr. Ritchey for the Yerkes Observatory. Probably the most celebrated, -as it was also the first of these monsters, was the 4-foot telescope -of Sir William Herschel, made by himself on the principle which goes -by his name. It was used by him to some extent in the discoveries -which have made his name famous, and nearly everyone who has ever -opened an astronomical book is familiar with the engraving of the huge -40-foot tube, with its cumbrous staging, which Oliver Wendell Holmes -has so quaintly celebrated in 'The Poet at the Breakfast Table' (Fig. -7). - -[Illustration: FIG. 7.--HERSCHEL'S 4-FOOT REFLECTOR.] - - - - -CHAPTER II - -THE TELESCOPE--PRACTICAL - - -Having thus briefly sketched the history of the telescope, we turn now -to consider the optical means which are most likely to be in the hands -or within the reach of the beginner in astronomical observation. Let -us, first of all, make the statement that any telescope, good, bad, or -indifferent, is better than no telescope. There are some purists who -would demur to such a statement, who make the beginner's heart heavy -with the verdict that it is better to have no telescope at all -than one that is not of the utmost perfection, and, of course, of -corresponding costliness, and who seem to believe that the performance -of an inferior glass may breed disgust at astronomy altogether. This -is surely mere nonsense. For most amateurs at the beginning of their -astronomical work the question is not between a good telescope and an -inferior one, it is between a telescope and no telescope. Of course, -no one would be so foolish as willingly to observe with an inferior -instrument if a better could be had; but even a comparatively poor -glass will reveal much that is of great interest and beauty, and -its defects must even be put up with sometimes for the sake of its -advantages until something more satisfactory can be obtained. An -instrument which will show fifty stars where the naked eye sees five -is not to be despised, even though it may show wings to Sirius that -have no business there, or a brilliant fringe of colours round Venus -to which even that beautiful planet can lay no real claim. Galileo's -telescope would be considered a shockingly bad instrument nowadays; -still, it had its own little influence upon the history of astronomy, -and the wonders which it first revealed are easily within the reach of -anyone who has the command of a shilling or two, and, what is perhaps -still more important, of a little patience. The writer has still in -his possession an object-glass made out of a simple single eyeglass, -such as is worn by Mr. Joseph Chamberlain. This, mounted in a -cardboard tube with another single lens in a sliding tube as an -eye-piece, proved competent to reveal the more prominent lunar -craters, a number of sunspots, the phases of Venus, and the existence, -though not the true form, of Saturn's ring. Its total cost, if memory -serve, was one shilling and a penny. Of course it showed, in addition, -a number of things which should not have been seen, such as a lovely -border of colour round every bright object; but, at the same time, -it gave a great deal more than thirteen pence worth of pleasure and -instruction. - -Furthermore, there is this to be said in favour of beginning with a -cheap and inferior instrument, that experience may thus be gained -in the least costly fashion. The budding astronomer is by nature -insatiably curious. He wants to know the why and how of all the things -that his telescope does or does not do. Now this curiosity, while -eminently laudable in itself, is apt in the end to be rather hard -upon his instrument. A fine telescope, whatever its size may be, is -an instrument that requires and should receive careful handling; it is -easily damaged, and costly to replace. And therefore it may be better -that the beginner should make his earlier experiments, and find -out the more conspicuous and immediately fatal of the many ways of -damaging a telescope, upon an instrument whose injury, or even whose -total destruction, need not cause him many pangs or much financial -loss. - -It is not suggested that a beginning should necessarily be made on -such a humble footing as that just indicated. Telescopes of the sizes -mainly referred to in these pages--_i.e._, refractors of 2 or 3 inches -aperture, and reflectors of 4-1/2 to 6 inches--may frequently be -picked up second-hand at a very moderate figure indeed. Of course, in -these circumstances the purchaser has to take his chance of defects -in the instrument, unless he can arrange for a trial of it, either by -himself, or, preferably, by a friend who has some experience; yet even -should the glass turn out far from perfect, the chances are that it -will at least be worth the small sum paid for it. Nor is it in the -least probable, as some writers seem to believe, that the use of -an inferior instrument will disgust the student and hinder him from -prosecuting his studies. The chances are that it will merely create -a desire for more satisfactory optical means. Even a skilled observer -like the late Rev. T. W. Webb had to confess of one of his telescopes -that 'much of its light went the wrong way'; and yet he was able to -get both use and pleasure out of it. The words of a well-known English -amateur observer may be quoted. After detailing his essays with -glasses of various degrees of imperfection Mr. Mee remarks: 'For the -intending amateur I could wish no other experience than my own. To -commence with a large and perfect instrument is a mistake; its owner -cannot properly appreciate it, and in gaining experience is pretty -sure to do the glass irreparable injury.' - -Should the beginner not be willing or able to face the purchase -of even a comparatively humble instrument, his case is by no means -desperate, for he will find facilities at hand, such as were not -thought of a few years ago, for the construction of his own telescope. -Two-inch achromatic object-glasses, with suitable lenses for the -making up of the requisite eye-pieces, are to be had for a few -shillings, together with cardboard tubes of sizes suitable for -fitting up the instrument; and such a volume as Fowler's 'Telescopic -Astronomy' gives complete directions for the construction of a glass -which is capable of a wonderful amount of work in proportion to -its cost. The substitution of metal tubes for the cardboard ones is -desirable, as metal will be found to be much more satisfactory if the -instrument is to be much used. The observer, however, will not long -be satisfied with such tools as these, useful though they may be. The -natural history of amateur astronomers may be summed up briefly in the -words 'they go from strength to strength.' The possessor of a small -telescope naturally and inevitably covets a bigger one; and when the -bigger one has been secured it represents only a stage in the search -for one bigger still, while along with the desire for increased size -goes that for increased optical perfection. No properly constituted -amateur will be satisfied until he has got the largest and best -instrument that he has money to buy, space to house, and time to use. - -Let us suppose, then, that the telescope has been acquired, and that -it is such an instrument as may very commonly be found in the hands -of a beginner--a refractor, say, of 2, 2-1/2, or 3 inches aperture -(diameter of object-glass). The question of reflectors will fall to -be considered later. Human nature suggests that the first thing to do -with it is to unscrew all the screws and take the new acquisition to -pieces, so far as possible, in order to examine into its construction. -Hence many glasses whose career of usefulness is cut short before -it has well begun. 'In most cases,' says Webb, 'a screw-driver is a -dangerous tool in inexperienced hands'; and Smyth, in the Prolegomena -to his 'Celestial Cycle,' utters words of solemn warning to the -'over-handy gentlemen who, in their feverish anxiety for meddling -with and making instruments, are continually tormenting them with -screw-drivers, files, and what-not.' Unfortunately, it is not only -the screw-driver that is dangerous; the most deadly danger to the most -delicate part of the telescope lies in the unarmed but inexperienced -hands themselves. You may do more irreparable damage to the -object-glass of your telescope in five minutes with your fingers than -you are likely to do to the rest of the instrument in a month with -a screw-driver. Remember that an object-glass is a work of art, -sometimes as costly as, and always much more remarkable than, the -finest piece of jewellery. It may be unscrewed, _carefully_, from -the end of its tube and examined. Should the examination lead to the -detection of bubbles or even scratches in the glass (quite likely the -latter if the instrument be second-hand), these need not unduly vex -its owner's soul. They do not necessarily mean bad performance, and -the amount of light which they obstruct is very small, unless the -case be an extreme one. But on no account should the two lenses of the -object-glass itself be separated, for this will only result in making -a good objective bad and a bad one worse. The lenses were presumably -placed in their proper adjustment to one another by an optician before -being sent out; and should their performance be so unsatisfactory -as to suggest that this adjustment has been disturbed, it is to an -optician that they should be returned for inspection. The glass may, -of course, be carefully and gently cleaned, using either soft chamois -leather, or preferably an old silk handkerchief, studiously kept -from dust; but the cleaning should never amount to more than a gentle -sweeping away of any dust which may have gathered on the surface. -Rubbing is not to be thought of, and the man whose telescope has been -so neglected that its object-glass needs rubbing should turn to some -other and less reprehensible form of mischief. For cleaning the -small lenses of the eye-pieces, the same silk may be employed; Webb -recommends a piece of blotting-paper, rolled to a point and aided by -breathing, for the edges which are awkward to get at. Care must, of -course, be taken to replace these lenses in their original positions, -and the easiest way to ensure this is to take out only one at a time. -In replacing them, see that the finger does not touch the surface of -the glass, or the cleaning will be all to do over again. - -[Illustration: FIG. 8. - -_a_, O.G. in perfect adjustment; _b_, O.G. defectively centred.] - -Next comes the question of testing the quality of the objective. (The -stand is meanwhile assumed, but will be spoken of later.) Point the -telescope to a star of about the third magnitude, and employ -the eye-piece of highest power, if more than one goes with the -instrument--this will be the shortest eye-piece of the set. If the -glass be of high quality, the image of the star will be a neat round -disc of small size, surrounded by one or two thin bright rings (Fig. -8, _a_). Should the image be elliptical and the rings be thrown to the -one side (Fig. 8, _b_), the glass may still be quite a good one, but -is out of square, and should be readjusted by an optician. Should -the image be irregular and the rings broken, the glass is of inferior -quality, though it may still be serviceable enough for many purposes. -Next throw the image of the star out of focus by racking the eye-piece -in towards the objective, and then repeat the process by racking it -again out of focus away from the objective. The image will, in either -case, expand into a number of rings of light, and these rings should -be truly circular, and should present precisely the same appearance at -equal distances within and without the focus. A further conception of -the objective's quality may be gained by observing whether the image -of a star or the detail of the moon or of the planets comes sharply -to a focus when the milled head for focussing is turned. Should it -be possible to rack the eye-tube in or out for any distance without -disturbing the distinctness of the picture to any extent, then the -glass is defective. A good objective will admit of no such range, but -will come sharply up to focus, and as sharply away from it, with any -motion of the focussing screw. A good glass will also show the details -of a planet like Saturn, such as are within its reach, that is, with -clearness of definition, while an inferior one will soften all the -outlines, and impart a general haziness to them. The observer may now -proceed to test the colour correction of his objective. No achromatic, -its name notwithstanding, ever gives an absolutely colourless image; -all that can be expected is that the colour aberration should have -been so far eliminated as not to be unpleasant. In a good instrument -a fringe of violet or blue will be seen around any bright object, such -as Venus, on a dark sky; a poor glass will show red or yellow. It -is well to make sure, however, should bad colour be seen, that the -eye-piece is not causing it; and, therefore, more than one eye-piece -should be tried before an opinion is formed. Probably more colour will -be seen at first than was expected, more particularly with an object -so brilliant as Venus. But the observer need not worry overmuch about -this. He will find that the eye gets so accustomed to it as almost -to forget that it is there, so that something of a shock may be -experienced when a casual star-gazing friend, on looking at some -bright object, remarks, as friends always do, 'What beautiful -colours!' Denning records a somewhat extreme case in which a friend, -who had been accustomed to observe with a refractor, absolutely -resented the absence of the familiar colour fringe in the picture -given by a reflector, which is the true achromatic in nature, though -not in name. The beginner is recommended to read the article 'The -Adjustment of a Small Equatorial,' by Mr. E. W. Maunder, in the -_Journal of the British Astronomical Association_, vol. ii., p. 219, -where he will find the process of testing described at length and with -great clearness. - -In making these tests, allowance has, of course, to be made for the -state of the atmosphere. A good telescope can only do its best on a -good night, and it is not fair to any instrument to condemn it until -it has been tested under favourable conditions. The ideal test would -be to have its performance tried along with that of another instrument -of known good quality and of as nearly the same size as possible. If -this cannot be arranged for, the tests must be made on a succession -of nights, and good performance on one of these is sufficient to -vindicate the reputation of the glass, and to show that any deficiency -on other occasions was due to the state of the air, and not to the -instrument. Should his telescope pass the above tests satisfactorily, -the observer ought to count himself a happy man, and will until he -begins to hanker after a bigger instrument. - -The mention of the pointing of the telescope to a star brings up the -question of how this is to be done. It seems a simple thing; as a -matter of fact, with anything like a high magnifying power it is next -to impossible; and there are few things more exasperating than to see -a star or a planet shining brightly before your eyes, and yet to find -yourself quite unable to get it into the field of view. The simple -remedy is the addition of a finder to the telescope. This is a small -telescope of low magnifying power which is fastened to the larger -instrument by means of collars bearing adjusting screws, which enable -it to be laid accurately parallel with the large tube (Fig. 10). Its -eye-piece is furnished with cross-threads, and a star brought to -the intersection of these threads will be in the field of the large -telescope. In place of the two threads crossing at right angles there -may be substituted three threads interlacing to form a little triangle -in the centre of the finder's field. By this device the star can -always be seen when the glass is being pointed instead of being -hidden, as in the other case, behind the intersection of the two -threads. A fine needle-point fixed in the eye-piece will also be found -an efficient substitute for the cross-threads. In the absence of -a finder the telescope may be pointed by using the lowest power -eye-piece and substituting a higher one when the object is in the -field; but beyond question the finder is well worth the small -addition which it makes to the cost of an instrument. A little care -in adjusting the finder now and again will often save trouble and -annoyance on a working evening. - -The question of a stand on which to mount the telescope now falls to -be considered, and is one of great importance, though apt to be rather -neglected at first. It will soon be found that little satisfaction or -comfort can be had in observing unless the stand adopted is steady. A -shaky mounting will spoil the performance of the best telescope that -ever was made, and will only tantalize the observer with occasional -glimpses of what might be seen under better conditions. Better have a -little less aperture to the object-glass, and a good steady mounting, -than an extra inch of objective and a mounting which robs you of all -comfort in the using of your telescope. Beginners are indeed rather -apt to be misled into the idea that the only matters of importance are -the objective and its tube, and that money spent on the stand is money -wasted. Hence many fearful and wonderful contrivances for doing badly -what a little saved in the size of the telescope and expended on the -stand would have enabled them to do well. It is very interesting, no -doubt, to get a view of Jupiter or Saturn for one field's-breadth, -and then to find, on attempting to readjust the instrument for another -look, that the mounting has obligingly taken your star-gazing into its -own hands, and is now directing your telescope to a different object -altogether; but repetition of this form of amusement is apt to pall. A -radically weak stand can never be made into a good one; the best plan -is to get a properly proportioned mounting at once, and be done with -it. - -[Illustration: FIG. 9.--SMALL TELESCOPE ON PILLAR AND CLAW STAND.] - -For small instruments, such as we are dealing with, the mounting -generally adopted is that known as the Altazimuth, from its giving two -motions, one in altitude and one in azimuth, or, to use more familiar -terms, one vertical and the other horizontal. There are various types -of the Altazimuth. If the instrument be of not more than 3 feet focal -length, the ordinary stand known as the 'pillar and claw' (Fig. 9) -will meet all the requirements of this form of motion. Should the -focal length be greater than 3 feet, it is advisable to have the -instrument mounted on a tripod stand, such as is shown in Fig. 10. In -the simpler forms of both these mountings the two motions requisite -to follow an object must be given by hand, and it is practically -impossible to do this without conveying a certain amount of tremor to -the telescope, which disturbs clearness of vision until it subsides, -by which time the object to be viewed is generally getting ready to -go out of the field again. To obviate this inconvenience as far as -possible, the star or planet when found should be placed just outside -the field of view, and allowed to enter it by the diurnal motion -of the earth. The tremors will thus have time to subside before the -object reaches the centre of the field, and this process must -be repeated as long as the observation continues. In making this -adjustment attention must be paid to the direction of the object's -motion through the field, which, of course, varies according to its -position in the sky. If it be remembered that a star's motion through -the telescopic field is the exact reverse of its true direction across -the sky, little difficulty will be found, and use will soon render -the matter so familiar that the adjustment will be made almost -automatically. - -[Illustration: FIG. 10.--TELESCOPE ON TRIPOD, WITH FINDER AND SLOW -MOTIONS.] - -A much more convenient way of imparting the requisite motions is by -the employment of tangent screws connected with Hooke's joint-handles, -which are brought conveniently near to the hands of the observer as -he sits at the eye-end. These screws clamp into circles or portions of -circles, which have teeth cut on them to fit the pitch of the screw, -and by means of them a slow and steady motion may be imparted to the -telescope. When it is required to move the instrument more rapidly, or -over a large expanse of sky, the clamps which connect the screws with -the circles are slackened, and the motion is given by hand. Fig. 10 -shows an instrument provided with these adjuncts, which, though not -absolutely necessary, and adding somewhat to the cost of the mounting, -are certainly a great addition to the ease and comfort of observation. - -[Illustration: FIG. 11.--EQUATORIAL MOUNTING FOR SMALL TELESCOPE.] - -The Altazimuth mounting, from its simplicity and comparative -cheapness, has all along been, and will probably continue to be, the -form most used by amateurs. It is, however, decidedly inferior in -every respect to the equatorial form of mount. In this form (Fig. 11) -the telescope is carried by means of two axes, one of which--the Polar -axis--is so adjusted as to be parallel to the pole of the earth's -rotation, its degree of inclination being therefore dependent upon the -latitude of the place for which it is designed. At the equator it will -be horizontal, will lie at an angle of 45 degrees half-way between -the equator and either pole, and will be vertical at the poles. At its -upper end it carries a cross-head with bearings through which there -passes another axis at right angles to the first (the declination -axis). Both these axes are free to rotate in their respective -bearings, and thus the telescope is capable of two motions, one of -which--that of the declination axis--enables the instrument to be set -to the elevation of the object to be observed, while the other--that -of the polar axis--enables the observer to follow the object, when -found, from its rising to its setting by means of a single movement, -the telescope sweeping out circles on the sky corresponding to those -which the stars themselves describe in their journey across the -heavens. This single movement may be given by means of a tangent screw -such as has already been described, and the use of a telescope thus -equipped is certainly much easier and more convenient than that of an -Altazimuth, where two motions have constantly to be imparted. To gain -the full advantage of the equatorial form of mounting, the polar axis -must be placed exactly in the North and South line, and unless the -mounting can be adjusted properly and left in adjustment, it is robbed -of much of its superiority. For large fixed instruments it is, of -course, almost universally used; and in observatories the motion in -Right Ascension, as it is called, which follows the star across the -sky, is communicated to the driving-wheel of the polar axis by means -of a clock which turns the rod carrying the tangent screw (Plate II.). -These are matters which in most circumstances are outside the sphere -of the amateur; it may be interesting for him, however, to see -examples of the way in which large instruments are mounted. The -frontispiece, accordingly, shows the largest and most perfect -instrument at present in existence, while Plate II., with Figs. 4 -and 12, give further examples of fine modern work. The student can -scarcely fail to be struck by the extreme solidity of the modern -mountings, and by the way in which all the mechanical parts of the -instrument are so contrived as to give the greatest convenience -and ease in working. Comparing, for instance, Plate II., a 6-inch -refractor by Messrs. Cooke, of York, available either for visual or -photographic work, with the Dorpat refractor (Fig. 3), it is seen that -the modern maker uses for a 6-inch telescope a stand much more -solid and steady than was deemed sufficient eighty years ago for an -instrument of 9-6/10 inches. Attention is particularly directed to the -way in which nowadays all the motions are brought to the eye-end so -as to be most convenient for the observer, and frequently, as in this -case, accomplished by electric power, while the declination circle is -read by means of a small telescope so that the large instrument can -be directed upon any object with the minimum of trouble. The -driving clock, well shown on the right of the supporting pillar, is -automatically controlled by electric current from the sidereal clock -of the observatory. - -[Illustration: - - PLATE II. - -6-inch Photo-Visual Refractor, equatorially mounted. Messrs. T. Cooke -& Sons.] - -We have now to consider the reflecting form of telescope, which, -especially in this country, has deservedly gained much favour, and has -come to be regarded as in some sense the amateur's particular tool. - -[Illustration: FIG. 12.--8-INCH REFRACTOR ON EQUATORIAL MOUNTING.] - -As a matter of policy, one can scarcely advise the beginner to make -his first essay with a reflector. Its adjustments, though simple -enough, are apt to be troublesome at the time when everything has -to be learned by experience; and its silver films, though much more -durable than is commonly supposed, are easily destroyed by careless -or unskilful handling, and require more careful nursing than the -objective of a refractor. But, having once paid his first fees to -experience, the observer, if he feel so inclined, may venture upon a -reflector, which has probably more than sufficient advantages to -make up for its weaker points. First and foremost of these advantages -stands the not inconsiderable one of cheapness. A 10-1/2-inch -reflector may be purchased new for rather less than the sum which will -buy a 4-inch refractor. True, the reflector has not the same command -of light inch for inch as the refractor, but a reflector of 10-1/2 -inches should at least be the match of an 8-inch refractor in this -respect, and will be immeasurably more powerful than the 4-inch -refractor, which comes nearest to it in price. Second stands the ease -and comfort so conspicuous in observing with a Newtonian. Instead -of having almost to break his neck craning under the eye-piece of a -telescope pointed to near the zenith, the observer with a Newtonian -looks always straight in front of him, as the eye-piece of a reflector -mounted as an altazimuth is always horizontal, and when the instrument -is mounted equatorially, the tube, or its eye-end, is made to rotate -so that the line of vision may be kept horizontal. Third is the -absence of colour. Colour is not conspicuous in a small refractor, -unless the objective be of very bad quality; but as the aperture -increases it is apt to become somewhat painfully apparent. The -reflector, on the other hand, is truly achromatic, and may be relied -upon to show the natural tints of all objects with which it deals. -This point is of considerable importance in connection with planetary -observation. The colouring of Jupiter, for instance, will be seen in a -reflector as a refractor can never show it. - -Against these advantages there have to be set certain disadvantages. -First, the question of adjustments. A small refractor requires -practically none; but a reflector, whatever its size, must be -occasionally attended to, or else its mirrors will get out of square -and bad performance will be the result. It is easy, however, to -make too much of this difficulty. The adjustments of the writer's -8-1/2-inch With reflector have remained for months at a time as -perfect as when they had been newly attended to. Second, the renewal -of the silver films. This may cause some trouble in the neighbourhood -of towns where the atmosphere is such as to tarnish silver quickly; -and even in the country a film must be renewed at intervals. But these -may be long enough. The film on the mirror above referred to has stood -without serious deterioration for five years at a time. Third, the -reflector, with its open-mouthed tube, is undoubtedly more subject -to disturbance from air currents and changes of temperature, and its -mirrors take longer to settle down into good definition after the -instrument has been moved from one point of the sky to another. This -difficulty cannot be got over, and must be put up with; but it is not -very conspicuous with the smaller sizes of telescopes, such as are -likely to be in the hands of an amateur at the beginning of his work. -There are probably but few nights when an 8-1/2-inch reflector will -not give quite a good account of itself in this respect by comparison -with a refractor of anything like equal power. On the whole, the state -of the question is this: If the observer wishes to have as much power -as possible in proportion to his expenditure, and is not afraid to -take the risk of a small amount of trouble with the adjustments and -films, the reflector is probably the instrument best suited to him. -If, on the other hand, he is so situated that his telescope has to be -much moved, or, which is almost as bad, has to stand unused for any -considerable intervals of time, he will be well advised to prefer -a refractor. One further advantage of the reflecting form is that, -aperture for aperture, it is very much shorter. The average refractor -will probably run to a length of from twelve to fifteen times the -diameter of its objective. Reflectors are rarely of a greater length -than nine times the diameter of the large mirror, and are frequently -shorter still. Consequently, size for size, they can be worked in less -space, which is often a consideration of importance. - -[Illustration: FIG. 13.--FOUR-FOOT REFLECTOR EQUATORIALLY MOUNTED.] - -The mountings of the reflector are in principle precisely similar to -those of the refractor already described. The greater weight, however, -and the convenience of having the body of the instrument kept as low -as possible, owing to the fact of the eye-piece being at the upper end -of the tube, have necessitated various modifications in the forms to -which these principles are applied. Plates III. and IV., and Fig. 13, -illustrate the altazimuth and equatorial forms of mounting as applied -to reflectors of various sizes, Fig. 13 being a representation of -Lassell's great 4-foot reflector. - -[Illustration: - - PLATE III. - -20-inch Reflector, Stanmore Observatory.] - -And now, having his telescope, whatever its size, principle, or -form of mounting, the observer has to proceed to use it. Generally -speaking, there is no great difficulty in arriving at the manner of -using either a refractor or a reflector, and for either instrument -the details of handling must be learned by experience, as nearly all -makers have little variations of their own in the form of clamps and -slow motions, though the principles in all instruments are the same. -With regard to these, the only recommendation that need be made is one -of caution in the use of the glass until its ways of working have -been gradually found out. With a knowledge of the principles of its -construction and a little application of common-sense, there is no -part of a telescope mounting which may not be readily understood. -Accordingly, what follows must simply take the form of general hints -as to matters which every telescopist ought to know, and which -are easier learned once and for all at the beginning than by slow -experience. These hints are of course the very commonplaces of -observation; but it is the commonplace that is the foundation of good -work in everything. - -If possible, let the telescope be fixed in the open air. Where -money is no object, a few pounds will furnish a convenient little -telescope-house, with either a rotating or sliding roof, which enables -the instrument to be pointed to any quarter of the heavens. Such -houses are now much more easily obtained than they once were, and -anyone who has tried both ways can testify how much handier it is to -have nothing to do but unlock the little observatory, and find the -telescope ready for work, than to have to carry a heavy instrument out -into the open. Plate IV. illustrates such a shelter, which has done -duty for more than twelve years, covering an 8-1/2-inch With, whose -tube and mounting are almost entirely the work of a local smith; and -in the _Journal of the British Astronomical Association_, vol. -xiv., p. 283, Mr. Edwin Holmes gives a simple description of a small -observatory which was put up at a cost of about £3, and has proved -efficient and durable. The telescope-house has also the advantage -of protecting the observer and his instrument from the wind, so that -observation may often be carried on on nights which would be quite too -windy for work in the open. - -[Illustration: - - PLATE IV. - -Telescope House and 8-1/2-inch 'with' Reflector.] - -Should it not be possible to obtain such a luxury, however, -undoubtedly the next best is fairly outside. No one who has garden -room should ever think of observing from within doors. If the -telescope be used at an open window its definition will be impaired -by air-currents. The floor of the room will communicate tremors to the -instrument, and every movement of the observer will be accompanied by -a corresponding movement of the object in the field, with results -that are anything but satisfactory. In some cases no other position is -available. If this be so, Webb's advice must be followed, the window -opened as widely and as long beforehand as possible, and the telescope -thrust out as far as is convenient. But these precautions only -palliate the evils of indoor observation. The open air is the best, -and with a little care in wrapping up the observer need run no risk. - -Provide the telescope, if a refractor, with a dew-cap. Without this -precaution dew is certain to gather upon the object-glass, with -the result of stopping all observation until it is removed, and the -accompanying risk of damage to the objective itself. Some instruments -are provided by their makers with dew-caps, and all ought to be; but -in the absence of this provision a cap may be easily contrived. A -tube of tin three or four times as long as the diameter of the -object-glass, made so as to slide fairly stiffly over the object end -of the tube where the ordinary cap fits, and blackened inside to a -dead black, will remove practically all risk. The blackening may be -done with lamp-black mixed with spirit varnish. Some makers--Messrs. -Cooke, of York, for instance--line both tube and dew-cap with black -velvet. This ought to be ideal, and might be tried in the case of the -dew-cap by the observer. Finders are rarely fitted with dew-caps, -but certainly should be; the addition will often save trouble and -inconvenience. - -Be careful to cover up the objective or mirror with its proper cap -before removing it into the house. If this is not done, dewing at once -results, the very proper punishment for carelessness. This may seem a -caution so elementary as scarcely to be worth giving; but it is easier -to read and remember a hint than to have to learn by experience, which -in the case of a reflector will almost certainly mean a deteriorated -mirror film. Should the mirror, if you are using a reflector, become -dewed in spite of all precautions, do not attempt to touch the film -while it is moist, or you will have the pleasure of seeing it scale -off under your touch. Bring it into a room of moderate temperature, -or stand it in a through draught of dry air until the moisture -evaporates; and should any stain be left, make sure that the mirror is -absolutely dry before attempting to polish it off. With regard to this -matter of polishing, touch the mirror as seldom as possible with the -polishing-pad. Frequent polishing does far more harm than good, and -the mirror, if kept carefully covered when not in use, does not need -it. A fold of cotton-wool between the cap and the mirror will, if -occasionally taken out and dried, help greatly to preserve the film. - -Next comes a caution which beginners specially need. Almost everyone -on getting his first telescope wants to see everything as big as -possible, and consequently uses the highest powers. This is an entire -mistake. For a telescope of 2-1/2 inches aperture two eye-pieces, -or at most three, are amply sufficient. Of these, one may be low in -power, say 25 to 40, to take in large fields, and, if necessary, to -serve in place of a finder. Such an eye-piece will give many star -pictures of surprising beauty. Another may be of medium power, say 80, -for general work; and a third may be as high as 120 for exceptionally -fine nights and for work on double stars. Nominally a 2-1/2 inch, if -of very fine quality, should bear on the finest nights and on stars -a power of 100 to the inch, or 250. Practically it will do nothing -of the sort, and on most nights the half of this power will be found -rather too high. Indeed, the use of high powers is for many reasons -undesirable. A certain proportion of light to size must be preserved -in the image, or it will appear faint and 'clothy.' Further, increased -magnifying power means also increased magnification of every tremor of -the atmosphere; and with high powers the object viewed passes through -the field so rapidly that constant shifting of the telescope is -required, and only a brief glimpse can be obtained before the -instrument has to be moved again. It is infinitely more satisfactory -to see your object of a moderate size and steady than to see it much -larger, but hazy, tremulous, and in rapid motion. 'In inquiring about -the quality of some particular instrument,' remarks Sir Howard Grubb, -'a tyro almost invariably asks, "What is the highest power you can -use?" An experienced observer will ask, "What is the lowest power with -which you can do so and so?"' - -Do not be disappointed if your first views of celestial objects do not -come up to your expectations. They seldom do, particularly in respect -of the size which the planets present in the field. A good deal of -the discouragement so often experienced is due to the idea that the -illustrations in text-books represent what ought to be seen by anyone -who looks through a telescope. It has to be remembered that these -pictures are, for one thing, drawn to a large scale, in order to -insure clearness in detail, that they are in general the results of -observation with the very finest telescopes, and the work of skilled -observers making the most of picked nights. No one would expect to -rival a trained craftsman in a first attempt at his trade; yet most -people seem to think that they ought to be able at their first essay -in telescopic work to see and depict as much as men who have -spent half a lifetime in an apprenticeship to the delicate art of -observation. Given time, patience, and perseverance, and the skill -will come. The finest work shown in good drawings represents, not -what the beginner may expect to see at his first view, but a standard -towards which he must try to work by steady practice both of eye and -hand. In this connection it may be suggested that the observer should -take advantage of every opportunity of seeing through larger and finer -instruments than his own. This will teach him two things at least. -First, to respect his own small telescope, as he sees how bravely it -stands up to the larger instrument so far as regards the prominent -features of the celestial bodies; and, second, to notice how the -superior power of the large glass brings out nothing startlingly -different from that which is shown by his own small one, but a wealth -of delicate detail which must be looked for (compare Plate XV. with -Fig. 22). A little occasional practice with a large instrument will be -found a great encouragement and a great help to working with a small -one, and most possessors of large glasses are more than willing to -assist the owners of small ones. - -Do not be ashamed to draw what you see, whether it be little or much, -and whether you can draw well or ill. At the worst the result will -have an interest to yourself which no representation by another hand -can ever possess; at the best your drawings may in course of time come -to be of real scientific value. There are few observers who cannot -make some shape at a representation of what they see, and steady -practice often effects an astonishing improvement. But draw only what -you see with certainty. Some observers are gifted with abnormal powers -of vision, others with abnormal powers of imagination. Strange to say, -the results attained by these two classes differ widely in appearance -and in value. You may not be endowed with faculties which will enable -you to take rank in the former class; but at least you need not -descend to the latter. It is after all a matter of conscience. - -Do not be too hasty in supposing that everybody is endowed with a zeal -for astronomy equal to your own. The average man or woman does not -enjoy being called out from a warm fireside on a winter's night, no -matter how beautiful the celestial sight to be seen. Your friend -may politely express interest, but to tempt him to this is merely to -encourage a habit of untruthfulness. The cause of astronomy is not -likely to be furthered by being associated in any person's mind with -discomfort and a boredom which is not less real because it is veiled -under quite inadequate forms of speech. It is better to wait until the -other man's own curiosity suggests a visit to the telescope, if you -wish to gain a convert to the science. - -When observing in the open be sure to wrap up well. A heavy ulster or -its equivalent, and some form of covering for the feet which will keep -them warm, are absolute essentials. See that you are thoroughly warm -before you go out. In all probability you will be cold enough before -work is over; but there is no reason why you should make yourself -miserable from the beginning, and so spoil your enjoyment of a fine -evening. - -Having satisfied his craving for a general survey of everything in -the heavens that comes within the range of his glass, the beginner -is strongly advised to specialize. This is a big word to apply to the -using of a 2-1/2- or 3-inch telescope, but it represents the only -way in which interest can be kept up. It does no good, either to -the observer or to the science of astronomy, for him to take out -his glass, have a glance at Jupiter and another at the Orion nebula, -satisfy himself that the two stars of Castor are still two, wander -over a few bright clusters, and then turn in, to repeat the same -dreary process the next fine night. Let him make up his mind to stick -to one, or at most two, objects. Lunar work presents an attractive -field for a small instrument, and may be followed on useful lines, as -will be pointed out later. A spell of steady work upon Jupiter will at -least prepare the way and whet the appetite for a glass more adequate -to deal with the great planet. Should star work be preferred, a fine -field is opened up in connection with the variable stars, the chief -requirement of work in this department being patience and regularity, -a small telescope being quite competent to deal with a very large -number of interesting objects. - -The following comments in Smyth's usual pungent style are worth -remembering: 'The furor of a green astronomer is to possess himself -of all sorts of instruments--to make observations upon everything--and -attempt the determination of quantities which have been again and -again determined by competent persons, with better means, and -more practical acquaintance with the subject. He starts with an -enthusiastic admiration of the science, and the anticipation of new -discoveries therein; and all the errors consequent upon the momentary -impulses of what Bacon terms "affected dispatch" crowd upon him. Under -this course--as soon as the more hacknied objects are "seen up"--and -he can decide whether some are greenish-blue or bluish-green--the -excitement flags, the study palls, and the zeal evaporates in -hyper-criticism on the instruments and their manufacturers.' - -This is a true sketch of the natural history, or rather, of the -decline and fall, of many an amateur observer. But there is no reason -why so ignominious an end should ever overtake any man's pursuit of -the study if he will only choose one particular line and make it his -own, and be thorough in it. Half-study inevitably ends in weariness -and disgust; but the man who will persevere never needs to complain of -sameness in any branch of astronomical work. - - - - -CHAPTER III - -THE SUN - - -From its comparative nearness, its brightness and size, and its -supreme importance to ourselves, the sun commands our attention; and -in the phenomena which it presents there is found a source of abundant -and constantly varying interest. Observation of these phenomena can -only be conducted, however, after due precautions have been taken. Few -people have any idea of the intense glow of the solar light and heat -when concentrated by the object-glass of even a small telescope, and -care must be exercised lest irreparable damage be done to the eye. -Galileo is said to have finally blinded himself altogether, and -Sir William Herschel to have seriously impaired his sight by solar -observation. No danger need be feared if one or other of the common -precautions be adopted, and some of these will be shortly described; -but before we consider these and the means of applying them, let us -gather together briefly the main facts about the sun itself. - -Our sun, then, is a body of about 866,000 miles in diameter, and -situated at a distance of some 92,700,000 miles from us. In bulk -it equals 1,300,000 of our world, while it would take about 332,000 -earths to weigh it down. Its density, as can be seen from these -figures, is very small indeed. Bulk for bulk, it is considerably -lighter than the earth; in fact, it is not very much denser than -water, and this has very considerable bearing upon our ideas of its -constitution. - -Natural operations are carried on in this immense globe upon a scale -which it is almost impossible for us to realize. A few illustrations -gathered from Young's interesting volume, 'The Sun,' may help to -make clearer to us the scale of the ruling body of our system. Some -conception of the immensity of its distance from us may first be -gained from Professor Mendenhall's whimsical illustration. Sensation, -according to Helmholtz's experiments, travels at a rate of about 100 -feet per second. If, then, an infant were born with an arm long enough -to reach to the sun, and if on his birthday he were to exercise this -amazing limb by putting his finger upon the solar surface, he would -die in blissful ignorance of the fact that he had been burned, for -the sensation of burning would take 150 years to travel along that -stupendous arm. Were the sun hollowed out like a gigantic indiarubber -ball and the earth placed at its centre, the enclosing shell would -appear like a far distant sky to us. Our moon would have room to -circle within this shell at its present distance of 240,000 miles, and -there would still be room for another satellite to move in an orbit -exterior to that of the moon at a further distance of more than -190,000 miles. The attractive power of this great body is no less -amazing than its bulk. It has been calculated that were the attractive -power which keeps our earth coursing in its orbit round the sun to -cease, and to be replaced by a material bond consisting of steel wires -of a thickness equal to that of the heaviest telegraph-wires, these -would require to cover the whole sunward side of our globe in the -proportion of nine to each square inch. The force of gravity at the -solar surface is such that a man who on the earth weighs 10 stone -would, if transported to the sun, weigh nearly 2 tons, and, if he -remained of the same strength as on earth, would be crushed by his own -weight. - -[Illustration: - - PLATE V. - -The Sun, February 3, 1905. Royal Observatory, Greenwich.] - -The first telescopic view of the sun is apt, it must be confessed, -to be a disappointment. The moon is certainly a much more attractive -subject for a casual glance. Its craters and mountain ranges catch -the eye at once, while the solar disc presents an appearance of almost -unbroken uniformity. Soon, however, it will become evident that the -uniformity is only apparent. Generally speaking, the surface will -quickly be seen to be broken up by one or more dark spots (Plate V.), -which present an apparently black centre and a sort of grey shading -round about this centre. The margin of the disc will be seen to be -notably less bright than its central portions; and near the margin, -and oftenest, though not invariably, in connection with one of the -dark spots, there will be markings of a brilliant white, and often of -a fantastically branched shape, which seem elevated above the general -surface; while as the eye becomes more used to its work it will be -found that even a small telescope brings out a kind of mottled or -curdled appearance over the whole disc. This last feature may often -be more readily seen by moving the telescope so as to cause the solar -image to sweep across the field of view, or by gently tapping the -tube so as to cause a slight vibration. Specks of dirt which may have -gathered upon the field lens of the eye-piece will also be seen; but -these may be distinguished from the spots by moving the telescope -a little, when they will shift their place relatively to the other -features; and their exhibition may serve to suggest the propriety of -keeping eye-pieces as clean as possible. - -[Illustration: PLATE VI. - -Photograph of Bridged Sunspot (Janssen). _Knowledge_, February, 1890.] - -The spots when more closely examined will be found to present -endless irregularities in outline and size, as will be seen from the -accompanying plates and figures. On the whole, there is comparative -fidelity to two main features--a dark central nucleus, known as the -umbra, and a lighter border, the penumbra; but sometimes there are -umbræ which have no penumbra, and sometimes there are spots which can -scarcely be called more than penumbral shadings. The shape of the spot -is sometimes fairly symmetrical; at other times the most fantastic -forms appear. The umbra appears dark upon the bright disc, but is in -reality of dazzling lustre, sending to us, according to Langley, 54 -per cent. of the amount of heat received from a corresponding area of -the brilliant unspotted surface. Within the umbra a yet darker deep, -if it be a deep, has been detected by various observers, but is -scarcely likely to be seen with the small optical means which we are -contemplating. The penumbra is very much lighter in colour than the -umbra, and invariably presents a streaked appearance, the lines all -running in towards the umbra, and resembling very much the edge of a -thatched roof. It will be seen to be very much lighter in colour on -the edge next the umbra, while it shades to a much darker tone on -that side which is next to the bright undisturbed part of the surface -(Figs. 14 and 15). Frequently a spot will be seen interrupted by a -bright projection from the luminous surface surrounding it which may -even extend from side to side of the spot, forming a bridge across -it (Plate VI., and Figs. 16, 17, and 18). These are the outstanding -features of the solar spots, and almost any telescope is competent to -reveal them. But these appearances have to be interpreted, so far as -that is possible, and to have some scale applied to them before their -significance can in the least be recognised. The observer will do -well to make some attempt at realizing the enormous actual size of the -seemingly trifling details which his instrument shows. For example, -the spot in Figs. 14 and 15 is identical with that measured by Mr. -Denning on the day between the dates of my rough sketches; and its -greatest diameter was then 27,143 miles. Spots such as those of 1858, -of February, 1892, and February, 1904, have approached or exceeded -140,000 miles in diameter, while others have been frequently recorded, -which, though not to be compared to these leviathans, have yet -measured from 40,000 to 50,000 miles in diameter, with umbræ of 25,000 -to 30,000 miles. Of course, the accurate measurement of the spots -demands appliances which are not likely to be in a beginner's hands; -but there are various ways of arriving at an approximation which is -quite sufficient for the purpose in view--namely, a realization of -the scale of any spot as compared with that of the sun or of our own -earth. - -[Illustration: FIG. 14.--SUN-SPOT, JUNE 18, 1889.] - -[Illustration: FIG. 15.--SUN-SPOT, JUNE 20, 1889.] - -Of these methods, the simplest on the whole seems to be that given -by Mr. W. F. Denning in his admirable volume, 'Telescopic Work for -Starlight Evenings.' Fasten on the diaphragm of an eye-piece (the -blackened brass disc with a central hole which lies between the field -and eye lenses of the eye-piece) a pair of fine wires at right angles -to one another. Bring the edge of the sun up to the vertical wire, the -eye-piece being so adjusted that the sun's motion is along the line -of the horizontal wire. This can easily be accomplished by turning the -eye-piece round until the solar motion follows the line of the wire. -Then note the number of seconds which the whole disc of the sun takes -to cross the vertical wire. Note, in the second place, the time which -the spot to be measured takes to cross the vertical wire; and, having -these two numbers, a simple rule of three sum enables the diameter of -the spot to be roughly ascertained. For the sun's diameter, 866,000 -miles, is known, and the proportion which it bears to the number of -seconds which it takes to cross the wire will be the same as that -borne by the spot to its time of transit. Thus, to take Mr. Denning's -example, if the sun takes 133 seconds to cross the wire, and the spot -takes 6·5, then 133: 866,000::6·5:42,323, which latter number will be, -roughly speaking, the diameter of the spot in miles. This, method -is only a very rough approximation; still, it at least enables the -observer to form some conception of the scale of what is being seen. -It will answer best when the sun is almost south, and is, of course, -less and less accurate as the spot in question is removed from -the centre of the disc; for the sun being a sphere, and not a flat -surface, foreshortening comes largely and increasingly into play as -spots near the edge (or limb) of the disc. - -Continued observation will speedily lead to the detection of the -exceedingly rapid changes which often affect the spots and their -neighbourhood. There are instances in which a spot passes across the -disc without any other apparent changes save those which are due to -perspective; and the same spot may even accomplish a complete rotation -and appear again with but little change. But, generally speaking, it -will be noticed that the average spot changes very considerably during -the course of a single rotation. Often, indeed, the changes are so -rapid as to be apparent within the course of a few hours. Figs. 14 -and 15 represent a spot which was seen on June 18 and 20, 1889, and -sketched by means of a 2-1/2-inch refractor with a power of 80. A -certain proportion of the change noticeable is due to perspective, but -there are also changes of considerable importance in the structure -of the spot which are actual, and due to motion of its parts. Mr. -Denning's drawing ('Telescopic Work,' p. 95) shows the spot on the day -between these two representations, and exhibits an intermediate stage -of the change. The late Professor Langley has stated that when he was -making the exquisite drawing of a typical sun-spot which has become so -familiar to all readers of astronomical text-books and periodicals, -a portion of the spot equal in area to the continent of South America -changed visibly during the time occupied in the execution of the -drawing; and this is only one out of many records of similar tenor. -Indeed, no one who has paid any attention to solar observation can -fail to have had frequent instances of change on a very large scale -brought under his notice; and when the reality of such change has been -actually witnessed, it brings home to the mind, as no amount of mere -statement can, the extraordinary mobility of the solar surface, and -the fact that we are here dealing with a body where the conditions are -radically different from those with which we are familiar on our own -globe. Changes which involve the complete alteration in appearance -of areas of many thousand square miles have to be taken into -consideration as things of common occurrence upon the sun, and must -vitally affect our ideas of his constitution and structure (Figs. 16, -17, 18). - -[Illustration: FIG. 16.--SUN-SPOT SEEN IN 1870.] - -Little more can be done by ordinary observation with regard to the -spots and the general surface. Common instruments are not likely to -have much chance with the curious structure into which the coarse -mottling of the disc breaks up when viewed under favourable -circumstances. This structure, compared by Nasmyth to willow-leaves, -and by others to rice-grains, is beautifully seen in a number of the -photographs taken by Janssen and others; but it is seldom that it can -be seen to full advantage. - -[Illustration: FIG. 17.--ANOTHER PHASE OF SPOT (FIG. 16).] - -[Illustration: FIG. 18.--PHASE OF SPOT (FIGS. 16 AND 17).] - -On the other hand, the spots afford a ready means by which the -observer may for himself determine approximately the rotation period -of the sun. A spot will generally appear to travel across the solar -disc in about 13 days 14-1/2 hours, and to reappear at the eastern -limb after a similar lapse of time, thus making the apparent -rotation-period 27 days 5 hours. This has to be corrected, as the -earth's motion round the sun causes an apparent slackening in the rate -of the spots, and a deduction of about 2 days has to be made for this -reason, the resulting period being about 25 days 7 hours. It will -quickly be found that no single spot can be relied upon to give -anything like a precise determination, as many have motions of their -own independent of that due to the sun's rotation; and, in addition, -there has been shown to be a gradual lengthening of the period in high -latitudes. Thus, spots near the equator yield a period of 25·09 days, -those in latitude 15° N. or S. one of 25·44, and those in latitude 30° -one of 26·53. - -This law of increase, first established by Carrington, has been -confirmed by the spectroscopic measures of Dunér at Upsala. His -periods, while uniformly in excess of those derived from ordinary -observations, show the same progression. For 0° his period is 25·46 -days, for 15° 26·35, and for 30° 27·57. Continuing his researches up -to 15° from the solar pole, Dunér has found that at that point the -period of rotation is protracted to 38.5 days. - -Reference has already been made to the bright and fantastically -branched features which diversify the solar surface, generally -appearing in connection with the spots, and best seen near the limb, -though existing over the whole disc. These 'faculæ,' as they are -called, will be readily seen with a small instrument--I have seen them -easily with a 2-inch finder and a power of 30. They suggest at once to -the eye the idea that they are elevations above the general surface, -and look almost like waves thrown up by the convulsions which -produce the spots. The rotation-period given by them has also been -ascertained, and the result is shorter than that given by the spots. -In latitude 0° it is 24·66 days, at 15° it is 25·26, at 30° 25·48. -These varieties of rotation show irresistibly that the sun cannot -in any sense of the term be called a rigid body. As Professor Holden -remarks: 'It is more like a vast whirlpool, where the velocities of -rotation depend on the situation of the rotating masses, not only as -to latitude, but also as to depth beneath the rotating surface.' Plate -VII., from a photograph of the sun taken by Mr. Hale, in which the -surface is portrayed by the light of one single calcium ray of the -solar spectrum, presents a view of the mottled appearance of the disc, -together with several bright forms which the author of the photograph -considers to be faculæ. M. Deslandres, of the Meudon Observatory, who -has also been very successful in this new branch of solar photography, -considers, however, that these forms are not faculæ, but distinct -phenomena, to which he proposes to assign the name 'faculides'; and -for various reasons his view appears to be the more probable. They -are, however, in any case, in close relation with the faculæ, and, as -Miss Clerke observes, 'symptoms of the same disturbance.' - -[Illustration: - - PLATE VII. - -Solar Surface with Faculæ. Yerkes Observatory.] - -The question of the nature of the sun spots is one that at once -suggests itself; but it must be confessed that no very satisfactory -answer can yet be given to it. None of the many theories put forward -have covered all the observed facts, and an adequate solution seems -almost as far off as ever. No one can fail to be struck with the -resemblance which the spots present to cavities in the solar surface. -Instinctively the mind seems to regard the umbra of the spot as being -the centre of a great hollow of which the penumbra represents the -sloping sides; and for long it was generally held that Wilson's -theory, which assumed this appearance to correspond to an actual fact, -was correct. Wilson found by observation of certain spots that when -the spot was nearest to one limb the penumbra disappeared, either -altogether or in part, on the side towards the centre, and that this -process was reversed as the spot approached the opposite limb, the -portion of the penumbra nearest the centre of the disc being always -the narrowest. - -This is the order of appearances which would naturally follow if the -spot in question were a cavity; and if it were invariable there could -scarcely be any doubt as to its significance. But while the Wilsonian -theory has been recognised in all the text-books for many years, -there has always been a suspicion that it was by no means adequately -established, and that it was too wide an inference from the number of -cases observed; and of late years it has been falling more and more -into discredit. Howlett, for example, an observer of great experience, -has asserted that the appearances on which the theory is based are not -the rule, but the exception, and that therefore it must be given up. -Numbers of spots seem to present the appearance of elevations rather -than of depressions, and altogether it seems as though no category has -yet been attained which will embrace all the varieties of spot-form. -On this point further observation is very much needed, and the -work that has to be done is well within the reach of even moderate -instruments. - -The fact that sun-spots wax and wane in numbers in a certain definite -period was first ascertained by the amateur observer Schwabe of -Dessau, whose work is a notable example of what may be accomplished by -steadfast devotion to one particular branch of research. Without any -great instrumental equipment, Schwabe effected the discovery of -this most important fact--a discovery second to none made in the -astronomical field during the last century--simply by the patient -recording of the state of the sun's face for a period of over thirty -years, during which he succeeded in securing an observation, on the -average, on about 300 days out of every year. The period now accepted -differs slightly from that assigned by him, and amounts to 11·11 -years. Beginning with a minimum, when few spots or none may be visible -for some time, the spots will be found to increase gradually in -number, until, about four and a half years from the minimum, a maximum -is reached; and from this point diminution sets in, and results, -in about 6·6 years, in a second minimum. The period is not one -of absolute regularity--a maximum or a minimum may sometimes -lag considerably behind its proper time, owing to causes as yet -unexplained. Still, on the whole, the agreement is satisfactory. - -This variation is also accompanied by a variation in the latitude of -the spots. Generally they follow certain definite zones, mostly lying -between 10° and 35° on either side of the solar equator. As a minimum -approaches, they tend to appear nearer to the equator than usual; and -when the minimum has passed the reappearance of the spots takes the -opposite course, beginning in high latitudes. - -It has further been ascertained that a close connection exists between -the activity which results in the formation of sun-spots, and the -electrical phenomena of our earth. Instances of this connection have -been so repeatedly observed as to leave no doubt of its reality, -though the explanation of it has still to be found. It has been -suggested by Young that there may be immediate and direct action in -this respect between the sun and the earth, an action perhaps kindred -with that solar repulsive force which seems to drive off the material -of a comet's tail. As yet not satisfactorily accounted for is the fact -that it does not always follow that the appearance of a great sun-spot -is answered by a magnetic storm on the earth. On the average the -connection is established; but there are many individual instances -of sun-spots occurring without any answering magnetic thrill from the -earth. To meet this difficulty, Mr. E. W. Maunder has proposed a view -of the sun's electrical influence upon our earth, which, whether it -be proved or disproved in the future, seems at present the most living -attempt to account for the observed facts. Briefly, he considers it -indubitably proved-- - - 1. That our magnetic disturbances are connected with the sun. - - 2. That the sun's action, of whatever nature, is not from the sun as - a whole, but from restricted areas. - - 3. That the sun's action is not radiated, but restricted in - direction. - -On his view, the great coronal rays or streamers seen in total -eclipses (Plate VIII.) are lines of force, and similarly the magnetic -influence which the sun exerts upon the earth acts along definite and -restricted lines. Thus a disturbance of great magnitude upon the sun -would only be followed by a corresponding disturbance on the earth -if the latter happened to be at or near the point where it would fall -within the sweep of the line of magnetic force emanating from the -sun. In proportion as the line of magnetic force approached to falling -perpendicularly on the earth, the magnetic disturbance would be large: -in proportion as it departed from the perpendicular it would diminish -until it vanished finally altogether. The suggestion seems an inviting -one, and has at least revived very considerably the interest in these -phenomena. - -Such, then, are the solar features which offer themselves to direct -observation by means of a small telescope. The spots, apart from their -own intrinsic interest, are seen to furnish a fairly accurate method -by which the observer can determine for himself the sun's rotation -period. Their size may be approximately measured, thus conveying to -the mind some idea of the enormous magnitude of the convulsions which -take place upon this vast globe. The spot zones may be noted, together -with the gradual shift in latitude as the period approaches or recedes -from minimum; while observations of individual spots may be conducted -with a view to gathering evidence which shall help either to confirm -or to confute the Wilsonian theory. In this latter department of -observation the main requisite is that the work should be done -systematically. Irregular observation is of little or no value; but -steady work may yield results of high importance. While, however, -systematic observation is desirable, it is not everyone who has the -time or the opportunity to give this; and to many of us daily solar -observation may represent an unattainable ideal. Even if this be the -case, there still remains an inexhaustible fund of beauty and interest -in the sun-spots. It does not take regular observation to enable one -to be interested in the most wonderful intricacy and beauty of the -solar detail, in its constant changes, and in the ideas which even -casual work cannot fail to suggest as to the nature and mystery of -that great orb which is of such infinite importance to ourselves. - -A small instrument, used in the infrequent intervals which may be all -that can be snatched from the claims of other work, will give the -user a far more intelligent interest in the sun, and a far better -appreciation of its features, than can be gained by the most careful -study of books. In this, and in all other departments of astronomy, -there is nothing like a little practical work to give life to the -subject. - -In the conduct of observation, however, regard must be paid to the -caution given at the beginning of this chapter. Various methods have -been adopted for minimizing the intense glare and heat. For small -telescopes--up to 2-1/2 inches or so--the common device of the -interposition of a coloured glass between the eye-piece and the eye -will generally be found sufficient on the score of safety, though -other arrangements may be found preferable. Such glasses are usually -supplied with small instruments, mounted in brass caps which screw or -slide on to the ends of the various eye-pieces. Neutral tint is -the best, though a combination of green and red also does well. -Red transmits too much heat for comfort. Should dark glasses not be -supplied, it is easy to make them by smoking a piece of glass to the -required depth, protecting it from rubbing by fastening over it a -covering glass which rests at each end on a narrow strip of cardboard. - -With anything larger than 2-1/2 inches, dark glass is never quite -safe. A 3-inch refractor will be found quite capable of cracking and -destroying even a fairly thick glass if observation be long continued. -The contrivance known as a polarizing eye-piece was formerly pretty -much beyond the reach of the average amateur by reason of its -costliness. Such eye-pieces are now becoming much cheaper, and -certainly afford a most safe and pleasant way of viewing the sun. They -are so arranged that the amount of light and heat transmitted can be -reduced at will, so as to render the use of a dark glass unnecessary, -thus enabling the observer to see all details in their natural -colouring. The ordinary solar diagonal, in which the bulk of the rays -is rejected, leaving only a small portion to reach the eye, is cheaper -and satisfactory, though a light screen-glass is still required with -it. But unquestionably the best general method of observing, and also -the least costly, is that of projecting the sun's image through the -telescope upon a prepared white surface, which may be of paper, or -anything else that may be found suitable. - -To accomplish this a light framework may be constructed in the shape -of a truncated cone. At its narrow end it slips or screws on to the -eye-end of the telescope, and it may be made of any length required, -in proportion to the size of solar disc which it is desired to obtain. -It should be covered with black cloth, and its base may be a board -with white paper stretched on it to receive the image, which is viewed -through a small door in the side. In place of the board with white -paper, other expedients may be tried. Noble recommends a surface of -plaster of Paris, smoothed while wet on plate glass, and this is very -good if you can get the plaster smooth enough. I have found white -paint, laid pretty thickly on glass and then rubbed down to a smooth -matt surface by means of cuttle-fish bone, give very satisfactory -results. Should it be desired to exhibit the sun's image to several -people at once, this can easily be done by projecting it upon a sheet -of paper fastened on a drawing-board, and supported at right angles to -the telescope by an easel. The framework, or whatever takes its place, -being in position, the telescope is pointed at the sun by means of -its shadow; when this is perfectly round, or when the shadow of the -framework perfectly corresponds to the shape of its larger end, the -sun's image should be in the field of view. - - - - -CHAPTER IV - -THE SUN'S SURROUNDINGS - - -We have now reached the point beyond which mere telescopic power will -not carry us, a point as definite for the largest instrument as for -the smallest. We have traced what can be seen on the visible sun, but -beyond the familiar disc, and invisible at ordinary seasons or with -purely telescopic means, there lie several solar features of the -utmost interest and beauty, the study of which very considerably -modifies our conception of the structure of our system's ruler. These -features are only revealed in all their glory and wonder during the -fleeting moments in which a total eclipse is central to any particular -portion of the earth's surface. - -A solar eclipse is caused by the fact that the moon, in her revolution -round the earth, comes at certain periods between us and the sun, and -obscures the light of the latter body either partially or totally. -Owing to the fact that the plane of the orbit in which the moon -revolves round the earth does not coincide with that in which the -earth revolves round the sun, the eclipse is generally only partial, -the moon not occupying the exact line between the centres of the sun -and the earth. The dark body of the moon then appears to cut off a -certain portion, larger or smaller, of the sun's light; but none of -the extraordinary phenomena to be presently described are witnessed. -Even during a partial eclipse, however, the observer may find -considerable interest in watching the outline of the dark moon, as -projected upon the bright background of the sun. It is frequently -jagged or serrated, the projections indicating the existence, on the -margin of the lunar globe, of lofty mountain ranges. - -[Illustration: FIG. 19.--ECLIPSES OF THE SUN AND MOON.] - -Occasionally the conditions are such that the moon comes centrally -between the earth and the sun (Fig. 19), and then an eclipse occurs -which may be either total or annular. The proportion between the -respective distances from us of the sun and the moon is such that -these two bodies, so vastly different in real bulk, are sensibly the -same in apparent diameter, so that a very slight modification of the -moon's distance is sufficient to reduce her diameter below that of -the sun. The lunar orbit is not quite circular, but has a small -eccentricity. It may therefore happen that an eclipse occurs when the -moon is nearest the earth, at which point she will cover the sun's -disc with a little to spare; or the eclipse may occur when she is -furthest away from the earth, in which case the lunar diameter will -appear less than that of the sun, and the eclipse will be only an -annular one, and a bright ring or 'annulus' of sunlight will be seen -surrounding the dark body of the moon at the time when the eclipse is -central. - -All conditions being favourable, however--that is to say, the eclipse -being central, and the moon at such a position in her orbit as to -present a diameter equal to, or slightly greater than, that of the -sun--a picture of extraordinary beauty and wonder reveals itself the -moment that totality has been established. The centre of the view is -the black disc of the moon. From behind it on every side there streams -out a wonderful halo of silvery light which in some of its furthest -streamers may sometimes extend to a distance of several million -miles. In the Indian Eclipse of 1898, for example, one streamer was -photographed by Mrs. Maunder, which extended to nearly six diameters -from the limb of the eclipsed sun (Plate VIII.). The structure of this -silvery halo is of the most remarkable complexity, and appears to be -subject to continual variations, which have already been ascertained -to be to some extent periodical and in sympathy with the sun-spot -period. At its inner margin this halo rests upon a ring of crimson -fire which extends completely round the sun, and throws up here and -there great jets or waves, which frequently assume the most fantastic -forms and rise to heights varying from 20,000 to 100,000 miles, or -in extreme instances to a still greater height. To these appearances -astronomers have given the names of the Corona, the Chromosphere, and -the Prominences. The halo of silvery light is the Corona, the ring -of crimson fire the Chromosphere, and the jets or waves are the -Prominences. - -[Illustration: PLATE VIII. - -Coronal Streamers: Eclipse of 1898. From Photographs by Mrs. Maunder.] - -The Corona is perhaps the most mysterious of all the sun's -surroundings. As yet its nature remains undetermined, though the -observations which have been made at every eclipse since attention was -first directed to it have been gradually suggesting and strengthening -the idea that there exists a very close analogy between the coronal -streamers and the Aurora or the tails of comets. The extreme rarity -of its substance is conclusively proved by the fact that such -insubstantial things as comets pass through it apparently unresisted -and undelayed. Its structure presents variations in different -latitudes. Near the poles it exhibits the appearance of brushes of -light, the rays shooting out from the sun towards each summit of -his axis, while the equatorial rays curve over, presenting a sort -of fish-tail appearance. These variations are modified, as already -mentioned, by some cause which is at all events coincident with the -sun-spot period. At minimum the corona presents itself with polar -brushes of light and fish-tail equatorial rays, the latter being -sometimes of the most extraordinary length, as in the case of the -eclipse of July 29, 1878, when a pair of these wonderful streamers -extended east and west of the eclipsed sun to a distance of at least -10,000,000 miles. - -When an eclipse occurs at a spot-maximum, the distribution of the -coronal features is found to have entirely changed. Instead of being -sharply divided into polar brushes and equatorial wings, the streamers -are distributed fairly evenly around the whole solar margin, in a -manner suggesting the rays from a star, or a compass-card ornament. -The existence of this periodic change has been repeatedly confirmed, -and there can be no doubt that the corona reflects in its structure -the system of variation which prevails upon the sun. 'The form of the -corona,' says M. Deslandres, 'undergoes periodical variations, which -follow the simultaneous periodical variations already ascertained for -spots, faculæ, prominences, and terrestrial magnetism.' Certainty -as to its composition has not yet been attained; nor is this to be -wondered at, for the corona is only to be seen in the all too brief -moments during which a total eclipse is central, and then only over -narrow tracts of country, and all attempts to secure photographs of -it at other times have hitherto failed. When examined with the -spectroscope, it yields evidence that its light is derived from three -sources--from the incandescence of solid or liquid particles, from -reflected sunshine, and from gaseous emissions. The characteristic -feature of the coronal spectrum is a bright green line belonging to an -unknown element which has been named 'coronium.' - -The Chromosphere and the Prominences, unlike the elusive corona, -may now be studied continuously by means of the spectroscope, and -instruments are now made at a comparatively moderate price, which, in -conjunction with a small telescope--3 inches will suffice--will enable -the observer to secure most interesting and instructive views of -both. The chromosphere is, to use Miss Clerke's expression, 'a solar -envelope, but not a solar atmosphere.' It surrounds the whole globe of -the sun to a depth of probably from 3,000 to 4,000 miles, and has -been compared to an ocean of fire, but seems rather to present the -appearance of a close bristling covering of flames which rise above -the surface of the visible sun like the blades of grass upon a lawn. -Any one of these innumerable flames may be elevated into unusual -proportions in obedience to the vast and mysterious forces which -are at work beneath, and then becomes a prominence. On the whole -the constitution of the chromosphere is the same as that of the -prominences. Professor Young has found that its normal constituents -are hydrogen, helium, coronium, and calcium. But whenever there is any -disturbance of its surface, the lines which indicate the presence of -these substances are at once reinforced by numbers of metallic -lines, indicating the presence of iron, sodium, magnesium, and other -substances. - -The scale to which these upheavals attain in the prominences is very -remarkable. For example, Young records the observation of a prominence -on October 7, 1880. When first seen, at about 10.30 a.m., it was about -40,000 miles in height and attracted no special attention. Half -an hour later it had doubled its height. During the next hour it -continued to soar upwards until it reached the enormous altitude of -350,000 miles, and then broke into filaments which gradually faded -away, until by 12.30 there was nothing left of it. On another occasion -he recorded one which darted upwards in half an hour from a moderate -elevation to a height of 200,000 miles, and in which clouds of -hydrogen must have been hurled aloft with a speed of at least -200 miles per second. (Plate IX. gives a representation of the -chromosphere and prominences from a photograph by M. Deslandres.) -Between the chromosphere and the actual glowing surface of the sun -which we see lies what is known as the 'reversing layer,' from the -fact that owing to its presence the dark lines of the solar spectrum -are reversed in the most beautiful way during the second at the -beginning and end of totality in an eclipse. Young, who was the first -to observe this phenomenon (December 22, 1870), remarks of it that as -soon as the sun has been hidden by the advancing moon, 'through the -whole length of the spectrum, in the red, the green, the violet, the -bright lines flash out by hundreds and thousands, almost startlingly; -as suddenly as stars from a bursting rocket-head, and as evanescent, -for the whole thing is over within two or three seconds.' - -[Illustration: - - PLATE IX. - -The Chromosphere and Prominences, April 11, 1894. Photographed by M. -H. Deslandres.] - -The spectrum of the reversing layer has since been photographed on -several occasions--first by Shackleton, at Novaya Zemlya, on August 9, -1896--and its bright lines have been found to be true reversals of the -dark lines of the normal solar spectrum. This layer may be described -as a thin mantle, perhaps 500 miles deep, of glowing metallic vapours, -surrounding the whole body of the sun, and normally, strange to say, -in a state of profound quiescence. Its presence was of course an -integral part of Kirchhoff's theory of the mode in which the dark -lines of the solar spectrum were produced. Such a covering was -necessary to stop the rays whose absence makes the dark lines; and it -was assumed that the rays so stopped would be seen bright, if only the -splendour of the solar light could be cut off. These assumptions have -therefore been verified in the most satisfactory manner. - -Thus, then, the structure of the sun as now known is very different -from the conception of it which would be given by mere naked-eye, -or even telescopic, observation. We have first the visible bright -surface, or photosphere, with its spots, faculæ, and mottling, and -surrounded by a kind of atmosphere which absorbs much of its light, as -is evidenced by the fact that the solar limb is much darker than the -centre of the disc (Plate V.); next the reversing layer, consisting of -an envelope of incandescent vapours, which by their absorption of the -solar rays corresponding to themselves give rise to the dark lines in -the spectrum. Beyond these again lies the chromosphere, rising into -gigantic eruptive or cloud-like forms in the prominences; and yet -further out the strange enigmatic corona. - -It must be confessed that the reversing layer, the chromosphere, and -the corona lie somewhat beyond the bounds and purpose of this volume; -but without mention of them any account of the sun is hopelessly -incomplete, and it is not at all improbable that a few years may see -the spectroscope so brought within the reach of ordinary observers as -to enable them in great measure to realize for themselves the facts -connected with the complex structure of the sun. In any case, the mere -recital of these facts is fitted to convey to the mind a sense of the -utter inadequacy of our ordinary conceptions of that great body which -governs the motions of our earth, and supplies to it and to the other -planets of our system life and heat, light and guidance. With the -unaided eye we view the sun as a small tranquil white disc; the -telescope reveals to us that it is a vast globe convulsed by storms -which involve the upheaval or submersion, within a few hours, of areas -far greater than our own world; the spectroscope or the total eclipse -adds to this revelation the further conception of a sweltering ocean -of flame surrounding the whole solar surface, and rising in great -jets of fire which would dissolve our whole earth as a drop of wax -is melted in the flame of a candle; while beyond that again the -mysterious corona stretches through unknown millions of miles its -streamers of silvery light--the great enigma of solar physics. Other -bodies in the universe present us with pictures of beautiful symmetry -and vast size: some even within our own system suggest by their -appearance the presence within their frame of tremendous forces -which are still actively moulding them; but the sun gives us the most -stupendous demonstration of living force that the mind of man can -apprehend. Of course there are many stars which are known to be suns -on which processes similar to those we have been considering are being -carried on on a yet vaster scale; but the nearness of our sun brings -the tremendous energy of these processes home to us in a way that -impresses the mind with a sense almost of fear. - -'Is it possible,' says Professor Newcomb, 'to convey to the mind any -adequate conception of the scale on which natural operations are here -carried on? If we call the chromosphere an ocean of fire, we must -remember that it is an ocean hotter than the fiercest furnace, and as -deep as the Atlantic is broad. If we call its movements hurricanes, we -must remember that our hurricanes blow only about 100 miles an hour, -while those of the chromosphere blow as far in a single second. They -are such hurricanes as, coming down upon us from the north, would, in -thirty seconds after they had crossed the St. Lawrence, be in the Gulf -of Mexico, carrying with them the whole surface of the continent in -a mass not simply of ruin, but of glowing vapour.... When we speak of -eruptions, we call to mind Vesuvius burying the surrounding cities in -lava; but the solar eruptions, thrown 50,000 miles high, would engulf -the whole earth, and dissolve every organized being on its surface in -a moment. When the mediæval poets sang, "Dies iræ, dies illa, solvet -sæclum in favilla," they gave rein to their wildest imagination -without reaching any conception of the magnitude or fierceness of the -flames around the sun.' - -The subject of the maintenance of the sun's light and heat is one that -scarcely falls within our scope, and only a few words can be devoted -to it. It is practically impossible for us to attain to any adequate -conception of the enormous amount of both which is continually being -radiated into space. Our own earth intercepts less than the two -thousand millionth part of the solar energy. It has been estimated -that if a column of ice 2-1/4 miles in diameter could be erected to -span the huge interval of 92,700,000 miles between the earth and the -sun, and if the sun could concentrate the whole of his heat upon it, -this gigantic pillar of ice would be dissolved in a single second; in -seven more it would be vaporized. The amount of heat developed on -each square foot of solar surface is 'equivalent to the continuous -evolution of about 10,000 horse-power'; or, as otherwise stated, -is equal to that which would be produced by the hourly burning of -nine-tenths of a ton of anthracite coal on the same area of 1 square -foot. - -It is evident, therefore, that mere burning cannot be the source of -supply. Lord Kelvin has shown that the sun, if composed of solid coal, -would burn itself out in about 6,000 years. - -Another source of heat may be sought in the downfall of meteoric -bodies upon the solar surface; and it has been calculated that the -inrush of all the planets of our system would suffice to maintain the -present energy for 45,604 years. But to suppose the existence near -the sun of anything like the amount of meteoric matter necessary to -account, on this theory, for the annual emission of heat involves -consequences which are quite at variance with observed facts, though -it is possible, or even practically certain, that a small proportion -of the solar energy is derived from this source. - -We are therefore driven back upon the source afforded by the slow -contraction of the sun. If this contraction happens, as it must, an -enormous amount of heat must be developed by the process, so much so -that Helmholtz has shown that an annual contraction of 250 feet would -account for the total present emission. This contraction is so -slow that about 9,500 years would need to elapse before it became -measurable with anything like certainty. In the meantime, then, we may -assume as a working hypothesis that the light and heat of the central -body of our system are maintained, speaking generally, by his steady -contraction. Of course this process cannot have gone on, and cannot go -on, indefinitely; but as the best authorities have hitherto regarded -the date when the sun shall have shrunk so far as to be no longer able -to support life on the earth as distant from us by some ten million -years, and as the latest investigations on the subject, those of Dr. -See, point in the direction of a very large extension of this limit, -we may have reasonable comfort in the conviction that the sun will -last our time. - - - - -CHAPTER V - -MERCURY - - -The planet nearest to the sun is not one which has proved itself -particularly attractive to observers in the past; and the reasons for -its comparative unattractiveness are sufficiently obvious. Owing to -the narrow limits of his orbit, he never departs further from the sun -either East or West than between 27° and 28°, and the longest period -for which he can be seen before sunrise or after sunset is two hours. -It follows that, when seen, he is never very far from the horizon, -and is therefore enveloped in the denser layers of our atmosphere, and -presents the appearance sadly familiar to astronomers under the name -of 'boiling,' the outlines of the planet being tremulous and confused. -Of course, observers who have powerful instruments provided with -graduated circles can find and follow him during the day, and it is in -daylight that nearly all the best observations have been secured. But -with humbler appliances observation is much restricted; and, in fact, -probably many observers have never seen the planet at all. - -Views of Mercury, however, such as they are, are by no means so -difficult to secure as is sometimes supposed. Denning remarks that -he has seen the planet on about sixty-five occasions with the naked -eye--that in May, 1876, he saw it on thirteen different evenings, and -on ten occasions between April 22 and May 11, 1890; and he states -it as his opinion that anyone who will make it a practice to obtain -naked-eye views should succeed from about twelve to fifteen times in -the year. During the spring of 1905, to take a recent example, Mercury -was quite a conspicuous object for some time in the Western sky, close -to the horizon, and there was no difficulty whatever in obtaining -several views of him both with the telescope and with the naked eye, -though the disc was too much disturbed by atmospheric tremors -for anything to be made of it telescopically. In his little book, -'Half-hours with the Telescope,' Proctor gives a method of finding the -planet which would no doubt prove quite satisfactory in practice, but -is somewhat needlessly elaborate. Anyone who takes the pains to -note those dates when Mercury is most favourably placed for -observation--dates easily ascertained from Whitaker or any other good -almanac--and to carefully scan the sky near the horizon after sunset -either with the naked eye, or, better, with a good binocular, will -scarcely fail to detect the little planet which an old English writer -more graphically than gracefully calls 'a squinting lacquey of the -sun.' - -Mercury is about 3,000 miles in diameter, and circles round the sun at -a mean distance of 36,000,000 miles. His orbit is very eccentric, so -that when nearest to the sun this distance is reduced to 28,500,000, -while when furthest away from him it rises to 43,500,000. The -proportion of sunlight which falls upon the planet must therefore vary -considerably at different points of his orbit. In fact, when he is -nearest to the sun he receives nine times as much light and heat -as would be received by an equal area of the earth; but when the -conditions are reversed, only four times the same amount. The bulk of -the planet is about one-nineteenth that of the earth, but its weight -is only one-thirtieth, so that its materials are proportionately less -dense than those of our own globe. It is about 3-1/2 times as dense as -water, the corresponding figure for the earth being rather more than -5-1/2. - -Further, it is apparent that the materials of which Mercury's globe -is composed reflect light very feebly. It has been calculated that the -planet reflects only 17 per cent. of the light which falls upon it, 83 -per cent. being absorbed; and this fact obviously carries with it the -conclusion that the atmosphere of this little world cannot be of any -great density. For clouds in full sunlight are almost as brilliantly -white as new-fallen snow, and if Mercury were surrounded with a -heavily cloud-laden atmosphere, he would reflect nearly five times the -amount of light which he at present sends out into space. - -As his orbit falls entirely within that of our own earth, Mercury, -like his neighbour Venus, exhibits phases. When nearest to us the -planet is 'new,' when furthest from us it is 'full,' while at the -stages intermediate between these points it presents an aspect like -that of the moon at its first and third quarters. It may thus be -seen going through the complete series from a thin crescent up to a -completely rounded disc. The smallness of its apparent diameter, -and the poor conditions under which it is generally seen, make the -observation of these phases by no means so easy as in the case of -Venus; yet a small instrument will show them fairly well. Observers -seem generally to agree that the surface has a dull rosy tint, and a -few faint markings have, by patient observation, been detected upon -it (Fig. 20); but these are far beyond the power of small telescopes. -Careful attention to them and to the rate of their apparent motion -across the disc has led to the remarkable conclusion that Mercury -takes as long to rotate upon his axis as he does to complete his -annual revolution in his orbit; in other words, that his day and his -year are of the same length--namely, eighty-eight of our days. This -conclusion, when announced in 1882 by the well-known Italian observer -Schiaparelli, was received with considerable hesitation. It has, -however, been confirmed by many observers, notably by Lowell at -Flagstaff Observatory, Arizona, and is now generally received, -though some eminent astronomers still maintain that really nothing is -certainly known as to the period of rotation. - -[Illustration: FIG. 20.--MERCURY AS A MORNING STAR. W. F. DENNING, -10-INCH REFLECTOR.] - -If the long period be accepted, it follows that Mercury must always -turn the same face to the sun--that one of his hemispheres must always -be scorching under intense heat, and the other held in the grasp of an -unrelenting cold of which we can have no conception. 'The effects of -these arrangements upon climate,' says Miss Agnes Clerke, 'must be -exceedingly peculiar.... Except in a few favoured localities, the -existence of liquid water must be impossible in either hemisphere. -Mercurian oceans, could they ever have been formed, should long ago -have been boiled off from the hot side, and condensed in "thick-ribbed -ice" on the cold side.' - -From what has been said it will be apparent that Mercury is scarcely -so interesting a telescopic object as some of the other planets. Small -instruments are practically ruled out of the field by the diminutive -size of the disc which has to be dealt with, and the average -observer is apt to be somewhat lacking in the patience without which -satisfactory observations of an object so elusive cannot be secured. -At the same time there is a certain amount of satisfaction and -interest in the mere detection of the little sparkling dot of light in -the Western sky after the sun has set, or in the Eastern before it has -risen; and the revelation of the planet's phase, should the telescope -prove competent to accomplish it, gives better demonstration than any -diagram can convey of the interior position of this little world. It -is consoling to think that even great telescopes have made very little -indeed of the surface of Mercury. Schiaparelli detected a number -of brownish stripes and streaks, which seemed to him sufficiently -permanent to be made the groundwork of a chart, and Lowell has made -a remarkable series of observations which reveal a globe seamed and -scarred with long narrow markings; but many observers question the -reality of these features altogether. - -It is perhaps just within the range of possibility that, even with -a small instrument, there may be detected that blunting of the South -horn of the crescent planet which has been noticed by several reliable -observers. But caution should be exercised in concluding that such a -phenomenon has been seen, or that, if seen, it has been more than -an optical illusion. Those who have viewed Mercury under ordinary -conditions of observation will be well aware how extremely difficult -it is to affirm positively that any markings on the surface or any -deformations of the outline of the disc are real and actual facts, and -not due to the atmospheric tremors which affect the little image. - -Interesting, though of somewhat rare occurrence, are the transits of -Mercury, when the planet comes between us and the sun, and passes as a -black circular dot across the bright solar surface. The first occasion -on which such a phenomenon was observed was November 7, 1631. The -occurrence of this transit was predicted by Kepler four years in -advance; and the transit itself was duly observed by Gassendi, though -five hours later than Kepler's predicted time. It gives some idea -of the uncertainty which attended astronomical calculations in those -early days to learn that Gassendi considered it necessary to begin -his observations two days in advance of the time fixed by Kepler. -If, however, the time of a transit can now be predicted with almost -absolute accuracy, it need not be forgotten that this result is -largely due to the labours of men who, like Kepler, by patient effort -and with most imperfect means, laid the foundations of the most -accurate of all sciences. - -The next transit of Mercury available for observation will take place -on November 14, 1907. It may be noted that during transits certain -curious appearances have been observed. The planet, for example, -instead of appearing as a black circular dot, has been seen surrounded -with a luminous halo, and marked by a bright spot upon its dark -surface. No satisfactory explanation of these appearances has been -offered, and they are now regarded as being of the nature of optical -illusions, caused by defects in the instruments employed, or by -fatigue of the eye. It might, however, be worth the while of any who -have the opportunity of observing the transit of 1907 to take notice -whether these features do or do not present themselves. For their -convenience it may be noted that the transit will begin about eleven -o'clock on the forenoon of November 14, and end about 12.45. - - - - -CHAPTER VI - -VENUS - - -Next in order to Mercury, proceeding outwards from the sun, comes the -planet Venus, the twin-sister, so to speak, of the earth, and familiar -more or less to everybody as the Morning and Evening Star. The -diameter of Venus, according to Barnard's measures with the 36-inch -telescope of the Lick Observatory, is 7,826 miles; she is therefore -a little smaller than our own world. Her distance from the sun is a -trifle more than 67,000,000 miles, and her orbit, in strong contrast -with that of Mercury, departs very slightly from the circular. Her -density is a little less than that of the earth. - -There is no doubt that, to the unaided eye, Venus is by far the most -beautiful of all the planets, and that none of the fixed stars can -for a moment vie with her in brilliancy. In this respect she is -handicapped by her position as an inferior planet, for she never -travels further away from the sun than 48°, and, even under the most -favourable circumstances, cannot be seen for much more than four hours -after sunset. Thus we never have the opportunity of seeing her, as -Mars and Jupiter can be seen, high in the South at midnight, and far -above the mists of the horizon. Were it possible to see her under such -conditions, she would indeed be a most glorious object. Even as it -is, with all the disadvantages of a comparatively low position and a -denser stratum of atmosphere, her brilliancy is extremely striking, -having been estimated, when at its greatest, at about nine times that -of Sirius, which is the brightest of all the fixed stars, and five -times that of Jupiter when the giant planet is seen to the best -advantage. It is, in fact, so great that, when approaching its -maximum, the shadows cast by the planet's light are readily seen, more -especially if the object casting the shadow have a sharply defined -edge, and the shadow be received upon a white surface--of snow, for -instance. This extreme brilliance points to the fact that the surface -of Venus reflects a very large proportion of the sunlight which falls -upon it--a proportion estimated as being at least 65 per cent., or -not very much less than that reflected by newly fallen snow. Such -reflective power at once suggests an atmosphere very dense and heavily -cloud-laden; and other observations point in the same direction. -So that in the very first two planets of the system we are at once -confronted with that diversity in details which coexists throughout -with a broad general likeness as to figure, shape of orbit, and other -matters. Mercury's reflective power is very small, that of Venus is -exceedingly great; Mercury's atmosphere seems to be very attenuated, -that of Venus, to all appearance, is much denser than that of our own -earth. - -Periodically, when Venus appears in all her splendour in the Western -sky, one meets with the suggestion that we are having a re-appearance -of the Star of Bethlehem; and it seems to be a perpetual puzzle to -some people to understand how the same body can be both the Morning -and the Evening Star. Those who have paid even the smallest attention -to the starry heavens are not, however, in the least likely to make -any mistake about the sparkling silver radiance of Venus; and it -would seem as though the smallest application of common-sense to the -question of the apparent motion of a body travelling round an almost -circular orbit which is viewed practically edgewise would solve for -ever the question of the planet's alternate appearance on either side -of the sun. Such an orbit must appear practically as a straight line, -with the sun at its middle point, and along this line the planet will -appear to travel like a bead on a wire, appearing now on one side of -the sun, now on another. If the reader will draw for himself a diagram -of a circle (sufficiently accurate in the circumstances), with the -sun in the centre, and divide it into two halves by a line supposed to -pass from his eye through the sun, he will see at once that when this -circle is viewed edgewise, and so becomes a straight line, a planet -travelling round it is bound to appear to move back and forward along -one half of it, and then to repeat the same movement along the other -half, passing the sun in the process. - -Like Mercury, and for the same reason of a position interior to our -orbit, Venus exhibits phases to us, appearing as a fully illuminated -disc when she is furthest from the earth, as a half-moon at the -two intermediate points of her orbit, and as a new moon when she is -nearest to us. The actual proof of the existence of these phases was -one of the first-fruits which Galileo gathered by means of his -newly invented telescope. It is said that Copernicus predicted their -discovery, and they certainly formed one of the conclusive proofs -of the correctness of his theory of the celestial system. It was the -somewhat childish custom of the day for men of science to put forth -the statement of their discoveries in the form of an anagram, over -which their fellow-workers might rack their brains; probably this was -done somewhat for the same reason which nowadays makes an inventor -take out a patent, lest someone should rob the discoverer of the -credit of his discovery before he might find it convenient to make -it definitely public. Galileo's anagram, somewhat more poetically -conceived than the barbarous alphabetic jumble in which Huygens -announced his discovery of the nature of Saturn's ring, read as -follows: 'Hæc immatura a me jam frustra leguntur o. y.' This, -when transposed into its proper order, conveyed in poetic form the -substance of the discovery: 'Cynthiæ figuras æmulatur Mater Amorum' -(The Mother of the Loves [Venus] imitates the phases of Cynthia). It -is true that two letters hang over the end of the original sentence, -but too much is not to be expected of an anagram. - -As a telescopic object, Venus is apt to be a little disappointing. Not -that her main features are difficult to see, or are not beautiful. A -2-inch telescope will reveal her phases with the greatest ease, and -there are few more exquisite sights than that presented by the silvery -crescent as she approaches inferior conjunction. It is a picture which -in its way is quite unique, and always attractive even to the most -hardened telescopist. - -Still, what the observer wants is not merely confirmation of the -statement that Venus exhibits phases. The physical features of a -planet are always the most interesting, and here Venus disappoints. -That very brilliant lustre which makes her so beautiful an object to -the naked eye, and which is even so exquisite in the telescopic view, -is a bar to any great progress in the detection of the planet's actual -features. For it means that what we are seeing is not really the -surface of Venus, but only the sunward side of a dense atmosphere--the -'silver lining' of heavy clouds which interpose between us and the -true surface of the planet, and render it highly improbable that -anything like satisfactory knowledge of her features will ever be -attained. Newcomb, indeed, roundly asserts that all markings hitherto -seen have been only temporary clouds and not genuine surface markings -at all; though this seems a somewhat absolute verdict in view of the -number of skilled observers who have specially studied the planet and -assert the objective reality of the markings they have detected. The -blunting of the South horn of the planet, visible in Mr. MacEwen's -fine drawing (Plate X.), is a feature which has been noted by so many -observers that its reality must be conceded. On the other hand, some -of the earlier observations recording considerable irregularities of -the terminator (margin of the planet between light and darkness), and -detached points of light at one of the horns, must seemingly be given -up. Denning, one of the most careful of observers, gives the following -opinion: 'There is strong negative evidence among modern observations -as to the existence of abnormal features, so that the presence of -very elevated mountains must be regarded as extremely doubtful.... -The detached point at the South horn shown in Schröter's telescope -was probably a false appearance due to atmospheric disturbances or -instrumental defects.' It will be seen, therefore, that the observer -should be very cautious in inferring the actual existence of any -abnormal features which may be shown by a small telescope; and -the more remarkable the features shown, the more sceptical he may -reasonably be as to their reality. The chances are somewhat heavily -in favour of their disappearance under more favourable conditions of -seeing. - -[Illustrations (2): - - PLATE X. - -Venus. H. MacEwen. 5-inch Refractor.] - -The same remark applies, with some modifications, to the dark markings -which have been detected on the planet by all sorts of observers -with all sorts of telescopes. There is no doubt that faint grey -markings, such as those shown in Plate X., are to be seen; the -observations of many skilled observers put this beyond all question. -Even Denning, who says that personally he has sometimes regarded the -very existence of these markings as doubtful, admits that 'the evidence -affirming their reality is too weighty and too numerously attested to -allow them to be set aside'; and Barnard, observing with the Lick -telescope, says that he has repeatedly seen markings, but always so -'vague and ill-defined that nothing definite could be made of them.' - -The observations of Lowell and Douglass at Flagstaff, Arizona, record -quite a different class of markings, consisting of straight, dark, -well-defined lines; as yet, however, confirmation of these remarkable -features is scanty, and it will be well for the beginner who, with a -small telescope and in ordinary conditions of observing, imagines he -has detected such markings to be rather more than less doubtful about -their reality. The faint grey areas, which are real features, at least -of the atmospheric envelope, if not of the actual surface, are -beyond the reach of small instruments. Mr. MacEwen's drawings, which -accompany this chapter, were made with a 5-inch Wray refractor, and -represent very well the extreme delicacy of these markings. I have -suspected their existence when observing with an 8-1/2-inch With -reflector in good air, but could never satisfy myself that they were -really seen. - -Up till the year 1890 the rotation period of Venus was usually stated -at twenty-three hours twenty-one minutes, or thereby, though this -figure was only accepted with some hesitation, as in order to -arrive at it there had to be some gentle squeezing of inconvenient -observations. But in that year Schiaparelli announced that his -observations were only consistent with a long period of rotation, -which could not be less than six months, and was not greater -than nine. The announcement naturally excited much discussion. -Schiaparelli's views were strongly controverted, and for a time the -astronomical world seemed to be almost equally divided in opinion. -Gradually, however, the conclusion has come to be more and more -accepted that Venus, like Mercury, rotates upon her axis in the same -time as she takes to make her journey round the sun--in other words, -that her day and her year are of the same length, amounting to about -225 of our days. In 1900 the controversy was to some extent reopened -by the statement of the Russian astronomer Bélopolsky that his -spectroscopic investigations pointed to a much more rapid rotation--to -a period, indeed, considerably shorter than twenty-four hours. It -is difficult, however, to reconcile this with the absence of polar -flattening in the globe of Venus. Lowell's spectroscopic observations -are stated by him to point to a period in accordance with his -telescopic results--namely, 225 days. The matter can scarcely be -regarded as settled in the meantime, but the balance of evidence seems -in favour of the longer period. - -Another curious and unexplained feature in connection with the planet -is what is frequently termed the 'phosphorescence' of the dark side. -This is an appearance precisely similar to that seen in the case of -the moon, and known as 'the old moon in the young moon's arms.' The -rest of the disc appears within the bright crescent, shining with a -dull rusty light. In the case of Venus, however, an explanation is -not so easily arrived at as in that of the moon, where, of course, -earth-light accounts for the visibility of the dark portion. Had the -planet been possessed of a satellite, the explanation might have lain -there; but Venus has no moon, and therefore no moonlight to brighten -her unilluminated portion; and our world is too far distant for -earth-shine to afford an explanation. It has been suggested that -electrical discharges similar to the aurora may be at the bottom of -the mystery; but this seems a little far-fetched, as does also the -attribution of the phenomenon to real phosphorescence of the oceans -of Venus. Professor Newcomb cuts the Gordian knot by observing: 'It -is more likely due to an optical illusion.... To whatever we might -attribute the light, it ought to be seen far better after the end of -twilight in the evening than during the daytime. The fact that it is -not seen then seems to be conclusive against its reality.' But the -appearance cannot be disposed of quite so easily as this, for it is -not accurate to say that it is only seen in the daytime, and against -Professor Newcomb's dictum may be set the judgment of the great -majority of the observers who have made a special study of the planet. - -We may, however, safely assign to the limbo of exploded ideas that of -the existence of a satellite of Venus. For long this object was one -of the most persistent of astronomical ghosts, and refused to be -laid. Observations of a companion to the planet, much smaller, and -exhibiting a similar phase, were frequent during the eighteenth -century; but no such object has presented itself to the far finer -instruments of modern times, and it may be concluded that the moon of -Venus has no real existence. - -Venus, like Mercury, transits the sun's disc, but at much longer -intervals which render her transits among the rarest of astronomical -events. Formerly they were also among the most important, as they were -believed to furnish the most reliable means for determining the sun's -distance; and most of the estimates of that quantity, up to within the -last twenty-five years, were based on transit of Venus observations. -Now, however, other methods, more reliable and more readily -applicable, are coming into use, and the transit has lost somewhat of -its former importance. The interest and beauty of the spectacle still -remain; but it is a spectacle not likely to be seen by any reader of -these pages, for the next transit of Venus will not take place until -June, 2004. - -As already indicated, Venus presents few opportunities for useful -observation to the amateur. The best time for observing, as in -the case of Mercury, is in broad daylight; and for this, unless in -exceptional circumstances, graduated circles and a fairly powerful -telescope are required. Practically the most that can be done by the -possessor of a small instrument is to convince himself of the reality -of the phases, and of the non-existence of a satellite of any size, -and to enjoy the exquisite and varying beauty of the spectacle -which the planet presents. Should his telescope be one of the small -instruments which show hard and definite markings on the surface, -he may also consider that he has learned a useful lesson as to the -possibility of optical illusion, and, incidentally, that he may be -well advised to procure a better glass when the opportunity of doing -so presents itself. The 'phosphorescence' of the dark side may be -looked for, and it may be noted whether it is not seen after dark, -or whether it persists and grows stronger. Generally speaking, -observations should be made as early in the evening as the planet can -be seen in order that the light of the sky may diminish as much as -possible the glare which is so evident when Venus is viewed against a -dark background. - - - - -CHAPTER VII - -THE MOON - - -Our attention is next engaged by the body which is our nearest -neighbour in space and our most faithful attendant and useful servant. -The moon is an orb of 2,163 miles in diameter, which revolves round -our earth in a slightly elliptical orbit, at a mean distance of about -240,000 miles. The face which she turns to us is a trifle greater -in area than the Russian Empire, while her total surface is almost -exactly equal to the areas of North and South America, islands -excluded. Her volume is about 2/99 of that of the earth; her materials -are, however, much less dense than those of which our world is -composed, so that it would take about eighty-one moons to balance the -earth. One result of these relations is that the force of gravity at -the lunar surface is only about one-sixth of that at the surface of -the earth, so that a twelve-stone man, if transported to the moon, -would weigh only two stone, and would be capable of gigantic feats -in the way of leaping and lifting weights. The fact of the diminished -force of gravity is of importance in the consideration of the question -of lunar surfacing. - -[Illustration: FIG. 21.--THE TIDES. - -A, Spring Tide (New Moon); B, Neap Tide.] - -The most conspicuous service which our satellite performs for us is -that of raising the tides. The complete statement of the manner in -which she does this would be too long for our pages; but the general -outline of it will be seen from the accompanying rough diagram (Fig. -21), which, it must be remembered, makes no attempt at representing -the scale either of the bodies concerned or of their distances from -one another, but simply pictures their relations to one another at the -times of spring and neap tides. The moon (M in Fig. 21, A) attracts -the whole earth towards it. Its attraction is greatest at the point -nearest to it, and therefore the water on the moonward side is drawn -up, as it were, into a heap, making high tide on that side of the -earth. But there is also high tide at the opposite side, the reason -being that the solid body of the earth, which is nearer to the moon -than the water on the further side, is more strongly attracted, and -so leaves the water behind it. Thus there are high tides at the two -opposite sides of the earth which lie in a straight line with the -moon, and corresponding low tides at the intermediate positions. Tides -are also produced by the attraction of the sun, but his vastly greater -distance causes his tide-producing power to be much less than that of -the moon. His influence is seen in the difference between spring and -neap tides. Spring tides occur at new or full moon (Fig. 21, A, case -of new moon). At these two periods the sun, moon, and earth, are all -in one straight line, and the pull of the sun is therefore added to -that of the moon to produce a spring tide. At the first and third -quarters the sun and moon are at right angles to one another; their -respective pulls therefore, to some extent, neutralize each other, and -in consequence we have neap tide at these seasons. - -[Illustration: - - PLATE XI. - -The Moon, April 5, 1900. Paris Observatory.] - -No one can fail to notice the beautiful set of phases through which -the moon passes every month. A little after the almanac has announced -'new moon,' she begins to appear as a thin crescent low down in the -West, and setting shortly after the sun. Night by night we can watch -her moving eastward among the stars, and showing more and more of -her illuminated surface, until at first quarter half of her disc is -bright. The reader must distinguish this real eastward movement from -the apparent east to west movement due to the daily rotation of the -earth. Its reality can readily be seen by noting the position of the -moon relatively to any bright star. It will be observed that if she -is a little west of the star on one night, she will have moved to a -position a little east of it by the next. Still moving farther East, -she reaches full, and is opposite to the sun, rising when he sets, and -setting when he rises. After full, her light begins to wane, till at -third quarter the opposite half of her disc is bright, and she is seen -high in the heavens in the early morning, a pale ghost of her evening -glories. Gradually she draws nearer to the sun, thinning down to the -crescent shape again until she is lost once more in his radiance, only -to re-emerge and begin again the same cycle of change. - -The time which the moon actually takes to complete her journey round -the earth is twenty-seven days, seven hours, and forty-three minutes; -and if the earth were fixed in space, this period, which is called the -_sidereal month_, would be the actual time from new moon to new moon. -While the moon has been making her revolution, however, the earth has -also been moving onwards in its journey round the sun, so that the -moon has a little further to travel in order to reach the 'new -moon' position again, and the time between two new moons amounts to -twenty-nine days, twelve hours, forty-four minutes. This period -is called a _lunar month_, and is also the _synodic period_ of our -satellite, a term which signifies generally the period occupied by any -planet or satellite in getting back to the same position with respect -to the sun, as observed from the earth. - -The fact that the moon shows phases signifies that she shines only -by reflected light; and it is surprising to notice how little of the -light that falls upon her is really reflected by her. On an ordinarily -clear night most people would probably say that the moon is much -brighter than any terrestrial object viewed in the daytime, when it -also is lit by the sun, as the moon is. Yet a very simple comparison -will show that this is not so. If the moon be compared during the -daytime with the clouds floating around her, she will be seen to be -certainly not brighter than they, generally much less bright; indeed, -even an ordinary surface of sandstone will look as bright as her -disc. In fact, the reason of her great apparent brightness at night is -merely the contrast between her and the dark background against which -she is seen; a fragment of our own world, put in her place, would -shine quite as brightly, perhaps even more so. It is possibly rather -difficult at first to realize that our earth is shining to the moon -and to the other planets as they do to us, but anyone who watches the -moon for a few days after new will find convincing evidence of the -fact. Within the arms of the thin crescent can be seen the whole body -of the lunar globe, shining with a dingy coppery kind of light--'the -ashen light,' as it is called. People talk of this as 'the old moon -in the young moon's arms,' and weather-wise (or foolish) individuals -pronounce it to be a sign of bad weather. It is, of course, nothing -of the sort, for it can be seen every month when the sky is reasonably -clear; but it is the sign that our world shines to the other worlds of -space as they do to her; for this dim light upon the part of the moon -unlit by the sun is simply the light which our own world reflects from -her surface to the moon. In amount it is thirteen times more than that -which the moon gives to us, as the earth presents to her satellite a -disc thirteen times as large as that exhibited by the latter. - -The moon's function in causing eclipses of the sun has already been -briefly alluded to. In turn she is herself eclipsed, by passing behind -the earth and into the long cone of shadow which our world casts -behind it into space (Fig. 19). It is obvious that such eclipses can -only happen when the moon is full. A total eclipse of the moon, though -by no means so important as a solar eclipse, is yet a very interesting -and beautiful sight. The faint shadow or penumbra is often scarcely -perceptible as the moon passes through it; but the passage of the dark -umbra over the various lunar formations can be readily traced, and -is most impressive. Cases of 'black eclipses' have been sometimes -recorded, in which the moon at totality has seemed actually to -disappear as though blotted out of the heavens; but in general this -is not the case. The lunar disc still remains visible, shining with -a dull coppery light, something like the ashen light, but of a -redder tone. This is due to the fact that our earth is not, like -its satellite, a next to airless globe, but is possessed of a pretty -extensive atmosphere. By this atmosphere those rays of the sun which -would otherwise have just passed the edge of the world are caught -and refracted so that they are directed upon the face of the eclipsed -moon, lighting it up feebly. The redness of the light is due to that -same atmospheric absorption of the green and blue rays which causes -the body of the setting sun to seem red when viewed through the dense -layer of vapours near the horizon. When the moon appears totally -eclipsed to us, the sun must appear totally eclipsed to an observer -stationed on the moon. A total solar eclipse seen from the moon must -present features of interest differing to some extent from those which -the similar phenomenon exhibits to us. The duration of totality will -be much longer, and, in addition to the usual display of prominences -and corona, there will be the strange and weird effect of the black -globe of our world becoming gradually bordered with a rim of ruddy -light as our atmosphere catches and bends the solar rays inwards upon -the lunar surface. - -In nine cases out of ten the moon will be the first object to which -the beginner turns his telescope, and he will find in our satellite -a never-failing source of interest, and a sphere in which, by patient -observation and the practice of steadily recording what is seen, he -may not only amuse and instruct himself, but actually do work that -may become genuinely useful in the furtherance of the science. The -possession of powerful instrumental means is not an absolute essential -here, for the comparative nearness of the object brings it well within -the reach of moderate glasses. The writer well remembers the keen -feeling of delight with which he first discovered that a very humble -and commonplace telescope--nothing more, in fact, than a small -ordinary spy-glass with an object-glass of about 1 inch in -aperture--was able to reveal many of the more prominent features of -lunar scenery; and the possessor of any telescope, no matter whether -its powers be great or small, may be assured that there is enough work -awaiting him on the moon to occupy the spare time of many years with -one of the most enthralling of studies. The view that is given by even -the smallest instrument is one of infinite variety and beauty; and its -interest is accentuated by the fact that the moon is a sphere where -practically every detail is new and strange. - -If the moon be crescent, or near one or other of her quarters at the -time of observation, the eye will at once be caught by a multitude -of circular, or nearly circular depressions, more clearly marked the -nearer they are to the line of division between the illuminated -and unilluminated portions of the disc. (This line is known as the -Terminator, the circular outline, fully illuminated, being called the -Limb). The margins of some of these depressions will be seen actually -to project like rings of light into the darkness, while their -interiors are filled with black shadow (Plates XI., XIII., XV., and -XVI.). At one or two points long bright ridges will be seen, extending -for many miles across the surface, and marking the line of one or -other of the prominent ranges of lunar mountains (Plates XI., XIII., -XVI., XVII.); while the whole disc is mottled over with patches of -varied colour, ranging from dark grey up to a brilliant yellow which, -in some instances, nearly approaches to white. - -If observation be conducted at or near the full, the conditions will -be found to have entirely changed. There are now very few ruggednesses -visible on the edge of the disc, which now presents an almost smooth -circular outline, nor are there any shadows traceable on the surface. -The circular depressions, formerly so conspicuous, have now almost -entirely vanished, though the positions and outlines of a few of them -may still be traced by their contrast in colour with the surrounding -regions. The observer's attention is now claimed by the extraordinary -brilliance and variety of the tones which diversify the sphere, and -particularly by the curious systems of bright streaks radiating from -certain well-marked centres, one of which, the system originating -near Tycho, a prominent crater not very far from the South Pole, is -so conspicuous as to give the full moon very much the appearance of a -badly-peeled orange (Plate XII.). - -[Illustration: - - PLATE XII. - -The Moon, November 13, 1902. Paris Observatory.] - -As soon as the moon has passed the full, the ruggedness of its margin -begins once more to become apparent, but this time on the opposite -side; and the observer, if he have the patience to work late at night -or early in the morning, has the opportunity of seeing again all -the features which he saw on the waxing moon, but this time with the -shadows thrown the reverse way--under evening instead of under morning -illumination. In fact the character of any formation cannot be truly -appreciated until it has been carefully studied under the setting as -well as under the rising and meridian sun. - -We must now turn our attention to the various types of formation which -are to be found upon the moon. These may be roughly summarized as -follows: (1) The great grey plains, commonly known as Maria, or seas; -(2) the circular or approximately circular formations, known generally -as the lunar craters, but divided by astronomers into a number of -classes to which reference will be made later; (3) the mountain -ranges, corresponding with more or less closeness to similar features -on our own globe; (4) the clefts or rills; (5) the systems of bright -rays, to which allusion has already been made. - -1. THE GREAT GREY PLAINS.--These are, of course, the most conspicuous -features of the lunar surface. A number of them can be easily seen -with the naked eye; and, so viewed, they unite with the brighter -portions to form that resemblance to a human face--'the man in -the moon'--with which everyone is familiar. A field-glass or small -telescope brings out their boundaries with distinctness, and suggests -a likeness to our own terrestrial oceans and seas. Hence the name -Maria, which was applied to them by the earlier astronomers, whose -telescopes were not of sufficient power to reveal more than their -broader outlines. But a comparatively small aperture is sufficient -to dispel the idea that these plains have any right to the title of -'seas.' The smoothness which at first suggests water proves to be only -relative. They are smooth compared with the brighter regions of the -moon, which are rugged beyond all terrestrial precedent; but they -would probably be considered no smoother than the average of our own -non-mountainous land surfaces. A 2 or 2-1/2-inch telescope will reveal -the fact that they are dotted over with numerous irregularities, some -of them very considerable. It is indeed not common to find a crater of -the largest size associated with them; but, at the same time, craters -which on our earth would be considered huge are by no means uncommon -upon their surface, and every increase of telescopic power reveals a -corresponding increase in the number of these objects (Plates XIII., -XV., XVII.). - -[Illustration: - - PLATE XIII. - -The Moon, September 12, 1903. Paris Observatory.] - -Further, the grey plains are characterized by features of which -instances may be seen with a very small instrument, though the more -delicate specimens require considerable power--namely, the long -winding ridges which either run concentrically with the margins of the -plains, or cross their surface from side to side. Of these the -most notable is the great serpentine ridge which traverses the Mare -Serenitatis in the north-west quadrant of the moon. As it runs, -approximately, in a north and south direction, it is well placed -for observation, and even a low power will bring out a good deal of -remarkable detail in connection with it. It rises in some places to a -height of 700 or 800 feet (Neison), and is well shown on many of the -fine lunar photographs now so common. Another point of interest in -connection with the Maria is the existence on their borders of a -number of large crater formations which present the appearance of -having had their walls breached and ruined on the side next the mare -by the action of some obscure agency. From consideration of these -ruined craters, and of the 'ghost craters,' not uncommon on the -plains, which present merely a faint outline, as though almost -entirely submerged, it has been suggested, by Elger and others, that -the Maria, as we see them represent, not the beds of ancient seas, -but the consolidated crust of some fluid or viscous substance such as -lava, which has welled forth from vents connected with the interior -of the moon, overflowing many of the smaller formations, and partially -destroying the walls of these larger craters. Notable instances of -these half-ruined formations will be found in Fracastorius (Plate -XIX., No. 78, and Plate XI.), and Pitatus (Plate XIX., No. 63, -and Plate XV.). The grey plains vary in size from the vast Oceanus -Procellarum, nearly 2,000,000 square miles in area, down to the Mare -Humboldtianum, whose area of 42,000 square miles is less than that of -England. - -2. THE CIRCULAR, OR APPROXIMATELY CIRCULAR FORMATIONS.--These, the -great distinguishing feature of lunar scenery, have been classified -according to the characteristics, more or less marked, which -distinguish them from one another, as walled-plains, mountain-rings, -ring-plains, craters, crater-cones, craterlets, crater-pits, and -depressions. For general purposes we may content ourselves with the -single title craters, using the more specific titles in outstanding -instances. - -[Illustration: - - PLATE XIV. - -Region of Maginus: Overlapping Craters. Paris Observatory.] - -To these strange formations we have scarcely the faintest analogy -on earth. Their multitude will at once strike even the most casual -observer. Galileo compared them to the 'eyes' in a peacock's tail, and -the comparison is not inapt, especially when the moon is viewed with -a small telescope and low powers. In the Southern Hemisphere -particularly, they simply swarm to such an extent that the district -near the terminator presents much the appearance of a honeycomb with -very irregular cells, or a piece of very porous pumice (Plate XIV.). -Their vast size is not less remarkable than their number. One of the -most conspicuous, for example, is the great walled-plain Ptolemäus, -which is well-placed for observation near the centre of the visible -hemisphere. It measures 115 miles from side to side of its great -rampart, which, in at least one peak, towers more than 9,000 feet -above the floor of the plain within. The area of this enormous -enclosure is about equal to the combined areas of Yorkshire, -Lancashire, and Westmorland--an extent so vast that an observer -stationed at its centre would see no trace of the mountain-wall which -bounds it, save at one point towards the West, where the upper part of -the great 9,000-feet peak already referred to would break the line of -the horizon (Plate XIX., No. 111; Plate XIII.). - -Nor is Ptolemäus by any means the largest of these objects. Clavius, -lying towards the South Pole, measures no less than 142 miles from -wall to wall, and includes within its tremendous rampart an area of at -least 16,000 square miles. The great wall which encloses this space, -itself no mean range of mountains, stands some 12,000 feet above the -surface of the plain within, while in one peak it rises to a height -of 17,000 feet. Clavius is remarkable also for the number of smaller -craters associated with it. There are two conspicuous ones, one on the -north, one on the south side of its wall, each about twenty-five -miles in diameter, while the floor is broken by a chain of four large -craters and a considerable number of smaller ones. - -Though unfavourably placed for observation, there is no lunar feature -which can compare in grandeur with Clavius when viewed either at -sunrise or sunset. At sunrise the great plain appears first as a huge -bay of black shadow, so large as distinctly to blunt the southern horn -of the moon to the naked eye. As the sun climbs higher, a few bright -points appear within this bay of darkness--the summits of the walls of -the larger craters--these bright islands gradually forming fine rings -of light in the shadow which still covers the floor of the great -plain. In the East some star-like points mark where the peaks of the -eastern wall are beginning to catch the dawn. Then delicate streaks -of light begin to stream across the floor, and the dark mass of shadow -divides itself into long pointed shafts, which stretch across the -plain like the spires of some great cathedral. The whole spectacle -is so magnificent and strange that no words can do justice to it; and -once seen it will not readily be forgotten. Even a small telescope -will enable the student to detect and draw the more important features -of this great formation; and for those whose instruments are more -powerful there is practically no limit to the work that may be done -on Clavius, which has never been studied with the minuteness that so -great and interesting an object deserves. (Clavius is No. 13, Plate -XIX. See also Plates XIII. and XV., and Fig. 22, the latter a rough -sketch with a 2-5/8-inch refractor.) - -From such gigantic forms as these, the craters range downwards in an -unbroken sequence through striking objects such as Tycho and the grand -Copernicus, both distinguished for their systems of bright rays, as -well as for their massive and regular ramparts, to tiny pits of black -shadow, a few hundred feet across, and with no visible walls, which -tax the powers of the very finest instruments. Schmidt's great map -lays down nearly 33,000 craters, and it is quite certain that these -are not nearly all which can be seen even with a moderate-sized -telescope. - -[Illustration: - - PLATE XV. - -Clavius, Tycho, and Mare Nubium. Yerkes Observatory.] - -As to the cause which has resulted in this multitude of circular -forms, there is no definite consensus of opinion. Volcanic action is -the agency generally invoked; but, even allowing for the diminished -force of gravity upon the moon, it is difficult to conceive of -volcanic action of such intensity as to have produced some of the -great walled-plains. Indeed, Neison remarks that such formations are -much more akin to the smaller Maria, and bear but little resemblance -to true products of volcanic action. But it seems difficult to tell -where a division is to be made, with any pretence to accuracy, between -such forms as might certainly be thus produced and those next above -them in size. The various classes of formation shade one into the -other by almost imperceptible degrees. - -[Illustration: FIG. 22. - -CLAVIUS, June 7, 1889, 10 p.m., 2-5/8 inch.] - -3. THE MOUNTAIN RANGES.--These are comparatively few in number, and -are never of such magnitude as to put them, like the craters, beyond -terrestrial standards of comparison. The most conspicuous range is -that known as the Lunar Apennines, which runs in a north-west and -south-east direction for a distance of upwards of 400 miles along -the border of the Mare Imbrium, from which its mass rises in a steep -escarpment, towering in one instance (Mount Huygens) to a height of -more than 18,000 feet. On the western side the range slopes gradually -away in a gentle declivity. The spectacle presented by the Apennines -about first quarter is one of indescribable grandeur. The shadows of -the great peaks are cast for many miles over the surface of the Mare -Imbrium, magnificently contrasting with the wild tract of hill-country -behind, in which rugged summits and winding valleys are mingled in a -scene of confusion which baffles all attempt at delineation. Two other -important ranges--the Caucasus and the Alps--lie in close proximity -to the Apennines; the latter of the two notable for the curious Alpine -Valley which runs through it in a straight line for upwards of eighty -miles. This wonderful chasm varies in breadth from about two miles, -at its narrowest neck, to about six at its widest point. It is closely -bordered, for a considerable portion of its length, by almost vertical -cliffs thousands of feet in height, and under low magnifying powers -appears so regular as to suggest nothing so much as the mark of -a gigantic chisel, driven by main force through the midst of the -mountain mass. The Alpine Valley is an easy object, and a power of -50 on a 2-inch telescope will show its main outlines quite clearly. -Indeed, the whole neighbourhood is one which will well repay the -student, some of the finest of the lunar craters, such as Plato, -Archimedes, Autolycus, and Aristillus, lying in the immediate vicinity -(Plates XIII. and XVII.). - -[Illustration: - - PLATE XVI. - -Region of Theophilus and Altai Mountains. Yerkes Observatory.] - -Among the other mountain-ranges may be mentioned the Altai Mountains, -in the south-west quadrant (Plate XVI.), the Carpathians, close to the -great crater Copernicus, and the beautiful semicircle of hills which -borders the Sinus Iridum, or Bay of Rainbows, to the east of the -Alpine range. This bay forms one of the loveliest of lunar landscapes, -and under certain conditions of illumination its eastern cape, the -Heraclides Promontory, presents a curious resemblance, which I have -only seen once or twice, to the head of a girl with long floating -hair--'the moon-maiden.' The Leibnitz and Doerfel Mountains, with -other ranges whose summits appear on the edge of the moon, are -seldom to be seen to great advantage, though they are sometimes -very noticeably projected upon the bright disc of the sun during the -progress of an eclipse.[*] They embrace some of the loftiest lunar -peaks reaching 26,000 feet in one of or two instances, according to -Schröter and Mädler. - -[Illustration: FIG. 23. - -ARISTARCHUS and HERODOTUS, February 20, 1891, 6.15 p.m., 3-7/8 inch.] - -4. THE CLEFTS OR RILLS.--In these, and in the ray-systems, we again -meet with features to which a terrestrial parallel is absolutely -lacking. Schröter of Lilienthal was the first observer to detect the -existence of these strange chasms, and since his time the number known -has been constantly increasing, till at present it runs to upwards of -a thousand. These objects range from comparatively coarse features, -such as the Herodotus Valley (Fig. 23), and the well-known Ariadæus -and Hyginus clefts, down to the most delicate threads, only to be seen -under very favourable conditions, and taxing the powers of the finest -instruments. They present all the appearance of cracks in a shrinking -surface, and this is the explanation of their existence which at -present seems to find most favour. In some cases, such as that of -the great Sirsalis cleft, they extend to a length of 300 miles; their -breadth varies from half a mile, or less, to two miles; their depth is -very variously estimated, Nasmyth putting it at ten miles, while Elger -only allows 100 to 400 yards. In a number of instances they appear -either to originate from a small crater, or to pass through one or -more craters in their course. The student will quickly find out for -himself that they frequently affect the neighbourhood of one or other -of the mountain ranges (as, for example, under the eastern face of the -Apennines, Plate XVII.), or of some great crater, such as Archimedes. -They are also frequently found traversing the floor of a great -walled-plain, and at least forty have been detected in the interior -of Gassendi (Plate XIX., No. 90). Smaller instruments are, of course, -incompetent to reveal more than a few of the larger and coarser of -these strange features. The Serpentine Valley of Herodotus, the cleft -crossing the floor of Petavius, and the Ariadæus and Hyginus rills -are among the most conspicuous, and may all be seen with a 2-1/2-inch -telescope and a power of 100. - -[Illustration: - - PLATE XVII. - -Apennines, Alps, and Caucasus. Paris Observatory.] - -5. THE SYSTEMS OF BRIGHT RAYS, radiating from certain craters, remain -the most enigmatic of the features of lunar scenery. Many of these -systems have been traced and mapped, but we need only mention the -three principal--those connected with Tycho, Copernicus, and -Kepler, all shown on Plate XII. The Tycho system is by far the most -noteworthy, and at once attracts the eye when even the smallest -telescope is directed towards the full moon. The rays, which are of -great brilliancy, appear to start, not exactly from the crater itself, -but from a greyish area surrounding it, and they radiate in all -directions over the surface, passing over, and almost completely -masking in their course some of the largest of the lunar craters. -Clavius, for example, and Maginus (Plate XIV.), become at full almost -unidentifiable from this cause, though Neison's statement that 'not -the slightest trace of these great walled-plains, with their extremely -lofty and massive walls, can be detected in full,' is certainly -exaggerated. The rays are not well seen save under a high sun--_i.e._, -at or near full, though some of them can still be faintly traced under -oblique illumination. - -In ordinary telescopes, and to most eyes, the Tycho rays appear to -run on uninterruptedly for enormous distances, one of them traversing -almost the whole breadth of the moon in a north-westerly direction, -and crossing the Mare Serenitatis, on whose dark background it is -conspicuous. Professor W. H. Pickering, who has made a special study -of the subject under very favourable conditions, maintains, however, -that this appearance of great length is an illusion, and that the -Tycho rays proper extend only for a short distance, being reinforced -at intervals by fresh rays issuing from small craters on their track. -The whole subject is one which requires careful study with the best -optical means. - -None of the other ray-systems are at all comparable with that of -Tycho, though those in connection with Copernicus and Kepler are -very striking. As to the origin and nature of these strange features, -little is known. There are almost as many theories as there are -systems; but it cannot be said that any particular view has commanded -anything like general acceptance. Nasmyth's well-known theory was -that they represented cracks in the lunar surface, caused by internal -pressure, through which lava had welled forth and spread to a -considerable distance on either side of the original chasm. Pickering -suggests that they may be caused by a deposit of white powder, pumice, -perhaps, emitted by the craters from which the rays originate. Both -ideas are ingenious, but both present grave difficulties, and neither -has commended itself to any very great extent to observers, a remark -which applies to all other attempts at explanation. - -Such are the main objects of interest upon the visible hemisphere of -our satellite. In observing them, the beginner will do well, after the -inevitable preliminary debauch of moon-gazing, during which he may -be permitted to range over the whole surface and observe anything -and everything, not to attempt an attack on too wide a field. Let -him rather confine his energies to the detailed study of one or two -particular formations, and to the delineation of all their features -within reach of his instrument under all aspects and illuminations. -By so doing he will learn more of the actual condition of the lunar -surface than by any amount of general and haphazard observation; and -may, indeed, render valuable service to the study of the moon. - -Neither let him think that observations made with a small telescope -are now of no account, in view of the number of large instruments -employed, and of the great photographic atlases which are at present -being constructed. It has to be remembered that the famous map of -Beer and Mädler was the result of observations made with a 3-3/4-inch -telescope, and that Lohrmann used an instrument of only 4-4/5 inches, -and sometimes one of 3-1/4. Anyone who has seen the maps of these -observers will not fail to have a profound respect for the work that -can be done with very moderate means. Nor have even the beautiful -photographs of the Paris, Lick, and Yerkes Observatories superseded -as yet the work of the human eye and hand. The best of the Yerkes -photographs, taken with a 40-inch refractor, are said to show detail -'sufficiently minute to tax the powers of a 6-inch telescope.' But -this can be said only of a very few photographs; and, generally -speaking, a good 3-inch glass will show more detail than can be seen -on any but a few exceptionally good negatives. - -In conducting his observations, the student should be careful to -outline his drawing on such a scale as will permit of the easy -inclusion of all the details which he can see, otherwise the sketch -will speedily become so crowded as to be indistinct and valueless. A -scale of 1 inch to about 20 miles, corresponding roughly to 100 inches -to the moon's diameter, will be found none too large in the case of -formations where much detail has to be inserted--that is to say, in -the case of the vast majority of lunar objects. Further, only such a -moderate amount of surface should be selected for representation as -can be carefully and accurately sketched in a period of not much over -an hour at most; for, though the lunar day is so much longer than our -own, yet the changes in aspect of the various formations due to the -increasing or diminishing height of the sun become very apparent if -observation be prolonged unduly; and thus different portions of the -sketch represent different angles of illumination, and the finished -drawing, though true in each separate detail, will be untrue as a -whole. - -Above all, care must be taken to set down only what is seen with -certainty, _and nothing more_. The drawing may be good or bad, but it -must be true. A coarse or clumsy sketch which is truthful to the -facts seen is worth fifty beautiful works of art where the artist has -employed imagination or recollection to eke out the meagre results of -observation. The astronomer's primary object is to record facts, not -to make pictures. If he is skilful in recording what he sees, his -sketch will be so much the more truthful; but the facts must come -first. Such practical falsehoods as the insertion of uncertain -details, or the practice of drawing upon one's recollection of the -work of other observers, or of altering portions of a sketch which -do not please the eye, are to be studiously avoided. The observer's -record of what he has seen should be above suspicion. It may -be imperfect; it should never be false. Such cautions may seem -superfluous, but a small acquaintance with the subject of astronomical -drawing will show that they are not. - -The want of a good lunar chart will speedily make itself felt. -Fortunately in these days it can be easily supplied. The great -photographic atlases now appearing are, of course, for the luxurious; -and the elaborate maps of Beer and Mädler or Schmidt are equally -out of the question for beginners. The smaller chart of the former -observers is, however, inexpensive and good, though a little crowded. -For a start there is still nothing much better than Webb's reduction -of Beer and Mädler's large chart, published in 'Celestial Objects for -Common Telescopes.' It can also be obtained separately; but requires -to be backed before use. Mellor's chart is also useful, and is -published in a handy form, mounted on mill-board. Those who wish -charts between these and the more elaborate ones will find their -wants met by such books as those of Neison or Elger. Neison's volume -contains a chart in twenty-two sections on a scale of 2 feet to -the moon's diameter. It includes a great amount of detail, and -is accompanied by an elaborate description of all the features -delineated. Its chief drawbacks are the fact that it was published -thirty years ago, and that it is an extremely awkward and clumsy -volume to handle, especially in the dim light of an observatory. -Elger's volume is, perhaps, for English students, the handiest general -guide to the moon. Its chart is on a scale of 18 inches to the moon's -diameter, and is accompanied by a full description. With either this -or Webb's chart, the beginner will find himself amply provided with -material for many a long and delightful evenings work. - -[Illustration: - - PLATE XVIII. - -Chart of the Moon. Nasmyth and Carpenter.] - -[Illustration: - - PLATE XIX. - -Key to Chart of Moon. Nasmyth and Carpenter.] - -The small chart which accompanies this chapter, and which, with its -key-map, I owe to the courtesy of Mr. John Murray, the publisher of -Messrs. Nasmyth and Carpenter's volume on the moon, is not in any -sense meant as a substitute for those already mentioned, but merely -as an introduction to some of the more prominent features of lunar -scenery. The list of 229 named and numbered formations will be -sufficient to occupy the student for some time; and the essential -particulars with regard to a few of the more important formations are -added in as brief a form as possible (Appendix I.). - -Before we leave our satellite, something must be said as to the -conditions prevailing on her surface. The early astronomers who -devoted attention to lunar study were drawn on in their labours -largely by the hope of detecting resemblances to our own earth, or -even traces of human habitation. Schröter and Gruithuisen imagined -that they had discovered not only indications of a lunar atmosphere, -but also evidence of change upon the surface, and traces of the -handiwork of lunarian inhabitants. Gruithuisen, in particular, was -confident that in due time it would become possible to trace the -cities and the works of the Lunarians. Gradually these hopes have -receded into the distance. The existence of a lunar atmosphere is, -indeed, no longer positively denied now, as it was a few years ago; -but it is certain that such atmosphere as may exist is of extreme -rarity, quite inadequate to support animal life as we understand such -a thing. Certain delicate changes of colour which take place within -some of the craters--Plato for instance--have been referred to -vegetation; and Professor Pickering has intimated his observation -of something which he considers to be the forming and melting of -hoar-frost within certain areas, Messier and a small crater near -Herodotus among others. But the observations at best are very delicate -and the inferences uncertain. It cannot be denied that the moon may -have an atmosphere; but positive traces of its existence are so faint -that, even if their reality be admitted, very little can be built upon -them. - -At the same time when the affirmation is made that the moon is 'a -world where there is no weather, and where nothing ever happens,' the -most careful modern students of lunar matters would be the first -to question such a statement. Even supposing it to be true that no -concrete evidence of change upon the lunar surface can be had, this -would not necessarily mean that no change takes place. The moon has -certainly never been studied to advantage with any power exceeding -1,000, and the average powers employed have been much less. Nasmyth -puts 300 as about the profitable limit, and 500 would be almost an -outside estimate for anything like regular work. But even assuming the -use of a power of 1,000, that means that the moon is seen as large as -though she were only 240 miles distant from us. The reader can judge -how entirely all but the very largest features of our world would -be lost to sight at such a distance, and how changes involving the -destruction of large areas might take place and the observer be none -the wiser. When it is remembered that even at this long range we are -viewing our object through a sea of troubled air of which every tremor -is magnified in proportion to the telescopic power employed, until the -finer details are necessarily blurred and indistinct, it will be seen -that the case has been understated. Indeed it may be questioned if the -moon has ever been as well seen as though it had been situated at a -distance of 500 miles from the earth. At such a distance nothing short -of the vastest cataclysms would be visible; and it is therefore going -quite beyond the mark to assume that nothing ever happens on the moon -simply because we do not see it happening. Moreover, the balance of -evidence does appear to be inclining, slightly perhaps, but still -almost unquestionably, towards the view that change does occur upon -the moon. Some of the observations which seem to imply change may -be explained on other grounds; but there is a certain residuum which -appears to defy explanation, and it is very noteworthy that while -those who at once dismiss the idea of lunar change are, generally -speaking, those who have made no special study of the moon's surface, -the contrary opinion is most strongly maintained by eminent observers -who have devoted much time to our satellite with the best modern -instruments to aid them in their work. - -The admission of the possibility of change does not, however, imply -anything like fitness for human habitation. The moon, to use Beer -and Mädler's oft-quoted phrase, is 'no copy of the earth'; and the -conditions of her surface differ widely from anything that we are -acquainted with. The extreme rarity of her atmosphere must render her, -were other conditions equally favourable, an ideal situation for an -observatory. From her surface the stars, which are hidden from us -in the daytime by the diffused light in our air, would be visible at -broad noonday; while multitudes of the smaller magnitudes which here -require telescopic power would there be plain to the unaided eye. The -lunar night would be lit by our own earth, a gigantic moon, presenting -a surface more than thirteen times as large as that which the full -moon offers us, and hanging almost stationary in the heavens, while -exhibiting all the effects of rapid rotation upon its own axis. Those -appendages of the sun, which only the spectroscope or the fleeting -total eclipse can reveal to us, the corona, the chromosphere, and the -prominences, would there be constantly visible. - -Our astronomers who are painfully wrestling with atmospheric -disturbance, and are gradually being driven from the plains to the -summits of higher and higher hills in search of suitable sites for -the giant telescopes of to-day, may well long for a world where -atmospheric disturbance must be unknown, or at least a negligible -quantity. - -[Footnote *: See drawings by Colonel Markwick with 2-3/4-inch -refractor, of the eclipse of August 30, 1905, 'The Total Solar -Eclipse, 1905,' British Astronomical Association, pp. 59, 60.] - - - - -CHAPTER VIII - -MARS - - -The Red Planet is our nearest neighbour on the further, as Venus is on -the hither side. He is also in some ways the planet best situated for -our observation; for while the greatest apparent diameter of his disc -is considerably less than that of Venus, he does not hide close to the -sun's rays like the inferior planets, but may be seen all night -when in opposition.[*] Not all oppositions, however, are equally -favourable. Under the best circumstances he may come as near to us as -35,000,000 miles; when less favourably situated, he may come no nearer -than 61,000,000. This very considerable variation in his distance -arises from the eccentricity of the planet's orbit, which amounts to -nearly one-tenth, and, so far as we are concerned, it means that his -disc is three times larger when he comes to opposition at his least -distance from the sun than it is when the conditions are reversed. -Under the most favourable circumstances--_i.e._, when opposition and -perihelion[†] occur together, he presents, it has been calculated, a -disc of the same diameter as a half sovereign held up 2,000 yards from -the spectator. Periods of opposition recur at intervals of about 780 -days, and at the more favourable ones the planet's brilliancy is very -striking. The 1877 opposition was very notable in this respect, and in -others connected with the study of Mars, and that which preceded the -Crimean War was also marked by great brilliancy. Readers of Tennyson -will remember how Maud - - 'Seem'd to divide in a dream from a band of the blest, - And spoke of a hope for the world in the coming wars-- - ... and pointed to Mars - As he glow'd like a ruddy shield on the Lion's breast.' - -Ancient records tell us of his brightness having been so great on -some occasions as to create a panic. Panics were evidently more easily -created by celestial phenomena then than they are now; but possibly -such statements have to be taken with a small grain of salt. - -The diameter of Mars is 4,200 miles. In volume he is equal to -one-seventh of the world; but his density is somewhat smaller, so that -nine globes such as Mars would be required to balance the earth. He -turns upon his axis in twenty-four hours thirty-seven minutes, and as -the inclination of the axis is not much different from that of our -own world he will experience seasonal effects somewhat similar to -the changes of our own seasons. The Martian seasons, however, will be -considerably longer than ours, as the year of Mars occupies 687 days, -and they will be further modified by the large variation which -his distance from the sun undergoes in the course of his year--the -difference between his greatest and least distances being no less than -26,500,000 miles. - -The telescopic view of Mars at once reveals features of considerable -interest. We are no longer presented with anything like the beautiful -phases of Venus, though Mars does show a slight phase when his -position makes a right angle with the sun and the earth. This phase, -however, never amounts to more than a dull gibbosity, like that of the -moon two or three days before or after full--the most uninteresting of -phases. But the other details which are visible much more than atone -for any deficiency in this respect. The brilliant ruddy star expands -under telescopic power into a broad disc whose ground tint is a warm -ochre. This tint is diversified in two ways. At the poles there are -brilliant patches of white, larger or smaller according to the Martian -season; while the whole surface of the remaining orange-tinted portion -is broken up by patches and lines of a dark greenish-grey tone. -The analogy with Arctic and Antarctic ice and snow-fields, and with -terrestrial continents and seas, is at once and almost irresistibly -suggested, although, as will be seen, there are strong reasons for not -pressing it too far. - -The dark markings, though by no means so sharply defined as the -outlines of lunar objects, are yet evidently permanent features; at -least this may be confidently affirmed of the more prominent among -them. Some of these can be readily recognised on drawings dating from -200 years back, and have served to determine with very satisfactory -accuracy the planet's rotation period. In accordance with the almost -irresistible evidence which the telescope was held to present, these -features were assumed to be seas, straits and bays, while the general -ochre-tinted portion of the planet's surface was considered to be dry -land. On this supposition the land area of Mars amounts to 5/7 of the -planet's surface, water being confined to the remaining 2/7. But it -is by no means to be taken as an accepted fact that the dark and light -areas do represent water and land. One fact most embarrassing to those -who hold this traditional view is that in the great wealth of detail -which observation with the huge telescopes of to-day has accumulated -the bulk belongs to the dark areas. Gradations of shade are seen -constantly in them; delicate details are far more commonly to be -observed upon them than upon the bright portions of the surface, and -several of the 'canals' have been traced clear through the so-called -seas. Speaking of his observations of Mars in 1894 with the 36-inch -refractor of the Lick observatory, Professor Barnard says: 'Though -much detail was shown on the bright "continental" regions, the -greater amount was visible on the so-called "seas."... During these -observations the impression seemed to force itself upon me that I was -actually looking down from a great altitude upon just such a surface -as that in which our observatory was placed. At these times there was -no suggestion that the view was one of far-away seas and oceans, but -exactly the reverse.' Such observations are somewhat disconcerting -to the old belief, which, nevertheless, continues to maintain itself, -though in somewhat modified form. - -It is indeed difficult, if not impossible, to explain the observed -facts with regard, for instance, to the white polar caps, on any other -supposition than that of the existence of at least a considerable -amount of water upon the planet. These caps are observed to be large -after the Martian winter has passed over each particular hemisphere. -As the season progresses, the polar cap diminishes, and has even been -seen to melt away altogether. In one of the fine drawings by the Rev. -T. E. R. Phillips, which illustrate this chapter (Plate XX.), the -north polar snow will be seen accompanied by a dark circular line, -concerning which the author of the sketch says: 'The _melting_ cap is -always girdled by a narrow and intensely dark line. This is not seen -when the cap is forming.' It is hard to believe that this is anything -else than the result of the melting of polar snows, and where there -is melting snow there must be water. Such results as those obtained by -Professor Pickering by photography point in the same direction. In one -of his photographs the polar cap was shown much shrunken; in another, -taken a few days later, it had very considerably increased in -dimensions--as one would naturally conclude, from a fall of snow in -the interval. The quantity of water may not be anything like so -great as was at one time imagined; still, to give any evidence of -its presence at all at a distance of 40,000,000 miles it must be very -considerable, and must play an important part in the economy of the -planet. - -[Illustration: PLATE XX. - -Mars: Drawing 1, January 30, 1899--12 hours. [lambda] = 301°, [phi] = -+10°. - -Drawing 2, April 22, 1903--10 hours. [lambda] = 200°, [phi] = +24°. - -Rev. T. E. R. Phillips.] - -In 1877 Schiaparelli of Milan announced that he had discovered that -the surface of Mars was covered with a network of lines running with -perfect straightness often for hundreds of miles across the surface, -and invariably connecting two of the dark areas. To these markings -he gave the name of 'canali,' a word which has been responsible for -a good deal of misunderstanding. Translated into our language by -'canals,' it suggested the work of intelligent beings, and imagination -was allowed to run riot over the idea of a globe peopled by Martians -of superhuman intelligence and vast engineering skill. The title -'canals' is still retained; but it should be noted that the term is -not meant to imply artificial construction any more than the term -'rill' on the moon implies the presence of water. - -At the next opposition of Mars, Schiaparelli not only rediscovered his -canals, but made the astonishing announcement that many of them were -double, a second streak running exactly parallel to the first at some -distance from it. His observations were received with a considerable -amount of doubt and hesitation. Skilled observers declared that -they could see nothing in the heavens the least corresponding to the -network of hard lines which the Italian observer drew across the globe -of Mars; and therein to some extent they were right, for the canals -are not seen with that hardness of definition with which they -are sometimes represented. But, at the same time, each successive -opposition has added fresh proof of the fact that Schiaparelli was -essentially right in his statement of what was seen. The question -of the doubling of the canals is still under dispute, and it seems -probable that it is not a real objective fact existing upon the -planet, but is merely an optical effect due to contrast. There can be -no question, however, about the positive reality of a great number -of the canals themselves; their existence is too well attested by -observers of the highest skill and experience. 'There is really no -doubt whatever,' says Mr. Denning, 'about the streaked or striated -configuration of the Northern hemisphere of Mars. The canals do not -appear as narrow straight deep lines in my telescope, but as soft -streams of dusky material with frequent condensations.' The drawings -by Mr. Phillips well represent the surface of the planet as seen with -an instrument of considerable power; and the reader will notice that -his representation of the canals agrees remarkably well with Denning's -description. The 'soft streams with frequent condensations' are -particularly well shown on the drawing of April 22, 1903, which -represents the region of 200° longitude (see Chart, Plate XXI.) on -the centre of the disc. 'The main results of Professor Schiaparelli's -work,' remarks Mr. Phillips, 'are imperishable and beyond question. -During recent years some observers have given to the so-called -"canals" a hardness and an artificiality which they do not possess, -with the result that discredit has been brought upon the whole canal -system.... But of the substantial accuracy and truthfulness (as a -basis on which to work) of the planet's configuration as charted by -the great Italian in 1877 and subsequent years, there is in my mind no -doubt.' The question of the reality of the canal system may almost -be said to have received a definite answer from the remarkable -photographs of Mars secured in May, 1905, by Mr. Lampland at the -Flagstaff Observatory, which prove that, whatever may be the nature -of the canals, the principal ones at all events are actual features of -the planet's surface. - -Much attention has been directed within the last few years to the -observations of Lowell, made with a fine 24-inch refractor at the same -observatory, which is situated at an elevation of over 7,000 feet. His -conclusion as to the reality of the canals is most positive; but in -addition to his confirmation of their existence, he has put forward -other views with regard to Mars which as yet have found comparatively -few supporters. He has pointed out that in almost all instances the -canals radiate from certain round spots which dot the surface of the -planet. These spots, which have been seen to a certain extent by -other observers, he calls 'oases,' using the term in its ordinary -terrestrial significance. His conclusions are, briefly, as follows: -That Mars has an atmosphere; that the dark regions are not seas, but -marshy tracts of vegetation; that the polar caps are snow and ice, and -the reddish portions of the surface desert land. The canals he holds -to be waterways, lined on either bank by vegetation, so that we see, -not the actual canal, but the green strip of fertilized land through -which it passes, while the round dark spots or 'oases' he believes to -be the actual population centres of the planet, where the inhabitants -cluster to profit by the fertility created by the canals. In support -of this view he adduces the observed fact that the canals and oases -begin to darken as the polar caps melt, and reasons that this implies -that the water set free by the melting of the polar snows is conveyed -by artificial means to make the wilderness rejoice. - -Lowell's theories may seem, very likely are, somewhat fanciful. It -must be remembered, however, that the ground facts of his argument are -at least unquestionable, whatever may be thought of his inferences. -The melting of the polar caps is matter of direct observation; nor -can it be questioned that it is followed by the darkening of the canal -system. It is probably wiser not to dogmatize upon the reasons and -purposes of these phenomena, for the very sufficient reason that we -have no means of arriving at any certitude. Terrestrial analogies -cannot safely be used in connection with a globe whose conditions are -so different from those of our own earth. The matter is well summed up -by Miss Agnes Clerke: 'Evidently the relations of solid and liquid in -that remote orb are abnormal; they cannot be completely explained by -terrestrial analogies. Yet a series of well-authenticated phenomena -are intelligible only on the supposition that Mars is, in some real -sense a terraqueous globe. Where snows melt there must be water; and -the origin of the Rhone from a great glacier is scarcely more evident -to our senses than the dissolution of the Martian ice-caps into pools -and streams.' - -[Illustration: - - PLATE XXI. - -Chart of Mars. 'Memoirs of the British Astronomical Association,' Vol. -XI., Part III., Plate VI.] - -Closely linked with the question of the existence of water on the -planet, and indeed a fundamental point in the settlement of it, is the -further question of whether there is any aqueous vapour in the Martian -atmosphere. The evidence is somewhat conflicting. It is quite apparent -that in the atmosphere of Mars there is nothing like the volume of -water vapour which is present in that of the earth, for if there were, -his features would be much more frequently obscured by cloud than -is found to be the case. Still there are many observations on record -which seem quite unaccountable unless the occasional presence of -clouds is allowed. Thus on May 21, 1903, Mr. Denning records that the -Syrtis Major (see Chart, Plate XXI.) being then very dark and sharply -outlined, a very bright region crossed its southern extremity. By May -23, the Syrtis Major, 'usually the most conspicuous object in Mars, -had become extremely feeble, as if covered with highly reflective -vapours.' On May 24, Mr. Phillips observed the region of Zephyria and -Aeolis to be also whitened, while the Syrtis Major was very faint; and -on the 25th, Mr. Denning observed the striking whiteness of the same -region observed by Mr. Phillips the day before. Illusion, so often -invoked to explain away inconvenient observations, seems here -impossible, in view of the prominence of the markings obscured, and -the experience of the observers; and the evidence seems strongly in -favour of real obscuration by cloud. It might have been expected that -the evidence of the spectroscope would in such a case be decisive, but -Campbell's negative conclusion is balanced by the affirmative result -reached by Huggins and Vogel. It is safe to say, however, that -whatever be the constitution of the Martian atmosphere, it is -considerably less dense than our own air mantle. - -During the last few years the public mind has been unusually exercised -over Mars, largely by reason of a misapprehension of the terms -employed in the discussion about his physical features. The talk of -'canals' has suggested human, or at all events intelligent, agency, -and the expectation arose that it might not be quite impossible to -establish communication between our world and its nearest neighbour on -the further side. The idea is, of course, only an old one furbished up -again, for early in last century it was suggested that a huge triangle -or ellipse should be erected on the Siberian steppes to show the -Lunarians or the Martians that we were intelligent creatures who -knew geometry. In these circumstances curiosity was whetted by the -announcement, first made in 1890, and since frequently repeated, of -the appearance of bright projections on the terminator of Mars. These -were construed, by people with vivid imaginations, as signals from -the Martians to us; while a popular novelist suggested a more sinister -interpretation, and harrowed our feelings with weird descriptions of -the invasion of our world by Martian beings of uncouth appearance and -superhuman intelligence, who were shot to our globe by an immense gun -whose flashes occasioned the bright projections seen. The projections -were, however, prosaically referred by Campbell to snow-covered -mountains, while Lowell believed that one very large one observed at -Flagstaff in May, 1903, was due to sunlight striking on a great cloud, -not of water-vapour, but of dust. - -As a matter of fact, Mars is somewhat disappointing to those who -approach the study of his surface with the hope of finding traces of -anything which might favour the idea of human habitation. He presents -an apparently enticing general resemblance to the earth, with his -polar caps and his bright and dark markings; and his curious network -of canals may suggest intelligent agency. But the resemblances are -not nearly so striking when examined in detail. The polar caps are -the only features that seem to hold their own beside their terrestrial -analogues, and even their resemblance is not unquestioned; the dark -areas, so long thought to be seas, are now proved to be certainly not -seas, whatever else they may be; and the canal system presents nothing -but the name of similarity to anything that we know upon earth. It is -quite probable that were Mars to come as near to us as our own moon, -the fancied resemblances would disappear almost entirely, and we -should find that the red planet is only another instance of the -infinite variety which seems to prevail among celestial bodies. That -being so, it need scarcely be remarked that any talk about Martian -inhabitants is, to say the least of it, premature. There may be such -creatures, and they may be anything you like to imagine. There is no -restraint upon the fancy, for no one knows anything about them, and no -one is in the least likely to know anything. - -The moons of Mars are among the most curious finds of modern -astronomy. When the ingenious Dr. Jonathan Swift, in editing the -travels of Mr. Lemuel Gulliver, of Wapping, wrote that the astronomers -of Laputa had discovered 'two lesser stars, or satellites, which -revolve about Mars,' the suggestion was, no doubt, put in merely -because some detail of their skill had to be given, and as well -one unlikely thing as another. Probably no one would have been more -surprised than the Dean of St. Patrick's, had he lived long enough, -or cared sixpence about the matter, to hear that his bow drawn at a -venture had hit the mark, and that Professor Asaph Hall had detected -two satellites of Mars. The discovery was one of the first-fruits of -the 26-inch Washington refractor, and was made in 1877, the year from -which the new interest in Mars may be said to date. The two moons -have been called Deimos and Phobos, or Fear and Panic, and are, in -all probability, among the very tiniest bodies of our system, as their -diameter can scarcely be greater than ten miles. Deimos revolves in an -orbit which takes him thirty hours eighteen minutes to complete, at -a distance of 14,600 miles from the centre of Mars. Phobos is much -nearer the planet, his distance from its centre being 5,800, while -from its surface he is distant only 3,700 miles. In consequence of -this nearness, he can never be seen by an observer on Mars from any -latitude higher than 69°, the bulge of the globe permanently shutting -him out from view. His period of revolution is only seven hours -thirty-nine minutes, so that to the Martian inhabitants, if there are -any, the nearer of the planet's moons must appear to rise in the west -and set in the east. By the combination of its own revolution and the -opposite rotation of Mars it will take about eleven hours to cross the -heavens; and during that period it will go through all its phases and -half through a second display. - -These little moons are certainly among the most curious and -interesting bodies of the solar system; but, unfortunately, the sight -of them is denied to most observers. That they were not seen by Sir -William Herschel with his great 4-foot reflector probably only points -to the superior defining power of the 26-inch Washington refractor as -compared with Herschel's celebrated but cumbrous instrument. Still, -they were missed by many telescopes quite competent to show them, and -of as good defining quality as the Washington instrument--a fact which -goes to add proof, if proof were needed, that the power which makes -discoveries is the product of telescope × observer, and that of the -two factors concerned the latter is the more important. It is said -that the moons have been seen by Dr. Wentworth Erck with a 7-1/3-inch -refractor. The ordinary observer is not likely to catch even a glimpse -of them with anything much smaller than a 12-inch instrument, and even -then must use precautions to exclude the glare of the planet, and may -count himself lucky if he succeed in the observation. - -A word or two may be said as to what a beginner may expect to see with -a small instrument. It has been stated that nothing under 6 inches can -make much of Mars; but this is a somewhat exaggerated statement of the -case. It is quite certain that the bulk of the more prominent markings -can be seen with telescopes of much smaller aperture. Some detail -has been seen with only 1-3/4-inch, while Grover has, with a 2-inch, -executed drawings which show how much can be done with but little -telescopic power. The fact is, that observers who are only in the -habit of using large telescopes are apt to be unduly sceptical of the -powers of small ones, which are often wonderfully efficient. The fine -detail of the canal system is, of course, altogether beyond small -instruments; and, generally speaking, it will take at least a 4-inch -to show even the more strongly marked of these strange features. At -the 1894 opposition, the writer, using a 3-7/8-inch Dollond of good -quality, was able to detect several of the more prominent canals, -but only on occasions of the best definition. The accompanying rough -sketch (Fig. 24) gives an idea of what may be expected to be seen, -under favourable conditions, with an instrument of between 2 and 3 -inches. It represents Mars as seen with a glass of 2-5/8-inch aperture -and fair quality. The main marking in the centre of the disc is -that formerly known as the Kaiser or Hour-glass Sea. Its name in -Schiaparelli's nomenclature, now universally used, is the Syrtis -Major. The same marking will also be seen in Mr. Phillips's drawing of -1899, January 30, in which it is separated by a curious bright bridge -from the Nilosyrtis to the North. The observer need scarcely expect to -see much more than is depicted in Fig. 24, with an instrument of -the class mentioned, but Plate XX. will give a very good idea of the -appearance of the planet when viewed with a telescope of considerable -power. The polar caps will be within reach, and sometimes present the -effect of projecting above the general level of the planet's surface, -owing, no doubt, to irradiation. - -[Illustration: FIG. 24. - -MARS, June 25, 1890, 10 hours 15 minutes; 2-5/8-inch, power 120.] - -To the intending observer one important caution may be suggested. In -observing and sketching the surface of Mars, do so _independently_. -The chart which accompanies this chapter is given for the purpose of -identifying markings which have been already seen, not for that of -enabling the observer to see details which are beyond the power of his -glass. No planet has been the cause of more illusion than Mars, and -drawings of him are extant which resemble nothing so much as the -photograph of an umbrella which has been turned inside out by a gust -of wind. In such cases it may reasonably be concluded that there is -something wrong, and that, unconsciously, 'the vision and the faculty -divine' have been exercised at the expense of the more prosaic, but -in this case more useful, quality of accuracy. By prolonged study of -a modern chart of Mars, and a little gentle stretching of the -imagination, the most unskilled observer with the smallest instrument -will detect a multitude of canals upon the planet, to which there is -but one objection, that they do not exist. There is enough genuine -interest about Mars, even when viewed with a small glass, without the -importation of anything spurious. In observation it will be noticed -that as the rotation period of Mars nearly coincides with that of the -earth, the change in the aspect presented from night to night will -be comparatively small, the same object coming to the meridian -thirty-seven minutes later each successive evening. Generally -speaking, Mars is an easier object to define than either Venus or -Jupiter, though perhaps scarcely bearing high powers so well as -Saturn. There is no planet more certain to repay study and to maintain -interest. He and Jupiter may be said to be at present the 'live' -planets of the solar system in an astronomical sense. - - - [Footnote *: The opposition of a planet occurs 'whenever - the sun, the earth, and the planet, as represented in their - projected orbits, are in a straight line, with the earth in - the middle.'] - - [Footnote †: That point in the orbit of a planet or comet - which is nearest to the sun.] - - - - -CHAPTER IX - -THE ASTEROIDS - - -In the year 1772 Bode of Berlin published the statement of a curiously -symmetrical relation existing among the planets of our system. The -gist of this relation, known as Bode's law, though it was really -discovered by Titius of Wittenberg, may be summed up briefly thus: -'The interval between the orbits of any two planets is about twice as -great as the inferior interval, and only half the superior one.' -Thus the distance between the orbits of the earth and Venus should, -according to Bode's law, be half of that between the earth and Mars, -which again should be half of that which separates Mars from the -planet next beyond him. Since the discovery of Neptune, this so-called -law has broken down, for Neptune is very far within the distance which -it requires; but at the time of its promulgation it represented with -considerable accuracy the actual relative positions of the planets, -with one exception. Between Mars and Jupiter there was a blank which -should, according to the law, have been filled by a planet, but to all -appearance was not. Noticing this blank in the sequence, Bode ventured -to predict that a planet would be found to fill it; and his foresight -was not long in being vindicated. - -Several continental astronomers formed a kind of planet-hunting -society to look out for the missing orb; but their operations were -anticipated by the discovery on January 1, 1801, of a small planet -which occupied a place closely approximating to that indicated for the -missing body by Bode's law. The news of this discovery, made by Piazzi -of Palermo in the course of observations for his well-known catalogue -of stars, did not reach Bode till March 20, and 'the delay just -afforded time for the publication, by a young philosopher of Jena -named Hegel, of a "Dissertation" showing, by the clearest light of -reason, that the number of the planets could not exceed seven, and -exposing the folly of certain devotees of induction who sought a new -celestial body merely to fill a gap in a numerical series.' - -The remarkable agreement of prediction and discovery roused a -considerable amount of interest, though the planet actually found, and -named Ceres after the patron-goddess of Sicily, seemed disappointingly -small. But before very long Olbers, one of the members of the original -planet-hunting society, surprised the astronomical world by the -discovery of a second planet which also fulfilled the condition of -Bode's law; and by the end of March, 1807, two other planets equally -obedient to the required numerical standard were found, the first by -Harding, the second by Olbers. Thus a system of four small planets, -Ceres, Pallas, Juno, and Vesta, was found to fill that gap in the -series which had originally suggested the search. To account for their -existence Olbers proposed the theory that they were the fragments of -a large planet which had been blown to pieces either by the disruptive -action of internal forces or by collision with a comet; and this -theory remained in favour for a number of years, though accumulating -evidence against it has forced its abandonment. - -It was not till 1845 that there was any addition to the number of the -asteroids, as they had come to be named. In that year, however, Hencke -of Driessen in Prussia, discovered a fifth, which has been named -Astræa, and in 1847 repeated his success by the discovery of a sixth, -Hebe. Since that time there has been a steady flow of discoveries, -until at the present time the number known to exist is close upon 700, -of which 569 have received permanent numbers as undoubtedly distinct -members of the solar system; and this total is being steadily added to -year by year, the average annual number of discoveries for the years -1902 to 1905 inclusive, being fifty-two. For a time the search for -minor planets was a most laborious business. The planet-hunter had -to construct careful maps of all the stars visible in a certain small -zone of the ecliptic, and to compare these methodically with the -actual face of the sky in the same zone, as revealed by his telescope. -Any star seen in the telescope, and not found to be marked upon the -chart, became forthwith an object of grave suspicion, and was watched -until its motion, or lack of motion, relatively to the other stars -either proved or disproved its planetary nature. At present this -lengthy and wearisome process has been entirely superseded by the -photographic method, in which a minor planet is detected by the fact -that, being in motion relatively to the fixed stars, its image will -appear upon the plate in the shape of a short line or trail, the -images of the fixed stars being round dots. Of course the trail may be -due to a planet which has already been discovered; but should there -be no known minor planet in the position occupied by the trail, then -a new member has been added to the system. Minor-planet hunting -has always been a highly specialized branch of astronomy, and a few -observers, such as Peters, Watson, Charlois and Palisa, and at present -Wolf, have accounted for the great majority of the discoveries. - -It was, however, becoming more and more a matter of question what -advantage was to be gained by the continuance of the hunt, when a -fresh fillip was given to interest by the discovery in 1898 of the -anomalous asteroid named Eros. Hitherto no minor planet had been known -to have the greater portion of its orbit within that of Mars, though -several do cross the red planet's borders; but the mean distance of -Eros from the sun proves to be about 135,000,000, while that of Mars -is 141,000,000 miles. In addition, the orbit of the new planet is -such that at intervals of sixty-seven years it comes within 15,000,000 -miles of the earth, or in other words nearer to us than any other -celestial body except the moon or a chance comet. It may thus come -to afford a means of revising estimates of celestial distances. Eros -presents another peculiarity. It has been found by E. von Oppolzer -to be variable in a period of two hours thirty-eight minutes; and the -theory has been put forward that the planet is double, consisting of -two bodies which revolve almost in contact and mutually eclipse one -another--in short, that Eros as a planet presents the same phenomenon -which we shall find as a characteristic of that type of variable stars -known as the Algol type. An explanation, in some respects more simple -and satisfactory, is that the variation in light is caused by the -different reflective power of various parts of its surface; but the -question is still open. - -The best results for the sizes of the four asteroids first discovered -are those of Barnard, from direct measurements with the Lick telescope -in 1894. He found the diameter of Ceres to be 485 miles, that of -Pallas 304, those of Vesta and Juno 243 and 118 miles respectively. -There appears to be as great diversity in the reflective power of -these original members of the group as in their diameters. Ceres is -large and dull, and, in Miss Clerke's words, 'must be composed of -rugged and sombre rock, unclothed probably by any vestige of air,' -while Vesta has a surface which reflects light with four times the -intensity of that of Ceres, and is, in fact, almost as brilliantly -white as newly fallen snow. - -In the place of Olbers' discredited hypothesis of an exploded planet, -has now been set the theory first suggested by Kirkwood, that instead -of having in the asteroids the remnants of a world which has become -defunct, we have the materials of one which was never allowed to -form, the overwhelming power of Jupiter's attraction having exerted -a disruptive influence over them while their formation was still only -beginning. - -So far as I am aware, they share with Mars the distinction of being -the only celestial bodies which have been made the subjects of a -testamentary disposition. In the case of Mars, readers may remember -that some years ago a French lady left a large sum of money to be -given to the individual who should first succeed in establishing -communication with the Planet of War; in that of the asteroids, the -late Professor Watson, a mighty hunter of minor planets in his day, -made provision for the supervision of the twenty-two planets captured -by him, lest any of them should get lost, stolen, or strayed. - -Small telescopes are, of course, quite impotent to deal with such -diminutive bodies as the asteroids; nor, perhaps, is it desirable -that the ranks of the minor-planet hunters should be reinforced to any -extent. - - - - -CHAPTER X - -JUPITER - - -Passing outwards from the zone of the minor planets, we come to the -greatest and most magnificent member of the solar system, the giant -planet Jupiter. To most observers, Jupiter will probably appear not -only the largest, but also the most interesting telescopic object -which our system affords. Some, no doubt, will put in a claim for -Mars, and some will share Sir Robert Ball's predilection for Saturn; -but the interest attaching to Mars is of quite a different character -from that which belongs to Jupiter, and while Saturn affords a picture -of unsurpassed beauty, there is not that interest of variety and -change in his exquisite system which is to be found in that of his -neighbour planet. Jupiter is constantly attractive by reason of the -hope, or rather the certainty, that he will always provide something -fresh to observe; and the perpetual state of flux in which the details -of his surface present themselves to the student offers to us the -only instance which can be conveniently inspected of the process of -world-formation. Jupiter is at the very opposite end of the scale -from such a body, for example, as our own moon. On the latter it would -appear as though all things were approaching the fixity of death; such -changes as are suspected are scarcely more than suspected, and, -even if established, are comparatively so small as to tax the utmost -resources of observation. On the former, such a thing as fixity or -stability appears to be unknown, and changes are constantly occurring -on a scale so gigantic as not to be beyond the reach of small -instruments, at least in their broader outlines. - -The main facts relating to the planet may be briefly given before -we go on to consider the physical features revealed to us by the -telescope. Jupiter then travels round the sun in a period of 11 -years, 314·9 days, at an average distance of almost 483,000,000 miles. -According to Barnard's measures, his polar diameter is 84,570, and his -equatorial diameter 90,190 miles. He is thus compressed at the poles -to the extent of 1/16th, and there is no planet which so conspicuously -exhibits to the eye the actual effect of this polar flattening, though -the compression of Saturn is really greater still. In volume he is -equal to more than 1,300 earths, but his density is so small that only -316 of our worlds would be needed to balance him. This low density, -not much greater than that of water, is quite in accordance with all -the other features which are revealed by observation, and appears to -be common to all the members of that group of large exterior planets -of which Jupiter is at once the first and the chief. - -The brilliancy of the great planet is exceedingly remarkable, far -exceeding that of Mars or Saturn, and only yielding to that of Venus. -In 1892 his lustre was double that of Sirius, which is by far the -brightest of all the fixed stars; and he has been repeatedly seen by -the unaided eye even when the sun was above the horizon. According to -one determination he reflects practically the same amount of light as -newly fallen snow; and even if this be rejected as impossibly high, -Zöllner's more moderate estimate, which puts his reflective power -at 62 per cent. of the light received, makes him almost as bright as -white paper. Yet to the eye it is very evident that his light has -a distinct golden tinge, and in the telescopic view this remains -conspicuous, and is further emphasized by the presence on his disc of -a considerable variety of colouring. - -Under favourable circumstances Jupiter presents to us a disc which -measures as much as 50″ in diameter. The very low magnifying power of -50 will therefore present him to the eye with a diameter of 2,500″, -which is somewhat greater than the apparent diameter of the moon. In -practice it is somewhat difficult to realize that this is the case, -probably owing to the want of any other object in the telescopic field -with which to compare the planet. But while there may be a little -disappointment at the seeming smallness of the disc even with a power -double that suggested, this will quickly be superseded by a growing -interest in the remarkable picture which is revealed to view. - -[Illustration: FIG. 25. - -JUPITER, October 9, 1891, 9.30 p.m.; 3-7/8-inch, power 120.] - -Some idea of the ordinary appearance of the planet may be gained from -Fig. 25, which reproduces a sketch made with a small telescope on -October 9, 1891. The first feature that strikes the eye on even the -most casual glance is the polar compression. The outline of the disc -is manifestly not circular but elliptical, and this is emphasized by -the fact that nearly all the markings which are visible run parallel -to one another in the direction of the longest diameter of the oval. -A little attention will reveal these markings as a series of dark -shadowy bands, of various breadths and various tones, which stretch -from side to side of the disc, fading a little in intensity as they -approach its margin, and giving the whole planet the appearance of -being girdled by a number of cloudy belts. The belts may be seen with -very low powers indeed, the presence of the more conspicuous ones -having repeatedly been evident to the writer with the rudimentary -telescope mentioned in Chapter II., consisting of a non-achromatic -double convex lens of 1-1/2-inch aperture, and a single lens eye-piece -giving a power of 36. Anything larger and more perfect than this will -bring them out with clearness, and an achromatic of from 2 to 3 inches -aperture will give views of the highest beauty and interest, and -will even enable its possessor to detect some of the more prominent -evidences of the changes which are constantly taking place. - -The number of belts visible varies very considerably. As many as -thirty have sometimes been counted; but normally the number is much -smaller than this. Speaking generally, two belts, one on either side -of a bright equatorial zone, will be found to be conspicuous, while -fainter rulings may be traced further north and south, and the dusky -hoods which cover the poles will be almost as easily seen as the -two main belts. It will further become apparent that this system -of markings is characterized by great variety of colouring. In this -respect no planet approaches Jupiter, and when seen under favourable -circumstances and with a good instrument, preferably a reflector, -some of the colour effects are most exquisite. Webb remarks: 'There is -often "something rich and strange" in the colouring of the disc. -Lord Rosse describes yellow, brick-red, bluish, and even full-blue -markings; Hirst, a belt edged with crimson lake; Miss Hirst, a small -sea-green patch near one of the poles.' The following notes of colour -were made on December 26, 1905: The south equatorial belt distinct -reddish-brown; the equatorial zone very pale yellow, almost white, -with faint slaty-blue shades in the northern portion; the north polar -regions a decided reddish-orange; while the south polar hood was of a -much colder greyish tone. But the colours are subject to considerable -change, and the variations of the two great equatorial belts appear, -according to Stanley Williams, to be periodic, maxima and minima of -redness being separated by a period of about twelve years, and the -maximum of the one belt coinciding with the minimum of the other. - -[Illustration: - - PLATE XXII. - -Jupiter, January 6, 1906--8 hours 20 minutes. Instrument, 9-1/4-inch -Reflector. - -[lambda] = 238° (System 1); [lambda] = 55° (System 2). - -Rev. T. E. R. Phillips.] - -These changes in colouring bring us to the fact that the whole system -of the Jovian markings is liable to constant and often very rapid -change. Anyone who compares drawings made a few years ago with those -made at the present time, such as Plates XXII. and XXIII., cannot fail -to notice that while there is a general similarity, the details have -changed so much that there is scarcely one individual feature which -has not undergone some modification. Indeed, this process of change is -sometimes so rapid that it can be actually watched in its occurrence. -Thus Mr. Denning remarks that 'on October 17, 1880, two dark spots, -separated by 20° of longitude, broke out on a belt some 25° north -of the equator. Other spots quickly formed on each side of the pair -alluded to, and distributed themselves along the belt, so that by -December 30 they covered three-fourths of its entire circumference.' -The dark belts, according to his observations, 'appear to be sustained -in certain cases by eruptions of dark matter, which gradually spread -out into streams.' - -Even the great equatorial belts are not exempt from the continual flux -which affects all the markings of the great planet, and the details -of their structure will be found to vary to a considerable extent at -different periods. At present the southern belt is by far the most -conspicuous feature of the surface, over-powering all other details by -its prominence, while its northern rival has shrunk in visibility to -a mere shadow of what it appears in drawings made in the seventies. -Through all the changes of the last thirty years, however, one very -remarkable feature of the planet has remained permanent at least in -form, though varying much in visibility. With the exception of the -canals of Mars, no feature of any of the planets has excited so -much interest as the great red spot on Jupiter. The history of this -extraordinary phenomenon as a subject of general study begins in 1878, -though records exist as far back as 1869 of a feature which almost -certainly was the same, and it has been suggested that it was observed -by Cassini two centuries ago. In 1878 it began to attract general -attention, which it well deserved. In appearance it was an enormous -ellipse of a full brick-red colour, measuring some 30,000 miles in -length by 7,000 in breadth, and lying immediately south of the south -equatorial belt. With this belt it appears to have some mysterious -connection. It is not actually joined to it, but seems, as Miss -Clerke observes, to be 'jammed down upon it'; at least, in the south -equatorial belt, just below where the spot lies, there has been formed -an enormous bay, bounded on the following side (_i.e._, the right -hand as the planet moves through the field), by a sharply upstanding -shoulder or cape. The whole appearance of this bay irresistibly -suggests to the observer that it has somehow or other been hollowed -out to make room for the spot, which floats, as it were, within it, -surrounded generally by a margin of bright material, which divides it -from the brown matter of the belt. The red spot, with its accompanying -bay and cape, is shown in Fig. 25 and in Plate XXII., which represents -the planet as seen by the Rev. T. E. R. Phillips on January 6, 1906. -The spot has varied very much in colour and in visibility, but on the -whole its story has been one of gradual decline; its tint has paled, -and its outline has become less distinct, as though it were being -obscured by an outflow of lighter-coloured matter, though there have -been occasional recoveries both of colour and distinctness. In 1891 it -was a perfectly easy object with 3-7/8 inches; at the present time -the writer has never found it anything but difficult with an 18-inch -aperture, though some observers have been able to see it steadily in -1905 and 1906 with much smaller telescopes. The continued existence -of the bay already referred to seems to indicate that it is only the -colour of the spot that has temporarily paled, and that observers may -in course of time witness a fresh development of this most interesting -Jovian feature. - -The nature of the red spot remains an enigma. It may possibly -represent an opening in the upper strata of Jupiter's dense -cloud-envelope, through which lower strata, or even the real body of -the planet, may be seen. The suggestion has also been made that it is -the glow of some volcanic fire on the body of the planet, seen through -the cloud-screen as the light of a lamp is seen through ground-glass. -But, after all, such ideas are only conjectures, and it is impossible -to say as yet even whether the spot is higher or lower than the -average level of the surface round it. A curious phenomenon which -was witnessed in 1891 suggested at first a hope that this question of -relative height would at least be determined. This phenomenon was the -overtaking of the red spot by a dark spot which had been travelling -after it on the same parallel, but with greater speed, for some -months. It appeared to be quite certain that the dark spot must either -transit the face of the red spot or else pass behind it; and in either -case interesting information as to the relative elevations of the two -features in question would have been obtained. The dark spot, however, -disappointed expectation by drifting round the south margin of the red -one, much as the current of a river is turned aside by the buttress of -a bridge. In fact, it would almost appear as though the red spot had -the power of resisting any pressure from other parts of the planet's -surface; yet in itself it has no fixity, for its period of rotation -steadily lengthened for several years until 1899, since when it has -begun to shorten again, so that it would appear to float upon the -surface of currents of variable speed rather than to be an established -landmark of the globe itself. The rotation period derived from it was, -in 1902, 9 hours 55 minutes 39·3 seconds. - -The mention of the changing period of rotation of the red spot lends -emphasis to the fact that no single period of rotation can be assigned -to Jupiter as a whole. It is impossible to say of the great planet -that he rotates in such and such a period: the utmost that can be said -is that certain spots upon his surface have certain rotation periods; -but these periods are almost all different from one another, and even -the period of an individual marking is subject, as already seen, to -variation. In fact, as Mr. Stanley Williams has shown, no fewer than -nine different periods of rotation are found to coexist upon the -surface; and though the differences in the periods seem small when -expressed in time, amounting in the extreme cases only to eight and -half minutes, yet their significance is very great indeed. In the case -of Mr. Williams's Zones II. and III., the difference in speed of these -two surface currents amounts to 400 miles per hour. Certain bright -spots near the equator have been found to move so much more rapidly -than the great red spot as to pass it at a speed of 260 miles an hour, -and to 'lap' it in forty-four and a half days, completing in that time -one whole rotation more than their more imposing neighbour. It cannot, -therefore, be said that Jupiter's rotation period is known; but the -average period of his surface markings is somewhere about nine hours -fifty-two minutes. - -Thus the rotation period adds its evidence to that already afforded -by the variations in colour and in form of the planet's markings that -here we are dealing with a body in a very different condition from -that of any of the other members of our system hitherto met with. We -have here no globe whose actual surface we can scrutinize, as we can -in the case of Mars and the moon, but one whose solid nucleus, if -it has such a thing, is perpetually veiled from us by a mantle which -seems more akin to the photosphere of the sun than to anything else -that we are acquainted with. The obvious resemblances may, and very -probably do, mask quite as important differences. The mere difference -in scale between the two bodies concerned must be a very important -factor, to say nothing of other causes which may be operative in -producing unlikeness. Still, there is a considerable and suggestive -general resemblance. - -In the sun and in Jupiter alike we have a view, not of the true -surface, but of an envelope which seems to represent the point of -condensation of currents of matter thrown up from depths below--an -envelope agitated in both cases, though more slowly in that of -Jupiter, by disturbances which bear witness to the operation of -stupendous forces beneath its veil. In both bodies there is a similar -small density: neither the sun nor Jupiter is much denser than water; -in both the determination of the rotation period is complicated by -the fact that the markings of the bright envelope by which the -determinations must be made move with entirely different speeds in -different latitudes. Here, however, there is a divergence, for while -in the case of the sun the period increases uniformly from the equator -to the poles, there is no such uniformity in the case of Jupiter. Thus -certain dark spots in 25° north latitude were found in 1880 to have a -shorter period than even the swift equatorial white markings. - -One further circumstance remains to be noted in pursuance of these -resemblances. Not only does the disc of Jupiter shade away at its -edges in a manner somewhat similar to that of the sun, being much -more brilliant in the centre than at the limb, but his remarkable -brilliancy, already noticed, has given rise to the suggestion that to -some small extent he may shine by his own inherent light. There are -certain difficulties, however, in the way of such a suggestion. The -satellites, for example, disappear absolutely when they enter the -shadow of their great primary--a fact which is conclusive against the -latter being self-luminous to anything more than a very small extent, -as even a small emission of native light from Jupiter would suffice -to render them visible. But even supposing that the idea of -self-luminosity has to be abandoned, everything points to the fact -that in Jupiter we have a body which presents much stronger analogies -to the sun than to those planets of the solar system which we have -so far considered. The late Mr. R. A. Proctor's conclusion probably -represents the true state of the case with regard to the giant planet: -'It may be regarded as practically proved that Jupiter's condition -rather resembles that of a small sun which has nearly reached the dark -stage than that of a world which is within a measurable time-interval -from the stage of orb-life through which our own Earth is passing.' - -Leaving the planet itself, we turn to the beautiful system of -satellites of which it is the centre. The four moons which, till 1892, -were thought to compose the complete retinue of Jupiter, were among -the first-fruits of Galileo's newly-invented telescope, and were -discovered in January, 1610. The names attached to them--Io, Europa, -Ganymede, and Callisto--have now been almost discarded in favour of -the more prosaic but more convenient numbers I., II., III., IV. The -question of their visibility to the unaided eye has been frequently -discussed, but with little result; nor is it a matter of much -importance whether or not some person exceptionally gifted with -keenness of sight may succeed in catching a momentary glimpse of one -which happens to be favourably placed. The smallest telescope or a -field-glass will show them quite clearly. They are, in fact, bodies -of considerable size, III., which is the largest, being 3,558 miles -in diameter, while IV. is only about 200 miles less; and a moderate -magnifying power will bring out their discs. - -[Illustration: PLATE XXIII. - -Jupiter, February 17, 1906. J. Baikie, 18-inch Reflector.] - -The beautiful symmetry of this miniature system was broken in 1892 by -Barnard's discovery of a fifth satellite--so small and so close to the -great planet that very few telescopes are of power sufficient to show -it. This was followed in 1904 by Perrine's discovery, from photographs -taken at the Lick Observatory with the Crossley reflector, of two -more members of the system, so that the train of Jupiter as at present -known numbers seven. The fifth, sixth, and seventh satellites are, of -course, far beyond the powers of any but the very finest instruments, -their diameters being estimated at 120, 100, and 30 miles -respectively. It will be a matter of interest, however, for the -observer to follow the four larger satellites, and to watch their -rapid relative changes of position; their occultations, when they pass -behind the globe of Jupiter; their eclipses, when they enter the great -cone of shadow which the giant planet casts behind him into space; -and, most beautiful of all, their transits. In occultations the -curious phenomenon is sometimes witnessed of an apparent flattening of -the planet's margin as the satellite approaches it at ingress or draws -away from it at egress. This strange optical illusion, which also -occurs occasionally in the case of transits, was witnessed by several -observers on various dates during the winter of 1905-1906. It is, of -course, merely an illusion, but it is curious why it should happen on -some occasions and not on others, when to all appearance the seeing is -of very much the same quality. The gradual fading away of the light of -the satellites as they enter into eclipse is also a very interesting -feature, but the transits are certainly the most beautiful objects of -all for a small instrument. The times of all these events are given in -such publications as the 'Nautical Almanac' or the 'Companion to the -Observatory'; but should the student not be possessed of either of -these most useful publications, he may notice that when a satellite is -seen steadily approaching Jupiter on the following side, a transit -is impending. The satellite will come up to the margin of the planet, -looking like a brilliant little bead of light as it joins itself to -it (a particularly exquisite sight), will glide across the margin, and -after a longer or shorter period will become invisible, being merged -in the greater brightness of the central portions of Jupiter's disc, -unless it should happen to traverse one of the dark belts, in which -case it may be visible throughout its entire journey. It will be -followed or preceded, according to the season, by its shadow, which -will generally appear as a dark circular dot. In transits which occur -before opposition the shadows precede the satellites; after opposition -they travel behind them. The transit of the satellite itself will -in most cases be a pretty sharp test of the performance of a 3-inch -telescope, or anything below that aperture; but the transit of the -shadow may be readily seen with a 2-1/2-inch, probably even with a -2-inch. There are certain anomalies in the behaviour of the shadows -which have never been satisfactorily explained. They have not always -been seen of a truly circular form, nor always of the same degree of -darkness, that of the second satellite being notably lighter in most -instances than those of the others. There are few more beautiful -celestial pictures than that presented by Jupiter with a satellite and -its shadow in transit. The swift rotation of the great planet and the -rapid motion of the shadow can be very readily observed, and the whole -affords a most picturesque illustration of celestial mechanics. - -A few notes may be added with regard to observation. In drawing the -planet regard must first of all be paid to the fact that Jupiter's -disc is not circular, and should never be so represented. It is easy -for the student to prepare for himself a disc of convenient size, say -about 2-1/2 inches in diameter on the major axis, and compressed -to the proper extent (1/16), which may be used in outlining all -subsequent drawings. Within the outline thus sketched the details must -be drawn with as great rapidity as is consistent with accuracy. The -reason for rapidity will soon become obvious. Jupiter's period of -rotation is so short that the aspect of his disc will be found to -change materially even in half an hour. Indeed, twenty minutes -is perhaps as long as the observer should allow himself for any -individual drawing, and a little practice will convince him that it -is quite possible to represent a good deal of detail in that time, and -that, even with rapid work, the placing of the various markings may be -made pretty accurate. The darker and more conspicuous features should -be laid down first of all, and the more delicate details thereafter -filled in, care being taken to secure first those near the preceding -margin of the planet before they are carried out of view by rotation. -The colours of the various features should be carefully noted at -the sides of the original drawings, and for this work twilight -observations are to be preferred. - -Different observers vary to some extent, as might be expected, in -their estimates of the planet's colouring, but on the whole there is -a broad general agreement. No planet presents such a fine opportunity -for colour-study as Jupiter, and on occasions of good seeing the -richness of the tones is perfectly astonishing. In showing the natural -colours of the planet the reflector has a great advantage over the -refractor, and observers using the reflecting type of instrument -should devote particular attention to this branch of the subject, as -there is no doubt that the colour of the various features is liable to -considerable, perhaps seasonal, variation, and systematic observation -of its changes may prove helpful in solving the mystery of Jupiter's -condition. The times of beginning and ending observation should be -carefully noted, and also the magnifying powers employed. These should -not be too high. Jupiter does not need, and will not stand, so much -enlargement as either Mars or Saturn. It is quite easy to secure -a very large disc, but over-magnifying is a great deal worse than -useless: it is a fertile source of mistakes and illusions. If the -student be content to make reasonable use of his means, and not to -overpress either his instrument or his imagination, he will find upon -Jupiter work full of absorbing interest, and may be able to make his -own contribution to the serious study of the great planet. - - - - -CHAPTER XI - -SATURN - - -At nearly double the distance of Jupiter from the sun circles the -second largest planet of our system, unique, so far as human knowledge -goes, in the character of its appendages. The orbit of Saturn has a -mean radius of 886,000,000 miles, but owing to its eccentricity, his -distance may be diminished to 841,000,000 or increased to 931,000,000. -This large variation may not play so important a part in his economy -as might have been supposed, owing to the fact that the sun heat -received by him is not much more than 1/100th of that received by the -earth. The planet occupies twenty-nine and a half years in travelling -round its immense orbit. Barnard's measures with the Lick telescope -give for the polar diameter 69,770, and for the equatorial 76,470 -miles. Saturn's polar compression is accordingly very great, amounting -to about 1/12th. Generally speaking, however, it is not so obvious -in the telescopic view as the smaller compression of Jupiter, being -masked by the proximity of the rings. - -[Illustration: PLATE XXIV. - -Saturn, July 2, 1894. E. E. Barnard, 36-inch Equatorial.] - -Saturn is the least dense of all the planets; in fact, this enormous -globe, nine times the diameter of the earth, would float in water. -This fact of extremely low density at once suggests a state of matters -similar to that already seen to exist, in all likelihood, in the case -of Jupiter; and all the evidence goes to support the view that Saturn, -along with the other three large exterior planets, is in the condition -of a semi-sun. - -The globe presents, on the whole, similar characteristics to those -already noticed as prevailing on Jupiter, but, as was to be expected, -in a condition enfeebled by the much greater distance across which -they are viewed and the smaller scale on which they are exhibited. -It is generally girdled by one or two tropical belts of a grey-green -tone; the equatorial region is yellow, and sometimes, like the -corresponding region of Jupiter, bears light spots upon it and a -narrow equatorial band of a dusky tone; the polar regions are of a -cold ashy or leaden colour. Professor Barnard's fine drawing (Plate -XXIV.) gives an admirable representation of these features as seen -with the 36-inch Lick telescope. Altogether, whether from greater -distance or from intrinsic deficiency, the colouring of Saturn is by -no means so vivid or so interesting as that of his larger neighbour. - -The period of rotation was, till within the last few years, thought -to be definitely and satisfactorily ascertained. Sir William Herschel -fixed it, from his observations, at ten hours sixteen minutes. -Professor Asaph Hall, from observations of a white spot near the -equator, reduced this period to ten hours fourteen minutes twenty-four -seconds. Stanley Williams and Denning, in 1891, reached results -differing only by about two seconds from that of Hall; but the former, -discussing observations of 1893, arrived at the conclusion that there -were variations of rotation presented in different latitudes and -longitudes of the planet's surface, the longest period being ten hours -fifteen minutes, and the shortest ten hours twelve minutes forty-five -seconds. Subsequently Keeler obtained, by spectroscopic methods, a -result exactly agreeing with that of Hall. It appeared, therefore, -that fairly satisfactory agreement had been reached on a mean period -of ten hours fourteen minutes twenty-four seconds. - -In 1903, however, a number of bright spots appeared in a middle north -latitude which, when observed by Barnard, Comas Solà, Denning, and -other observers, gave a period remarkably longer than that deduced -from spots in lower latitudes--namely, about ten hours thirty-eight -minutes. Accordingly, it follows that the surface of Saturn's -equatorial regions rotates much more rapidly than that of the regions -further north--a state of affairs which presents an obvious likeness -to that prevailing on Jupiter. But in the case of Saturn the -equatorial current must move relatively to the rest of the surface at -the enormous rate of from 800 to 900 miles an hour, a speed between -three and four times greater than that of the corresponding current on -Jupiter! - -The resemblance between the two great planets is thus very marked -indeed. Great size, coupled with small density; very rapid rotation, -with its accompaniment of large polar compression; and, even more -markedly in the case of the more distant planet than in that of -Jupiter, a variety of rotation periods for different markings, which -indicates that these features have been thrown up from different -strata of the planet's substance--such points of likeness are too -significant to be ignored. It is not at all likely that Saturn has -any solidity to speak of, any more than Jupiter; the probabilities all -point in the direction of a comparatively small nucleus of somewhat -greater solidity than the rest, surrounded by an immense condensation -shell, where the products of various eruptions are represented. - -Were this all that can be said about Saturn, the planet would scarcely -be more than a reduced and somewhat less interesting edition of -Jupiter. As it is, he possesses characteristics which make him -Jupiter's rival in point of interest, and, as a mere telescopic -picture, perhaps even his superior. When Galileo turned his telescope -upon Saturn, he was presented with what seemed an insoluble enigma. -It appeared to him that, instead of being a single globe, the planet -consisted of three globes in contact with one another; and this -supposed fact he intimated to Kepler in an anagram, which, when -rearranged, read: 'Altissimum planetam tergeminum observavi'--'I -have observed the most distant planet to be threefold.' Under better -conditions of observation, he remarked subsequently, the planet -appeared like an olive, as it still does with low powers. This was -sufficiently puzzling, but worse was to follow. After an interval, -on observing Saturn again, he found that the appearances which had so -perplexed him had altogether disappeared; the globe was single, like -those of the other planets. In his letter to Welser, dated December -4, 1612, the great astronomer describes his bewilderment, and his -fear lest, after all, it should turn out that his adversaries had been -right, and that his discoveries had been mere illusions. - -Then followed a period when observers could only command optical power -sufficient to show the puzzling nature of the planet's appendages, -without revealing their true form. It appeared that Saturn had 'ansæ,' -or handles, on either side of him, between which and his body the -sky could be seen; and many uncouth figures are still preserved which -eloquently testify to the bewilderment of those who drew them, though -some of them are wonderfully accurate representations of the planet's -appearance when seen with insufficient means. The bewilderment was -sometimes veiled, in amusing cuttle-fish fashion, under an inky cloud -of sesquipedalian words. Thus Hevelius describes the aspects of Saturn -in the following blasting flight of projectiles: 'The mono-spherical, -the tri-spherical, the spherico-ansated, the elliptico-ansated, -and the spherico-cuspidated,' which is very beautiful no doubt, but -scarcely so simple as one could wish a popular explanation to be. - -In the year 1659, however, Huygens, who had been observing Saturn with -a telescope of 2-1/3 inches aperture and 23 feet focal length, bearing -a magnifying power of 100, arrived at the correct solution of the -mystery, which he announced to, or rather concealed from, the world -in a barbarous jumble of letters, which, when properly arranged, read -'annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam -inclinato'--'he (Saturn) is surrounded by a thin flat ring, nowhere -touching (him, and) inclined to the ecliptic.' Huygens also discovered -the first and largest of Saturn's satellites, Titan. His discoveries -were followed by those of Cassini, who in 1676 announced his -observation of that division in the ring which now goes by his name. -From Cassini's time onwards to the middle of the nineteenth century, -nothing was observed to alter to any great extent the conception -of the Saturnian system which had been reached; though certain -observations were made, which, though viewed with some suspicion, -seemed to indicate that there were more divisions in the ring than -that which Cassini had discovered, and that the system was thus a -multiple one. In particular a marking on the outer ring was detected -by Encke, and named after him, though generally seen, if seen at all, -rather as a faint shading than as a definite division. (It is not -shown in Barnard's drawing, Plate XXIV.). But in 1850 came the last -great addition to our knowledge of the ring system, W. C. Bond in -America, and Dawes in England making independently the discovery of -the faint third ring, known as the Crape Ring, which lies between the -inner bright ring and the globe. - -The extraordinary appendages thus gradually revealed present a -constantly varying aspect according to the seasons of the long -Saturnian year. At Saturn's equinoxes they disappear, being turned -edgewise; then, reappearing, they gradually broaden until at the -solstice, 7-1/3 years later, they are seen at their widest expansion; -while from this point they narrow again to the following equinox, -and repeat the same process with the opposite side of the ring -illuminated, the whole set of changes being gone through in 29 years -167·2 days. Barnard's measures give for the outer diameter of the -outer ring 172,310 miles; while the clear interval between the inner -margin of the Crape Ring and the ball is about 5,800 miles, and the -width of the great division in the ring-system (Cassini's) 2,270 -miles. In sharp contrast to these enormous figures is the fact -that the rings have no measurable thickness at all, and can only be -estimated at not more than 50 miles. They disappear absolutely when -seen edgewise; even the great Lick telescope lost them altogether for -three days in October, 1891.[*] - -The answer to the question of what may be the constitution of these -remarkable features may now be given with a moderate approach to -certainty. It has been shown successively that the rings could not be -solid, or liquid, and in 1857 Clerk-Maxwell demonstrated that the only -possible constitution for such a body is that of an infinite number -of small satellites. The rings of Saturn thus presumably consist of -myriads of tiny moonlets, each pursuing its own individual orbit -in its individual period, and all drawn to their present form of -aggregation by the attraction of Saturn's bulging equator. The -appearances presented by the rings are explicable on this theory, and -on no other. Thus the brightness of the two rings A and B would arise -from the closer grouping of the satellites within these zones; while -the semi-darkness of the Crape Ring arises from the sparser sprinkling -of the moonlets, which allows the dark sky to be seen between them. -Cassini's division corresponds to a zone which has been deprived of -satellites; and as it has been shown that this vacant zone occupies a -position where a revolving body would be subject to disturbance from -four of Saturn's satellites, the force which cleared this gap in the -ring is obvious. It has been urged as an objection to the satellite -theory that while the thin spreading of the moonlets would account for -the comparative darkness of the Crape Ring when seen against the sky, -it by no means accounts for the fact that this ring is seen as a dark -stripe upon the body of the planet. Seeliger's explanation of this is -both satisfactory and obvious, when once suggested--namely, that the -darkness of the Crape Ring against the planet is due to the fact that -what we see is not the actual transits of the satellites themselves, -but the perpetual flitting of their shadows across the ball. The final -and conclusive argument in favour of this theory of the constitution -of the rings was supplied by the late Professor Keeler by means of the -spectroscope. It is evident that if the rings were solid, the speed -of rotation should increase from their inner to their outer -margin--_i.e._, the outer margin must move faster, in miles per -second, than the inner does. If, on the contrary, the rings are -composed of a great number of satellites, the relation will be exactly -reversed, and, owing to the superior attractive force exercised upon -them by the planet through their greater nearness to him, the inner -satellites will revolve faster than the outer ones. Now, this point -is capable of settlement by spectroscopic methods involving the -application of the well-known Doppler's principle, that the speed of a -body's motion produces definite and regular effects upon the pitch of -the light emitted or reflected by it. The measurements were of extreme -delicacy, but the result was to give a rate of motion of 12-1/2 miles -per second for the inner edge of ring B, and of 10 miles for the outer -edge of A, thus affording unmistakable confirmation of the satellite -theory of the rings. Keeler's results have since been confirmed by -Campbell and others; and it may be regarded as a demonstrated fact -that the rings, as already stated, consist of a vast number of small -satellites. - -It has been maintained that the ball of Saturn is eccentrically placed -within the ring, and further, that this eccentricity is essential to -the stability of the system; while the suggestion has also been made -that the ring-system is undergoing progressive change, and that the -interval between it and the ball is lessening. It has to be noticed, -however, that the best measures, those of Barnard, indicate that the -ball is symmetrically placed within the rings; and the suggestion of a -diminishing interval between the ring-system and the ball receives no -countenance from comparison of the measures which have been made at -different times. - -There can be no question that of all objects presented to observation -in the solar system, there is not one, which, for mere beauty and -symmetry can be for a moment compared with Saturn, even though, as -already indicated, Mars and Jupiter present features of more lasting -interest. To quote Proctor's words: 'The golden disc, faintly striped -with silver-tinted belts; the circling rings, with their various -shades of brilliancy and colour; and the perfect symmetry of the -system as it sweeps across the dark background of the field of -view, combine to form a picture as charming as it is sublime and -impressive.' Fortunately the main features of this beautiful picture -are within the reach of very humble instruments. Webb states that -when the ring system was at its greatest breadth he has seen it with -a power of about twenty on only 1-1/3-inch aperture. A beginner cannot -expect to do so much with such small means; but at all events a 2-inch -telescope with powers of from 50 to 100 will reveal the main outlines -of the ring very well indeed, and, with careful attention will show -the shadow of the ring upon the ball, and that of the ball upon the -ring. When we come to the question of the division in the ring, we are -on somewhat more doubtful ground. Proctor affirms that 'the division -in the ring (Cassini's) can be seen in a good 2-inch aperture in -favourable weather.' One would have felt inclined to say that the -weather would require to be very favourable indeed, were it not that -Proctor's statement is corroborated by Denning, who remarks that -'With a 2-inch refractor, power about ninety, not only are the rings -splendidly visible, but Cassini's division is readily glimpsed, as -well as the narrow dark belt on the body of the planet.' The student -may, however, be warned against expecting that such statements will -apply to his own individual efforts. There are comparatively few -observers whose eyes have had such systematic training as to qualify -them for work like this, and those who begin by expecting to see all -that skilled observers see with an instrument of the same power are -only laying up for themselves stores of disappointment. Mr. Mee's -frank confession may be commended to the notice of those who hope to -see at the first glance all that old students have learned to see -by years of hard work. 'The first time I saw Saturn through a large -telescope, a fine 12-inch reflector, I confess I could not see -the division (Cassini's), though the view of the planet was one of -exquisite beauty and long to be remembered, and notwithstanding the -fact that the much fainter division of Encke was at the moment visible -to the owner of the instrument!' It is extremely unlikely that the -beginner will see the division with anything much less than 3 inches, -and even with that aperture he will not see it until the rings are -well opened. The writer's experience is that it is not by any means so -readily seen as is sometimes supposed. Three inches will show it under -good conditions; with 3-7/8 it can be steadily held, even when the -rings are only moderately open (steady holding is a very different -thing from 'glimpsing'), but even with larger apertures the division -becomes by no means a simple object as the rings close up (Fig. 26). -In fact, there is nothing better fitted to fill the modern observer's -mind with a most wholesome respect for the memory of a man like -Cassini, than the thought that with his most imperfect appliances this -great observer detected the division, a much more difficult feat than -the mere seeing it when its existence and position are already known, -and discovered also four of the Saturnian satellites. As for the minor -divisions in the ring, if they are divisions, they are out of the -question altogether for small apertures, and are often invisible even -to skilled observers using the finest telescopes. Barnard's drawing -(Plate XXIV.), as already noted, shows no trace of Encke's division; -but nine months later the same observer saw it faintly in both ansæ of -the ring. The conclusion from this and many similar observations seems -to be that the marking is variable, as may very well be, from the -constitution of the ring. The Crape Ring is beyond any instrument -of less than four inches, and even with such an aperture requires -favourable circumstances. - -[Illustration: FIG. 26. - -SATURN, 3-7/8-inch.] - -With regard to a great number of very remarkable details which of -late years have been seen and drawn by various observers, it may be -remarked that the student need not be unduly disappointed should his -small instrument fail, as it almost certainly will, to show these. -This is a defect which his telescope shares with an instrument of such -respectable size and undoubted optical quality as the Lick 36-inch. -Writing in January, 1895, concerning the beautiful drawing which -accompanies this chapter, Professor Barnard somewhat caustically -observes: 'The black and white spots lately seen upon Saturn by -various little telescopes were totally beyond the reach of the -36-inch--as well as of the 12-inch--under either good or bad -conditions of seeing.... The inner edge (of the Crape Ring) was a -uniform curve; the serrated or saw-toothed appearance of its inner -edge which had previously been seen with some small telescopes -was also beyond the reach of the 36-inch.' Such remarks should be -consoling to those who find themselves and their instruments unequal -to the remarkable feats which are sometimes accomplished, or recorded. - -So far as one's personal experience goes, Saturn is generally the -most easily defined of all the planets. Of late years he has been very -badly placed for observers in the Northern Hemisphere, and this has -considerably interfered with definition. But when well placed the -planet presents a sharpness and steadiness of outline which render -him capable of bearing higher magnifying powers than Jupiter, and even -than Mars, though a curious rippling movement will often be noticed -passing along the rings. It can scarcely be said, however, that -there is much work for small instruments upon Saturn--the seeing -of imaginary details being excluded. Accordingly, in spite of the -undoubted beauty of the ringed planet, Jupiter will on the whole be -found to be an object of more permanent interest. Yet, viewed merely -as a spectacle, and as an example of extraordinary grace and symmetry, -Saturn must always command attention. The sight of his wonderful -system can hardly fail to excite speculation as to its destiny; and -the question of the permanence of the rings is one that is almost -thrust upon the spectator. With regard to this matter it may be noted -that, according to Professor G. H. Darwin, the rings represent merely -a passing stage in the evolution of the Saturnian system. At present -they are within the limit proved by Roche, in 1848, to be that within -which no secondary body of reasonable size could exist; and thus the -discrete character of their constituents is maintained by the strains -of unequal attraction. Professor Darwin believes that in time the -inner particles of the ring will be drawn inwards, and will eventually -fall upon the planet's surface, while the outer ones will disperse -outwards to a point beyond Roche's limit, where they may eventually -coalesce into a satellite or satellites--a poor compensation for the -loss of appendages so brilliant and unique as the rings. - -Saturn's train of satellites is the most numerous and remarkable in -our system. As already mentioned, Huygens, the discoverer of the true -form of the ring, discovered also the first and brightest satellite, -Titan, which is a body somewhat larger than our own moon, having a -diameter of 2,720 miles. A few years later came Cassini's discoveries -of four other satellites, beginning in 1671 and ending in 1684. For -more than 100 years discovery paused there, and it was not until -August and September, 1789, that Sir William Herschel added the sixth -and seventh to our knowledge of the Saturnian system. - -In 1848 Bond in America and Lassell in England made independently the -discovery of the eighth satellite--another of the coincidences which -marked the progress of research upon Saturn, and in both of which Bond -was concerned. Then followed another pause of fifty years broken by -the discovery, in 1898, by Professor Pickering, of a ninth, whose -existence was not completely confirmed till 1904. The motion of this -satellite has proved to be retrograde, unlike that of the earlier -discovered members of the family, so that its discovery has introduced -us to a new and abnormal feature of the Saturnian system. The -discoverer of Ph[oe]be, as the ninth satellite has been named, has -followed up his success by the discovery of a tenth member of Saturn's -retinue, known provisionally as Themis. Accordingly the system, as at -present known, consists of a triple ring and ten satellites. The last -discovered moons are very small bodies, the diameter of Ph[oe]be, for -instance, being estimated at 150 miles; while its distance from Saturn -is 8,000,000 miles. From the surface of the planet Ph[oe]be would -appear like a star of fifth or sixth magnitude; to observers on our -own earth its magnitude is fifteenth or sixteenth. The ten satellites -have been named as follows: 1, Titan, discovered by Huygens; 2, -Japetus; 3, Rhea; 4, Dione; 5, Tethys, all discovered by Cassini; 6, -Enceladus; and 7, Mimas, Sir William Herschel; 8, Hyperion, Bond and -Lassell; 9, Ph[oe]be; and 10, Themis, W. H. Pickering. Titan, the -largest satellite, has been found to be considerably denser than -Saturn himself. - -The most of these little moons are, of course, beyond the power of -small glasses; but a 2-inch will show Titan perfectly well. Japetus -also is not a difficult object, but is much easier at his western than -at his eastern elongation, a fact which probably points to a surface -of unequal reflective power. Rhea, Dione, and Tethys are much more -difficult. Kitchiner states that a friend of his saw them with -2-7/10-inch aperture, the planet being hidden; but probably his friend -had been amusing himself at the quaint old gentleman's expense. -Noble concludes that with a first-class 3-inch and under favourable -circumstances four, or as a bare possibility even five, satellites may -be seen; and I have repeatedly seen all the five with 3-7/8-inches. -The only particular advantages of seeing them are the test which they -afford of the instrument used, and the accompanying practice of the -eye in picking up minute points of light. There is a considerable -interest in watching the gradual disappearance of the brilliant disc -of Saturn behind the edge of the field, or of the thick wire which may -be placed in the eye-piece to hide the planet, and then catching the -sudden flash up of the tiny dots of light which were previously lost -in the glare of the larger body. For purposes of identification, -recourse must be had to the 'Companion to the Observatory,' which -prints lists of the elongations of the various satellites and a -diagram of their orbits which renders it an easy matter to identify -any particular satellite seen. Transits are, with the exception -of that of Titan, beyond the powers of such instruments as we are -contemplating. The shadow of Titan has, however, been seen in transit -with a telescope of only 2-7/8-inch aperture. - - - [Footnote *: The plane of the rings passes through the earth - on April 13, and through the sun on July 27, 1907, at - which periods it is probable that the rings will altogether - disappear.] - - - - -CHAPTER XII - -URANUS AND NEPTUNE - - -Hitherto we have been dealing with bodies which, from time immemorial, -have been known to man as planets. There must have been a period when -one by one the various members of our system known to the ancients -were discriminated from the fixed stars by unknown but patient and -skilful observers; but, from the dawn of historical astronomy, up to -the night of March 13, 1781, there had been no addition to the number -of those five primary planets the story of whose discovery is lost in -the mists of antiquity. - -It may be questioned whether any one man, Kepler and Newton being -possible exceptions, has ever done so much for the science of -astronomy as was accomplished by Sir William Herschel. Certainly -no single observer has ever done so much, or, which is almost more -important than the actual amount of his achievement, has so completely -revolutionized methods and ideas in observing. - -A Hanoverian by birth, and a member of the band of the Hanoverian -Guards, Herschel, after tasting the discomforts of war in the shape -of a night spent in a ditch on the field of Hastenbeck, where that -egregious general the Duke of Cumberland was beaten by the French, -concluded that he was not designed by Nature for martial distinction, -and abruptly solved the problem of his immediate destiny by recourse -to the simple and unheroic expedient of desertion. He came to England, -got employment after a time as organist of the Octagon Chapel at Bath, -and was rapidly rising into notice as a musician, when the force of -his genius, combined with a discovery which came certainly unsought, -but was grasped as only a great man can grasp the gifts of Fortune, -again changed the direction of his life, and gave him to the science -of astronomy. - -He had for several years employed his spare time in assiduous -observation; and, finding that opticians' prices were higher than he -could well afford, had begun to make Newtonian reflectors for -himself, and had finally succeeded in constructing one of 6-1/2 inches -aperture, and of high optical quality. With this instrument, on the -night of March 13, 1781, he was engaged in the execution of a plan -which he had formed of searching the heavens for double stars, with a -view to measuring their distance from the earth by seeing whether the -apparent distance of the members of the double from one another varied -in any degree in the course of the earth's journey round the sun. He -was working through the stars in the constellation Gemini, when his -attention was fixed by one which presented a different appearance from -the others which had passed his scrutiny. - -In a good telescope a fixed star shows only a very small disc, which -indeed should be but a point of light; and the finer the instrument -the smaller the disc. The disc of this object, however, was -unmistakably larger than those of the fixed stars in its -neighbourhood--unmistakably, that is, to an observer of such skill as -Herschel, though those who have seen Uranus under ordinary powers will -find their respect considerably increased for the skill which at -once discriminated the tiny greenish disc from that of a fixed star. -Subsequent observation revealed to Herschel that he was right in -supposing that this body was not a star, for it proved to be in motion -relatively to the stars among which it was seen. But, in spite of -poetic authority, astronomical discoveries do not happen quite so -dramatically as the sonnet 'On First looking into Chapman's Homer' -suggests. - - 'Then felt I like some watcher of the skies, - When a new planet swims into his ken' - -is a noble simile, were it only true to the facts. But new planets -do not swim around promiscuously in this fashion; and in the case of -Uranus, which more nearly realizes the thought of Keats than any other -in the history of astronomy, the 'watcher of the skies' felt probably -more puzzlement than anything else. Herschel was far from realizing -that he had found a new planet. When unmistakable evidence was -forthcoming that the newly discovered body was not a fixed star, he -merely felt confirmed in the first conjecture which had been suggested -by the size of its disc--namely, that he had discovered a new comet; -and it was as a new comet that Uranus was first announced to the -astronomical world. - -It quickly became evident, however, that the new discovery moved in no -cometary orbit, but in one which marked it out as a regular member -of the solar system. A search was then instituted for earlier -observations of the planet, and it was found to have been observed -and mistaken for a fixed star on twenty previous occasions! One -astronomer, Lemonnier, had actually observed it no fewer than twelve -times, several of them within a few weeks of one another, and, had he -but reduced and compared his observations, could scarcely have failed -to have anticipated Herschel's discovery. But perhaps an astronomer -who, like Lemonnier, noted some of his observations on a paper-bag -which had formerly contained hair-powder, and whose astronomical -papers have been described as 'the image of Chaos,' scarcely deserved -the honour of such a discovery! - -When it became known that this new addition to our knowledge of the -solar system had been made by the self-taught astronomer at Bath, -Herschel was summoned to Court by George III., and enabled to devote -himself entirely to his favourite study by the bestowal of the not -very magnificent pension of £200 a year, probably the best investment -that has ever been made in the interests of astronomical science. In -gratitude to the penurious monarch who had bestowed on him this meagre -competence, Herschel wished to call his planet the Georgium Sidus--the -Georgian Star, and this title, shortened in some instances to the -Georgian, is still to be found in some ancient volumes on astronomy. -The astronomers of the Continent, however, did not feel in the least -inclined to elevate Farmer George to the skies before his due time, -and for awhile the name of Herschel was given to the new planet, which -still bears as its symbol the first letter of its discoverer's name -with a globe attached to the cross-bar [Uranus]. Finally, the name -Urᾰnus ('a' short) prevailed, and has for long been in universal -use. - -Uranus revolves round the sun at a distance from him of about -1,780,000,000 miles, in an orbit which takes eighty-four of our years -to complete. Barnard gives his diameter at 34,900 miles, and if this -measure be correct, he is the third largest planet of the system. -Other measures give a somewhat smaller diameter, and place Neptune -above him in point of size. - -Subsequent observers have been able to see but little more than -Herschel saw upon the diminutive disc to which even so large a body -is reduced at so vast a distance. When near opposition, Uranus can -readily be seen with the naked eye as a star of about the sixth -magnitude, and there is no difficulty in picking him up with the -finder of an ordinary telescope by means of an almanac and a good star -map, nor in raising a small disc by the application of a moderately -high power, say 200 and upwards. (Herschel was using 227 at the time -of his discovery.) But small telescopes do little more than give their -owners the satisfaction of seeing, pretty much as Herschel saw it, -the object on which his eye was the first to light. Nor have even the -largest instruments done very much more. Rings, similar to those of -Saturn, were once suspected, but have long since been disposed of, -and most of the observations of spots and belts have been gravely -questioned. The Lick observers in 1890 and 1891 describe the belts as -'the merest shades on the planet's surface.' - -The spectrum of Uranus is marked by peculiarities which distinguish -it from that of the other planets. It is crossed by six dark -absorption-bands, which indicate at all events that the medium -through which the sunlight which it reflects to us has passed is of -a constitution markedly different from that of our own atmosphere. It -was at first thought that the spectrum gave evidence of the planet's -self-luminosity; but this has not proved to be the case, though -doubtless Uranus, like Jupiter and Saturn, is in the condition of -a semi-sun. Like the other members of the group of large exterior -planets, his density is small, being only 1/5 greater than that of -water. - -Six years after his great discovery, Herschel, with the 40-foot -telescope of 4 feet in aperture which he had now built, discovered two -satellites, and believed himself to have discovered four more. Later -observations have shown that, in the case of the four, small stars -near the planet had been mistaken for satellites. Subsequently two -more were discovered, one by Lassell, and one by Otto Struve, making -the number of the Uranian retinue up to four, so far as our present -knowledge goes. These four satellites, known as Ariel, Umbriel, -Oberon, and Titania, are distinguished by the fact that their orbits -are almost perpendicular to the plane of the orbit of Uranus, and that -the motions of all of them are retrograde. Titania and Oberon, the two -discovered by Herschel, are the easiest objects; but although they are -said to have been seen with a 4·3-inch refractor, this is a feat which -no ordinary observer need hope to emulate. An 8-inch is a more likely -instrument for such a task, and a 12-inch more likely still; -the average observer will probably find the latter none too big. -Accordingly, they are quite beyond the range of such observation as we -are contemplating. The rotation period of Uranus is not known. - -In a few years after the discovery of Uranus, it became apparent that -by no possible ingenuity could his places as determined by present -observation be satisfactorily combined with those determined by the -twenty observations available, as already mentioned, from the period -before he was recognised as a planet. Either the old observations were -bad, or else the new planet was wandering from the track which it had -formerly followed. It appeared to Bouvard, who was constructing the -tables for the motions of Uranus, the simplest course to reject the -old observations as probably erroneous, and to confine himself to the -modern ones. Accordingly this course was pursued, and his tables were -published in 1821, but only for it to be found that in a few years -they also began to prove unsatisfactory; discrepancies began to appear -and to increase, and it quickly became apparent that an attempt must -be made to discover the cause of them. - -Bouvard himself appears to have believed in the existence of a planet -exterior to Uranus whose attraction was producing these disturbances, -but he died in 1843 before any progress had been made with the -solution of the enigma. In 1834 Hussey approached Airy, the Astronomer -Royal, with the suggestion that he might sweep for the supposed -exterior planet if some mathematician would help him as to the most -likely region to investigate. Airy, however, returned a sufficiently -discouraging answer, and Hussey apparently was deterred by it from -carrying out a search which might very possibly have been rewarded -by success. Bessel, the great German mathematician, had marked the -problem for his own, and would doubtless have succeeded in solving -it, but shortly after he had begun the gathering of material for his -researches, he was seized with the illness which ultimately proved -fatal to him. - -The question was thus practically untouched when in 1841, John Couch -Adams, then an undergraduate of St. John's College, Cambridge, jotted -down a memorandum in which he indicated his resolve to attack it and -attempt the discovery of the perturbing planet, 'as soon as possible -after taking my degree.' The half-sheet of notepaper on which the -memorandum was made is still extant, and forms part of the volume -of manuscripts on the subject preserved in the library of St. John's -College. - -On October 21, 1845, Adams, who had taken his degree (Senior Wrangler) -in 1843, communicated to Airy the results of his sixth and final -attempt at the solution of the problem, and furnished him with the -elements and mass of the perturbing planet, and an indication of its -approximate place in the heavens. Airy, whose record in the matter -reads very strangely, was little more inclined to give encouragement -to Adams than to Hussey. He replied by propounding to the young -investigator a question which he considered 'a question of vast -importance, an _experimentum crucis_,' which Adams seemingly -considered of so little moment, that strangely enough he never -troubled to answer it. Then the matter dropped out of sight, though, -had the planet been sought for when Adams's results were first -communicated to the Astronomer Royal, it would have been found within -3-1/2 lunar diameters of the place assigned to it. - -Meanwhile, in France, another and better-known mathematician had taken -up the subject, and in three memoirs presented to the French Academy -of Sciences in 1846, Leverrier furnished data concerning the new -planet which agreed in very remarkable fashion with those furnished by -Adams to Airy. The coincidence shook Airy's scepticism, and he asked -Dr. Challis, director of the Cambridge Observatory, to begin a search -for the planet with the large Northumberland equatorial. Challis, who -had no complete charts of the region to be searched, began to make -observations for the construction of a chart which would enable him to -detect the planet by means of its motion. It is more than likely that -had he adopted Hussey's suggestion of simply sweeping in the vicinity -of the spot indicated, he would have been successful, for the -Northumberland telescope was of 11 inches aperture, and would have -borne powers sufficient to distinguish readily the disc of Neptune -from the fixed stars around it. However, Challis chose the more -thorough, but longer method of charting; and even to that he did -not devote undivided attention. 'Some wretched comet,' says Proctor, -'which he thought it his more important duty to watch, prevented -him from making the reductions which would have shown him that the -exterior planet had twice been recorded in his notes of observations.' - -Indeed, a certain fatality seems to have hung over the attempts made -in Britain to realize Adams's discovery. In 1845, the Rev. W. R. -Dawes, one of the keenest and most skilful of amateur observers, was -so much impressed by some of Adams's letters to the Astronomer Royal -that he wrote to Lassell, asking him to search for the planet. When -Dawes's letter arrived, Lassell was suffering from a sprained ankle, -and laid the letter aside till he should be able to resume work. In -the meantime the letter was burned by an officious servant-maid, and -Lassell lost the opportunity of a discovery which would have crowned -the fine work which he accomplished as an amateur observer. - -A very different fate had attended Leverrier's calculations. On -September 23, 1846, a letter from Leverrier was received at the Berlin -Observatory, asking that search should be made for the planet in the -position which his inquiries had pointed out. The same night Galle -made the search, and within a degree of the spot indicated an object -was found with a measurable disc of between two and three seconds -diameter. As it was not laid down on Bremiker's star-chart of the -region, it was clearly not a star, and by next night its planetary -nature was made manifest. The promptitude with which Leverrier's -results were acted upon by Encke and Galle is in strong contrast to -the sluggishness which characterized the British official astronomers, -who, indeed, can scarcely be said to have come out of the business -with much credit. - -A somewhat undignified controversy ensued. The French astronomers, -very naturally, were eager to claim all the laurels for their -brilliant countryman, and were indignant when a claim was put in on -behalf of a young Englishman whose name had never previously been -heard of. Airy, however, displayed more vigour in this petty squabble -than in the search for Neptune, and presented such evidence in support -of his fellow-countryman's right to recognition that it was impossible -to deny him the honour which, but for official slackness, would have -fallen to him as the actual as well as the potential discoverer of the -new planet. Adams himself took no part in the strife; spoke, indeed, -no words on the matter, except to praise the abilities of Leverrier, -and gave no sign of the annoyance which most men in like circumstances -would have displayed. - -Galle suggested that the new planet should be called Janus; but the -name of the two-faced god was felt to be rather too pointedly suitable -at the moment, and that of Neptune was finally preferred. Neptune -is about 32,900 miles in diameter, his distance from the sun is -2,792,000,000 miles, and he occupies 165 years in the circuit of his -gigantic orbit. The spectroscopic evidence, such as it is, seems to -point to a condition somewhat similar to that of Uranus. - -Neptune had only been discovered seventeen days when Lassell found him -to be attended by one satellite. First seen on October 10, 1846, it -was not till the following July that the existence of this body -was verified by Lassell himself and also by Otto Struve and Bond -of Harvard. From the fact that it is visible at such an enormous -distance, it is evident that this satellite must be of considerable -size--probably at least equal to our own moon. - -Small instruments can make nothing of Neptune beyond, perhaps, -distinguishing the fact that, whatever the tiny disc may be, it is -not that of a star. His satellite is an object reserved for the very -finest instruments alone. - -Should Neptune have any inhabitants, their sky must be somewhat barren -of planets. Jupiter's greatest elongation from the sun would be about -10°, and he would be seen under somewhat less favourable conditions -than those under which we see Mercury; while the planets between -Jupiter and the sun would be perpetually invisible. Saturn and Uranus, -however, would be fairly conspicuous, the latter being better seen -than from the earth. - -Suspicions have been entertained of the existence of another planet -beyond Neptune, and photographic searches have been made, but hitherto -without success. So far as our present knowledge goes, Neptune is the -utmost sentinel of the regular army of the solar system. - - - - -CHAPTER XIII - -COMETS AND METEORS - - -There is one type of celestial object which seldom fails to stir up -the mind of even the most sluggishly unastronomical member of the -community and to inspire him with an interest in the science--an -interest which is usually conspicuous for a picturesque inaccuracy -in the details which it accumulates, for a pathetic faith in the most -extraordinary fibs which may be told in the name of science, and for -a subsidence which is as rapid as the changes in the object which gave -the inspiration. The sun may go on shining, a perpetual mystery and -miracle, without attracting any attention, save when a wet spring -brings on the usual talk of sun-spots and the weather; Jupiter -and Venus excite only sufficient interest to suggest an occasional -question as to whether that bright star is the Star of Bethlehem; but -when a great comet spreads its fiery tail across the skies everybody -turns astronomer for the nonce, and normally slumber-loving people -are found willing, or at least able, to desert their beds at the -most unholy hours to catch a glimpse of the strange and mysterious -visitant. And, when the comet eventually withdraws from view again, as -much inaccurate information has been disseminated among the public as -would fill an encyclopædia, and require another to correct. - -Comets are, however, really among the most interesting of celestial -objects. Though we no longer imagine them to foretell wars, famines, -and plagues, or complacently to indicate the approbation of heaven -upon some illustrious person deceased or about to decease, and have -almost ceased to shiver at the possibilities of a collision between a -comet and the earth, they have within the last half century taken on a -new and growing interest of a more legitimate kind, and there are few -departments of science in which the advance of knowledge has been more -rapid or which promise more in the immediate future, given material to -work upon. - -The popular idea of a comet is that it is a kind of bright wandering -star with a long tail. Indeed, the star part of the conception is -quite subsidiary to the tail part. The tail is _the_ thing, and a -comet without a tail is not worthy of attention, if it is not rather -guilty of claiming notice on false pretences. As a matter of fact, the -tail is absent in many comets and quite inconspicuous in many more; -and a comet may be a body with any degree of resemblance or want of -resemblance to the popular idea, from the faint globular stain of -haze, scarcely perceptible in the telescopic field against the dark -background of the sky, up to a magnificent object, which, like the -dragon in the Revelation, seems to draw the third part of the stars -of heaven after it--an object like the Donati comet of 1858, with a -nucleus brighter than a first-magnitude star, and a tail like a great -feathery plume of light fifty millions of miles in length. It seems -as impossible to set limits to the variety of form of which comets are -capable as it is to set limits to their number. - -Generally speaking, however, a comet consists of three parts: The -nucleus--which appears as a more or less clearly defined star-like -point, and is the only part of the comet which will bear any -magnification to speak of--the coma, and the tail. In many telescopic -comets the nucleus is entirely absent, and, in the comets in which -it is present, it is of very varied size, and often presents curious -irregularities in shape, and even occasionally the appearance of -internal motions. It frequently changes very much in size during the -period of the comet's visibility. The nucleus is the only part of a -comet's structure which has even the most distant claim to solidity; -but even so the evidence which has been gradually accumulated all goes -to show that while it may be solid in the sense of being composed of -particles which have some substance, it is not solid in the sense of -being one coherent mass, but rather consists of something like a swarm -of small meteoric bodies. Surrounding the nucleus is the coma, from -which the comet derives its name. This is a sort of misty cloud -through which the nucleus seems to shine like a star in a nebula or -a gas-lamp in a fog. Its boundaries are difficult to trace, as it -appears to fade away gradually on every side into the background; but -generally its appearance is more or less of a globular shape except -where the tail streams away from it behind. Sometimes the coma is of -enormous extent--the Great Comet of 1811 showed a nucleus of 428 miles -diameter, enclosed within a nebulous globe 127,000 miles across, which -in its turn was wrapped in a luminous atmosphere of four times -greater diameter, with an outside envelope covering all, and extending -backwards to form the tail. But it is also of the most extraordinary -tenuity, the light of the very faintest stars having been frequently -observed to shine undimmed through several millions of miles of coma. -Finally, there is the tail, which may be so short as to be barely -distinguishable; or may extend, as in the case of Comet 1811 (ii.), to -130,000,000 miles; or, as in that of Comet 1843 (i.), to 200,000,000. -The most tenuous substances with which we are acquainted seem to be -solidity itself compared with the material of a comet's tail. It is -'such stuff as dreams are made of.' - -Comets fall into two classes. There are those whose orbits follow -curves that are not closed, like the circle or the ellipse, but appear -to extend indefinitely into space. A comet following such an orbit -(parabolic or hyperbolic) seems to come wandering in from the depths -of space, passes round the sun, and then gradually recedes into the -space from which it came, never again to be seen of human eye. It is -now becoming questionable, however, whether any comet can really -be said to come in from infinite space; and the view is being more -generally held that orbits which to us appear portions of unclosed -curves may in reality be only portions of immensely elongated -ellipses, and that all comets are really members of the solar system, -travelling away, indeed, to distances that are immense compared with -even the largest planetary orbit, but yet infinitely small compared -with the distances of the fixed stars. - -Second, there are those comets whose orbits form ellipses with a -greater or less departure from the circular form. Such comets must -always return again, sooner or later, to the neighbourhood of the sun, -which occupies one of the foci of the ellipse, and they are known as -Periodic Comets. The orbits which they follow may have any degree of -departure from the circular form, from one which does not differ very -notably from that of such a planet as Eros, up to one which may be -scarcely distinguishable from a parabola. Thus we have Periodic Comets -again divided into comets of short and comets of long period. In the -former class, the period ranges from that of Encke's comet which never -travels beyond the orbit of Jupiter, and only takes 3·29 years to -complete its journey, up to that of the famous comet whose periodicity -was first discovered by Halley, whose extreme distance from the sun -is upwards of 3,200,000,000 miles, and whose period is 76·78 years. -Comets of long period range from bodies which only require a paltry -two or three centuries to complete their revolution, up to others -whose journey has to be timed by thousands of years. In the case of -these latter bodies, there is scarcely any distinction to be made -between them and those comets which are not supposed to be periodic; -the ellipse of a comet which takes three or four thousand years to -complete its orbit is scarcely to be distinguished, in the small -portion of it that can be traced, from a parabola. - -Several comets have been found to be short period bodies, which, -though bright enough to have been easily seen, have yet never been -noticed at any previous appearance. It is known that some at least of -these owe their present orbits to the fact that having come near to -one or other of the planets they have been, so to speak, captured, and -diverted from the track which they formerly pursued. Several of the -planets have more or less numerous flocks of comets associated with -them which they have thus captured and introduced into a short period -career. Jupiter has more than a score in his group, while Saturn, -Uranus and Neptune have smaller retinues. There can be no question -that a comet of first-class splendour, such as that of 1811, that of -1858, or that of 1861, is one of the most impressive spectacles that -the heavens have to offer. Unfortunately it is one which the present -generation, at least in the northern hemisphere, has had but little -opportunity of witnessing. Chambers notices 'that it may be taken as -a fact that a bright and conspicuous comet comes about once in ten -years, and a very remarkable comet once every thirty years;' and adds, -'tested then by either standard of words "bright and conspicuous," or -"specially celebrated," it may be affirmed that a good comet is now -due.' It is eleven years since that hopeful anticipation was penned, -and we are still waiting, not only for the 'specially celebrated,' but -even for the 'bright and conspicuous' comet; so that on the whole we -may be said to have a grievance. Still, there is no saying when the -grievance may be removed, as comets have a knack of being unexpected -in their developments; and it may be that some unconsidered little -patch of haze is even now drawing in from the depths which may yet -develop into a portent as wonderful as those that astonished the -generation before us in 1858 and 1861. - -The multitude of comets is, in all probability, enormous. Between -the beginning of the Christian era and 1888 the number recorded was, -according to Chambers, 850; but the real number for that period must -have been indefinitely greater, as, for upwards of 1600 out of the -1888 years, only those comets which were visible to the naked eye -could have been recorded--a very small proportion of the whole. The -period 1801 to 1888 shows 270, so that in less than one century -there has been recorded almost one-third of the total for nineteen -centuries. At present no year goes by without the discovery of several -comets; but very few of them become at all conspicuous. For example, -in 1904, six comets were seen--three of these being returns of comets -previously observed, and three new discoveries; but none of these -proved at all notable objects in the ordinary sense, though Comet 1904 -(_a_), discovered by Brooks, was pretty generally observed. - -It would serve no useful purpose to repeat here the stories of any of -the great comets. These may be found in considerable detail in such -volumes as Chambers's 'Handbook of Astronomy,' vol. i., or Miss Agnes -Clerke's 'History of Astronomy.' Attention must rather be turned to -the question, 'What are comets?' It is a question to which no answer -of a satisfactory character could be given till within the last fifty -years. Even the great comet of 1858, the Donati, which made so deep an -impression on the public mind, and was so closely followed and -studied by astronomers, was not the medium of any great advance in -the knowledge of cometary nature. The many memoirs which it elicited -disclosed nothing fundamentally new, and broke out no new lines of -inquiry. Two things have since then revolutionized the study of the -subject--the application of the spectroscope to the various comets -that have appeared in the closing years of the nineteenth century, and -the discovery of the intimate connection between comets and meteors. - -It was in 1864, a year further made memorable astronomically by Sir -William Huggins's discovery of the gaseous nature of some of the -nebulæ, that the spectroscope was first applied to the study of -a comet. The celestial visitor thus put to the question, a comet -discovered by Tempel, was in nowise a distinguished object, appearing -like a star of the second magnitude, or less, with a feeble though -fairly long tail. When analyzed by Donati, it was found to yield a -spectrum consisting of three bright bands, yellow, green, and blue, -separated by dark spaces. This observation at once modified ideas as -to cometary structure. Hitherto it had been supposed that comets shone -by reflected light; but Donati's observation revealed beyond question -that the light of the 1864 comet at all events was inherent, and that, -so far as the observation went, the comet consisted of glowing gas. - -[Illustration: - - PLATE XXV. - -Great Comet. Photographed May 5, 1901, with the 13-inch Astrographic -Refractor of the Royal Observatory, Cape of Good Hope.] - -In 1868 Sir William Huggins carried the matter one step further by -showing that the spectrum of Winnecke's comet of that year agreed -with that of olefiant gas rendered luminous by electricity; and the -presence of the hydrocarbon spectrum has since been detected in -a large number of comets. The first really brilliant comet to be -analyzed by the spectroscope was Coggia's (1874), and it presented not -only the three bright bands that had been already seen, but the whole -range of five bands characteristic of the hydrocarbon spectrum. In -certain cases, however--notably, that of Holmes's comet of 1892 and -that of the great southern comet of 1901 (Plate XXV.)--the spectrum -has not exhibited the usual bright band type, but has instead shown -merely a continuous ribbon of colour. From these analyses certain -facts emerge. First, that the gaseous surroundings of comets consist -mainly of hydrogen and carbon, and that in all probability their -luminosity is due, not to mere solar heat, but to the effect of some -electric process acting upon them during their approach to the sun; -and second, that, along with these indications of the presence -of luminous hydrocarbon compounds, there is also evidence of the -existence of solid particles, mainly in the nucleus, but also to some -extent in the rest of the comet, which shine by reflected sunlight. It -is further almost certain, from the observation by Elkin and Finlay -of the beginning of the transit of Comet 1882 (iii.) across the sun's -face, that this solid matter is not in any sense a solid mass. The -comet referred to disappeared absolutely as soon as it began to pass -the sun's edge. Had it been a solid mass or even a closely compacted -collection of small bodies it would have appeared as a black spot upon -the solar surface. The conclusion, then, is obvious that the solid -matter must be very thinly and widely spread, while its individual -particles may have any size from that of grains of sand up to that of -the large meteoric bodies which sometimes reach our earth. - -Thus the state of the case as regards the constitution of comets is, -roughly speaking, this: They consist of a nucleus of solid -matter, held together, but with a very slack bond, by the power of -gravitation. From this nucleus, as the comet approaches perihelion, -the electric action of the sun, working in a manner at present -unknown, drives off volumes of luminous gas, which form the tail; and -in some comets the waves of this vapour have been actually seen rising -slowly in successive pulses from the nucleus, and then being driven -backwards much as the smoke of a steamer is driven. It has been found -also by investigation of Comet Wells 1882 and the Great Comet of 1882 -that in some at least of these bodies sodium and iron are present. - -The question next arises, What becomes of comets in the end? Kepler -long ago asserted his belief that they perished, as silkworms perish -by spinning their own thread, exhausting themselves by the very -efforts of tail-production which render them sometimes so brilliant to -observation; and this seems to be pretty much the case. Thus Halley's -comet, which was once so brilliant and excited so much attention, -was at its last visit a very inconspicuous object indeed. At its -apparition in 1845-1846 Biela's comet was found to have split into -two separate bodies, which were found at their return in 1852 to have -parted company widely. Since that year it has never been observed -again in the form of a comet, though, as we shall see, it has -presented itself in a different guise. The same fate has overtaken the -comets of De Vico (1844), and Brorsen (1846). The former should have -returned in 1850, but failed to keep its appointment; and the latter, -after having established a character for regularity by returning to -observation on four occasions, failed to appear in 1890, and has never -since been seen. - -The mystery of such disappearances has been at least partially -dispelled by the discovery, due to Schiaparelli and other workers in -the same field, that various prominent meteor-showers travel in orbits -precisely the same as those of certain comets. Thus the shower of -meteors which takes place with greater or less brilliancy every year -from a point in the constellation Perseus has been proved to follow -the orbit of the bright comet of 1862; while the great periodic shower -of the Leonids follows the track of the comet of 1866; the orbit of -the star-shower of April 20--the Lyrids--corresponds with that of a -comet seen in 1861; and the disappearance of Biela's comet appears to -be accounted for by the other November shower whose radiant point is -in the constellation Andromeda. In fact, the state of the matter is -well summed up by Kirkwood's question: 'May not our periodic meteors -be the débris of ancient but now disintegrated comets, whose matter -has become distributed round their orbits?' The loosely compacted mass -which forms the nucleus of the comet appears to gradually lose its -cohesion under the force of solar tidal action, and its fragments -come to revolve independently in their orbit, for a time in a loosely -gathered swarm, and then gradually, as the laggards drop behind, in -the form of a complete ring of meteoric bodies, which are distributed -over the whole orbit. The Leonid shower is in the first condition, or, -rather, was when it was last seen, for it seems to be now lost to us; -the Perseid shower is in the second. The shower of the Andromedes has -since confirmed its identity with the lost comet of Biela by displays -in 1872, 1885, and 1892, at the seasons when that comet should -have returned to the neighbourhood of the sun. It appears to be -experiencing the usual fate of such showers, and becoming more -widely distributed round its orbit, and the return in 1905 was very -disappointing, the reason apparently being that the dense group in -close attendance on the comet has suffered disturbance from Jupiter -and Saturn, and now passes more than a million miles outside the -earth's orbit. - -In 1843 there appeared one of the most remarkable of recorded comets. -It was not only of conspicuous brilliancy and size, though its tail -at one stage reached the enormous length of 200,000,000 miles, but was -remarkable for the extraordinarily close approach which it made to the -sun. Its centre came as near to the sun as 78,000 miles, leaving no -more than 32,000 miles between the surfaces of the two bodies; it -must, therefore, have passed clear through the corona, and very -probably through some of the prominences. Its enormous tail was -whirled, or rather appeared to be whirled, right round the sun in a -little over two hours, thus affording conclusive proof that the -tail of a comet cannot possibly be an appendage, but must consist of -perpetually renewed emanations from the nucleus. But in addition to -these wonders, the comet of 1843 proved the precursor of a series of -fine comets travelling in orbits which were practically identical. The -great southern comet of 1880 proved, when its orbit had been computed, -to follow a path almost exactly the same as that of its predecessor -of thirty-seven years before. It seemed inconceivable that a body so -remarkable as the 1843 comet should have a period of only thirty-seven -years, and yet never previously have attracted attention. Before -the question had been fairly discussed, it was accentuated by -the discovery, in 1881, of a comet whose orbit was almost -indistinguishable from that of the comet of 1807. But the 1807 comet -was not due to return till A.D. 3346. Further, the comet of 1881 -proved to have a period, not of seventy-four years, as would have been -the case had it been a return of that of 1807, but of 2,429 years. -The only possible conclusion was that here were two comets which were -really fragments of one great comet which had suffered disruption, -as Biela's comet visibly did, and that one fragment followed in the -other's wake with an interval of seventy-four years. - -Meanwhile, the question of the 1843 and 1880 comets was still -unsettled, and it received a fresh complication by the appearance -of the remarkable comet of 1882, whose transit of the sun has been -already alluded to, for the orbit of this new body proved to be a -reproduction, almost, but not quite exact, of those of the previous -two. Astronomers were at a greater loss than ever, for if this were a -return of the 1880 comet, then the conclusion followed that something -was so influencing its orbit as to have shortened its period from -thirty-seven to two years. The idea of the existence of some medium -round the sun, capable of resisting bodies which passed through it, -and thus causing them to draw closer to their centre of attraction and -shortening their periods, was now revived, and it seemed as though, -at its next return, this wonderful visitant must make the final plunge -into the photosphere, with what consequences none could foretell. -These forebodings proved to be quite baseless. The comet passed so -close to the sun (within 300,000 miles of his surface), that it must -have been sensibly retarded at its passage by the resisting medium, -had such a thing existed; but not the slightest retardation was -discernible. The comet suffered no check in its plunge through the -solar surroundings, and consequently the theory of the resisting -medium may be said to have received its quietus. - -Computation showed that the 1882 comet followed nearly the same orbit -as its predecessors; and thus we are faced by the fact of families -of comets, travelling in orbits that are practically identical, and -succeeding one another at longer or shorter intervals. The idea that -these families have each sprung from the disruption of some much -larger body seems to be most probable, and it appears to be confirmed -by the fact that in the 1882 comet the process of further disruption -was actually witnessed. Schmidt of Athens detected one small offshoot -of the great comet, which remained visible for several days. Barnard -a few days later saw at least six small nebulous bodies close to their -parent, and a little later Brooks observed another. 'Thus,' as Miss -Agnes Clerke remarks, 'space appeared to be strewn with the filmy -débris of this beautiful but fragile structure all along the track of -its retreat from the sun.' - -The state of our knowledge with regard to comets may be roughly summed -up. We have extreme tenuity in the whole body, even the nucleus -being apparently not solid, but a comparatively loose swarm of -solid particles. The nucleus, in all likelihood, shines by reflected -sunlight--in part, at all events. The nebulous surroundings and tail -are produced by solar action upon the matter of which the comet is -composed, this action being almost certainly electrical, though heat -may play some part in it. The nebulous matter appears to proceed in -waves from the nucleus, and to be swept backward along the comet's -track by some repellent force, probably electrical, exerted by -the sun. This part of the comet's structure consists mainly of -self-luminous gases, generally of the hydrocarbon type, though sodium -and iron have also been traced. Comets, certainly in many cases, -probably in all, suffer gradual degradation into swarms of meteors. -The existence of groups of comets, each group probably the outcome of -the disruption of a much larger body, is demonstrated by the fact of -successive comets travelling in almost identically similar orbits. -Finally, comets are all connected with the solar system, so far, at -least, that they accompany that system in its journey of 400,000,000 -miles a year through space. Our system does not, as it were, pick up -the comets as it sweeps along upon its great journey; it carries them -along with it. - -A few words may be added as to cometary observation. It is scarcely -likely that any very great number of amateur observers will ever -be attracted by the branch of comet-hunting. The work is somewhat -monotonous and laborious, and seems to require special aptitudes, -and, above all, an enormous endowment of patience. Probably the true -comet-hunter, like the poet, is born, not made; and it is not likely -that there are, nor desirable that there should be, many individuals -of the type of Messier, the 'comet-ferret.' 'Messier,' writes a -contemporary, 'is at all events a very good man, and simple as a -child. He lost his wife some years ago, and his attendance upon her -death-bed prevented his being the discoverer of a comet for which he -had been lying in wait, and which was snatched from him by Montaigne -de Limoges. This made him desperate. A visitor began to offer him -consolation for his recent bereavement, when Messier, thinking only of -the comet, answered, "I had discovered twelve; alas! to be robbed of -the thirteenth by that Montaigne!" and his eyes filled with tears. -Then, recollecting that it was necessary to deplore his wife, he -exclaimed, "Ah! cette pauvre femme!" and again wept for his comet.' In -addition to the fact that few have reached such a degree of scientific -detachment as to put a higher value upon a comet than upon the nearest -of relatives, there is the further fact that the future of cometary -discovery, and of the record of cometary change seems to lie almost -entirely with photography, which is wonderfully adapted for the work -(Plate XXVI.). - -[Illustration: - - PLATE XXVI. - - 1 2 - -Photographs of Swift's Comet. By Professor E. E. Barnard. - -1. Taken April 4, 1892; exposure 1 hour. 2. Taken April 6, 1892; -exposure 1 hour 5 minutes.] - -Anyone who desires to become a comet-hunter must, in addition to -the possession of the supreme requisites, patience and perseverance, -provide himself with an instrument of at least 4 inches aperture, -together with a good and comprehensive set of star-charts and the New -General Catalogue of nebulæ with the additions which have been made to -it. The reason for this latter item of equipment is the fact that many -telescopic comets are scarcely to be distinguished from nebulæ, and -that an accurate knowledge of the nebulous objects in the regions to -be searched for comets, or at least a means of quickly identifying -such objects, is therefore indispensable. The portions of the heavens -which afford the most likely fields for discovery will naturally be -those in the vicinity of where the sun has set at evening, or where he -is about to rise in the early morning, all comets having of necessity -to approach the sun more or less closely at their perihelion passage. -Other parts of the heavens should not be neglected; but these are the -most likely neighbourhoods. - -Most of us, however, will be content to discover our comets in the -columns of the daily newspaper, or by means of a post-card from -some obliging friend. The intimation, in whatever way received, will -generally contain the position of the comet at a certain date, given -in right ascension and declination, and either a statement of its -apparent daily motion, or else a provisional set of places for several -days ahead. Having either of these, the comet's position must be -marked down on the star-map, and the course which it is likely to -pursue must be traced out in pencil by means of the data--a perfectly -simple matter of marking down the position for each day by its -celestial longitude and latitude as given. The observer will next note -carefully the alignment of the comet with the most conspicuous stars -in the neighbourhood of the particular position for the day of his -observation; and, guiding his telescope by means of these, will point -it as nearly as possible to that position. He may be lucky enough -to hit upon his object at once, especially if it be a comparatively -bright one. More probably, he will have to 'sweep' for it. In this -case the telescope must be pointed some little distance below and to -one side of the probable position of the comet, and moved slowly and -gently along, careful watch being kept upon the objects which pass -through the field, until a similar distance on the opposite side of -the position has been reached. Then raise the instrument by not more -than half a field's breadth, estimating this by the stars in the -field, and repeat the process in the opposite direction, going on -until the comet appears in the field, or until it is obvious that it -has been missed. A low power should be used at first, which may be -changed for a somewhat higher one when the object has been found. But -in no case will the use of really high magnifiers be found advisable. -It is, of course, simply impossible with the tail, for which the -naked eye is the best instrument, nor can the coma bear any degree -of magnification, though occasionally the nucleus may be sufficiently -sharply defined to bear moderate powers. The structure of the latter -should be carefully observed, with particular attention to the -question of whether any change can be seen in it, or whether there -seem any tendency to such a multiplication of nuclei as characterized -the great comet of 1882. It is possible that the pulses of vapour -sunwards from the nucleus may also be observed. - -Appearance of motion, wavy or otherwise, in the tail, should also be -looked for, and carefully watched if seen. Beyond this there is not -very much that the ordinary observer can do; the determination of -positions requires more elaborate appliances, and the spectroscope is -necessary for any study of cometary constitution. It only remains -to express a wish for the speedy advent of a worthy subject for -operations. - - -We turn now to those bodies which, as has been pointed out, appear to -be the débris of comets which have exhausted their cometary destiny, -and ceased to have a corporate existence. Everyone is familiar with -the phenomenon known as a meteor, or shooting-star, and there are few -clear nights on which an observer who is much in the open will not see -one or more of these bodies. Generally they become visible in the form -of a bright point of light which traverses in a straight line a longer -or shorter path across the heavens, and then vanishes, sometimes -leaving behind it for a second or two a faintly luminous train. The -shooting-stars are of all degrees of brightness, from the extremely -faint streaks which sometimes flash across the field of the telescope, -up to brilliant objects, brighter than any of the planets or fixed -stars, and sometimes lighting up the whole landscape with a light like -that of the full moon. - -The prevailing opinion, down to a comparatively late date, was that -shooting-stars were mere exhalations in the earth's atmosphere, -arising as one author expressed it, 'from the fermentation of acid -and alkaline bodies, which float in the atmosphere'; and it was -also suggested by eminent astronomers that they were the products -of terrestrial volcanoes, returning, after long wanderings, to their -native home. - -The true study of meteoric astronomy may be said to date from the year -1833, when a shower of most extraordinary splendour was witnessed. -The magnificence of this display was the means of turning greater -attention to the subject; and it was observed as a fact, though the -importance of the observation was scarcely realized, that the meteors -all appeared to come from nearly the one point in the constellation -Leo. The fact of there being a single radiant point implied that -the meteors were all moving in parallel lines, and had entered -our atmosphere from a vast distance. Humboldt, who had witnessed a -previous appearance of this shower in 1799, suggested that it might be -a periodic phenomenon; and his suggestion was amply confirmed when -in 1866 the shower made its appearance again in scarcely diminished -splendour. Gradually other showers came to be recognised, and their -radiant points fixed; and meteoric astronomy began to be established -upon a scientific basis. - -In 1866 Schiaparelli announced that the shower which radiates in -August from the constellation Perseus follows the same track as that -of Swift's comet (1862 iii.); and in the following year the great -November shower from Leo, already alluded to, was proved to have a -similar connection with Tempel's comet (1866 i.). The shower which -comes from the constellation Lyra, about April 20, describes the -same orbit as that of Comet 1861 i.; while, as already mentioned, -the mysterious disappearance of Biela's comet received a reasonable -explanation by its association with the other great November -shower--that which radiates from the constellation Andromeda. With -regard to the last-named shower, it has not only been shown that -the meteors are associated with Biela's comet, but also that they -separated from it subsequent to 1841, in which year the comet's orbit -was modified by perturbations from Jupiter. The Andromeda meteors -follow the modified orbit, and hence must have been in close -association with the comet when the perturbation was exercised. - -The four outstanding meteor radiants are those named, but there are -very many others. Mr. Denning, to whom this branch of science owes so -much, estimates the number of distinct radiants known at about 4,400; -and it seems likely that every one of these showers, some of them, of -course very feeble, represents some comet deceased. The history of a -meteor shower would appear to be something like this: When the comet, -whose executor it is, has but recently deceased, it will appear as a -very brilliant periodic shower, occurring on only one or two nights -exactly at the point where the comet in its journeying would have -crossed the earth's track, and appearing only at the time when the -comet itself would have been there. Gradually the meteors get more and -more tailed out along the orbit, as runners of unequal staying powers -get strung out over a track in a long race, until the displays may -be repeated, with somewhat diminished splendour, year after year for -several years before and after the time when the parent comet is -due. At last they get thinly spread out over the whole orbit, and -the shower becomes an annual one, happening each year when the earth -crosses the orbit of the comet. This has already happened to the -Perseid shower; at least 500,000,000 miles of the orbit of Biela's -comet are studded with representatives of the Andromedes; and the -Leonid shower had already begun to show symptoms of the same process -at its appearance in 1866. Readers will remember the disappointment -caused by the failure of the Leonid shower to come up to time in 1899, -and it seems probable that the action of some perturbing cause has so -altered the orbit of this shower that it now passes almost clear -of the earth's path, so that we shall not have the opportunity of -witnessing another great display of the Leonid meteors. - -So far as is known, no member of one of these great showers has -ever fallen to the earth. There are two possible exceptions to this -statement, as in 1095 a meteor fell to the ground during the progress -of a shower of Lyrids, and in 1885 another fell during a display of -the Andromedes. In neither case, however, was the radiant point noted, -and unless it was the same as that of the shower the fall of the -meteor was a mere coincidence. It seems probable that this is the -case, and the absence of any evidence that a specimen from a cometary -shower has reached the earth points to the extreme smallness of the -various members of the shower and also to the fine division of the -matter of the original comet. - -In addition to the meteors originating from systematic showers, there -are also to be noted frequent and sometimes very brilliant single -meteors. Specimens of these have in many instances been obtained. They -fall into three classes--'Those in which iron is found in considerable -amount are termed siderites; those containing an admixture of iron and -stone, siderolites; and those consisting almost entirely of stone are -known as aerolites' (Denning). The mass of some of these bodies is -very considerable. Swords have been forged out of their iron, one of -which is in the possession of President Diaz of Mexico, while diamonds -have been found in meteoric irons which fell in Arizona. It may -be interesting to know that, according to a grave decision of the -American courts, a meteor is 'real estate,' and belongs to the person -on whose ground it has fallen; the alternative--that it is 'wild -game,' and the property of its captor--having been rejected by the -court. So far as I am aware, the legal status of these interesting -flying creatures has not yet been determined in Britain. - -The department of meteoric astronomy is one in which useful work can -be done with the minimum of appliances. The chief requisites are a -good set of star-maps, a sound knowledge of the constellations, a -straight wand, and, above all, patience. The student must make himself -familiar with the constellations (a pleasant task, which should be -part of everyone's education), so that when a meteor crosses his -field of view he may be able to identify at once with an approach to -accuracy its points of appearance and disappearance. It is here that -the straight wand comes into play. Mr. Denning advises the use of it -as a means of guiding the eye. It is held so as to coincide with the -path of the meteor just seen, and will thus help the eye to estimate -the position and slope of the track relatively to the stars of the -constellations which it has crossed. This track should be marked as -quickly as possible on the charts. Mere descriptions of the appearance -of meteors, however beautiful, are quite valueless. It is very -interesting to be told that a meteor when first seen was 'of the size -and colour of an orange,' but later 'of the apparent size of the -full moon, and surrounded by a mass of glowing vapour which further -increased its size to that of the head of a flour-barrel'; but the -description is scarcely marked by sufficient precision of statement -for scientific purposes. The observer must note certain definite -points, of which the following is a summary: (1) Date, hour, and -minute of appearance. (2) Brightness, compared with some well-known -star, planet, or, if exceptionally bright, with the moon. (3) Right -ascension and declination of point of first appearance. (4) The -same of point of disappearance. (5) Length of track. (6) Duration of -visibility. (7) Colour, presence of streak or train, and any other -notable features. (8) Radiant point. - -When these have been given with a reasonable approach to accuracy, -the observer has done his best to provide a real, though small, -contribution to the sum of human knowledge; nor is the determination -of these points so difficult as would at first appear from their -number. The fixing of the points of appearance and disappearance and -of the radiant will present a little difficulty to start with; but in -this, as in all other matters, practice will bring efficiency. It may -be mentioned that the efforts of those who take up this subject would -be greatly increased in usefulness by their establishing a connection -with the Meteor Section of the British Astronomical Association. - -One curious anomaly has been established by Mr. Denning's patient -labour--the existence, namely, of what are termed 'stationary -radiants.' It is obvious that if meteors have the cometary connection -already indicated, their radiant point should never remain fixed; as -the showers move onwards in their orbit they should leave the original -radiant behind. Mr. Denning has conclusively proved, however, that -there are showers which do not follow the rule in this respect, but -proceed from a radiant which remains the same night after night, some -feeble showers maintaining the same radiant for several months. It is -not easy to see how this fact is to be reconciled with the theory of -cometary origin; but the fact itself is undeniable. - - - - -CHAPTER XIV - -THE STARRY HEAVENS - - -We now leave the bounds of our own system, and pass outwards towards -the almost infinite spaces and multitudes of the fixed stars. In doing -so we are at once confronted with a wealth and profusion of beauty and -a vastness of scale which are almost overwhelming. Hitherto we have -been dealing almost exclusively with bodies which, though sometimes -considerably larger than our world, were yet, with the exception of -the sun, of the same class and comparable with it; and with distances -which, though very great indeed, were still not absolutely beyond the -power of apprehension. But now all former scales and standards have to -be left behind, for even the vast orbit of Neptune, 5,600,000,000 -of miles in diameter, shrinks into a point when compared with the -smallest of the stellar distances. Even our unit of measurement has -to be changed, for miles, though counted in hundreds of millions, are -inadequate; and, accordingly, the unit in which our distance from the -stars is expressed is the 'light year,' or the distance travelled by a -ray of light in a year. - -Light travels at the rate of about 186,000 miles a second, and -therefore leaps the great gulf between our earth and the sun in -about eight minutes. But even the nearest of the fixed stars--Alpha -Centauri, a star of the first magnitude in the Southern Hemisphere--is -so incredibly distant that light takes four years and four months to -travel to us from it; while the next nearest, a small star in Ursa -Major, is about seven light-years distant, and the star 61 of the -constellation Cygnus, the first northern star whose distance was -measured, is separated from us by two years more still. - -At present the distances of about 100 stars are known approximately; -but it must be remembered that the approximation is a somewhat -rough one. The late Mr. Cowper Ranyard once remarked of measures of -star-distances that they would be considered rough by a cook who was -in the habit of measuring her salt by the cupful and her pepper by the -pinch. And the remark has some truth--not because of any carelessness -in the measurements, for they are the results of the most minute and -scrupulous work with the most refined instrumental means that modern -skill can devise and construct--but because the quantities to be -measured are almost infinitely small. - -It is at present considered that the average distance from the earth -of stars of the first magnitude is thirty-three light years, that of -stars of the second fifty-two, and of the third eighty-two. In other -words, when we look at such stars on any particular evening, we are -seeing them, not as they are at the moment of observation, but as they -were thirty-three, fifty-two, or eighty-two years ago, when the rays -of light which render them visible to us started on their almost -inconceivable journey. The fact of the average distance of -first-magnitude stars being less than that of second, and that of -second in turn less than that of third, is not to be held as implying -that there are not comparatively small stars nearer to us than some -very bright ones. Several insignificant stars are considerably nearer -to us than some of the most brilliant objects in the heavens--_e.g._, -61 Cygni, which is of magnitude 4·8, is almost infinitely nearer to us -than the very brilliant first magnitude star Rigel in Orion. The rule -holds only on the average. - -The number of the stars is not less amazing than their distance. It is -true that the number visible to the unaided eye is not by any means -so great as might be imagined on a casual survey. On a clear night the -eye receives the impression that the multitude of stars is so great -as to be utterly beyond counting; but this is not the case. The -naked-eye, or 'lucid,' stars have frequently been counted, and it -has been found that the number visible to a good average eye in -both hemispheres together is about 6,000. This would give for each -hemisphere 3,000, and making allowance for those lost to sight in -the denser air near the horizon, or invisible by reason of restricted -horizon, it is probable that the number of stars visible at any one -time to any single observer in either hemisphere does not exceed -2,500. In fact Pickering estimates the total number visible, down to -and including the sixth magnitude, to be only 2,509 for the Northern -Hemisphere, and on that basis it may safely be assumed that 2,000 -would be the extreme limit for the average eye. - -[Illustration: - - PLATE XXVII. - -Region of the Milky Way in Sagittarius. Photographed by Professor E. -E. Barnard.] - -But this somewhat disappointing result is more than atoned for when -the telescope is called in and the true richness of the heavenly host -begins to appear. Let us take for illustration a familiar group of -stars--the Pleiades. The number of stars visible to an ordinary eye -in this little group is six; keen-sighted people see eleven, or even -fourteen. A small telescope converts the Pleiades into a brilliant -array of luminous points to be counted not by units but by scores, -while the plates taken with a modern photographic telescope of 13 -inches aperture show 2,326 stars. The Pleiades, of course, are a -somewhat notable group; but those who have seen any of the beautiful -photographs of the heavens, now so common, will know that in many -parts of the sky even this great increase in number is considerably -exceeded; and that for every star the eye sees in such regions a -moderate telescope will show 1,000, and a great instrument perhaps -10,000. It is extremely probable that the number of stars visible with -the largest telescopes at present in use would not be overstated at -100,000,000 (Plate XXVII.). - -It is evident, on the most casual glance at the sky, that in the words -of Scripture, 'One star differeth from another star in glory.' There -are stars of every degree of brilliancy, from the sparkling white -lustre of Sirius or Vega, down to the dim glimmer of those stars -which are just on the edge of visibility, and are blotted out by the -faintest wisp of haze. Accordingly, the stars have been divided into -'magnitudes' in terms of scales which, though arbitrary, are yet found -to be of general convenience. Stars of the first six magnitudes come -under the title of 'lucid' stars; below the sixth we come to the -telescopic stars, none of which are visible to the naked eye, and -which range down to the very last degree of faintness. Of stars of the -first magnitude there are recognised about twenty, more or less. -By far the brightest star visible to us in the Northern Hemisphere, -though it is really below the Equator, is Sirius, whose brightness -exceeds by no fewer than fourteen and a half times that of Regulus, -the twentieth star on the list. The next brightest stars, Canopus and -Alpha Centauri, are also Southern stars, and are not visible to us -in middle latitudes. The three brightest of our truly Northern stars, -Vega, Capella, and Arcturus, come immediately after Alpha Centauri, -and opinions are much divided as to their relative brightness, their -diversity in colour and in situation rendering a comparison somewhat -difficult. The other conspicuous stars of the first magnitude visible -in our latitudes are, in order of brightness, Rigel, Procyon, Altair, -Betelgeux, Aldebaran, Pollux, Spica Virginis, Antares, Fomalhaut, -Arided (Alpha Cygni), and Regulus, the well-known double star Castor -following not far behind Regulus. The second magnitude embraces, -according to Argelander, 65 stars; the third, 190; fourth, 425; fifth, -1,100; sixth, 3,200; while for the ninth magnitude the number leaps -up to 142,000. It is thus seen that the number of stars increases with -enormous rapidity as the smaller magnitudes come into question, and, -according to Newcomb, there is no evidence of any falling off in -the ratio of increase up to the tenth magnitude. In the smaller -magnitudes, however, the ratio of increase does not maintain itself. -The number of the stars, though very great, is not infinite. - -A further fact which quickly becomes apparent to the naked eye is that -the stars are not all of the same colour. Sirius, for example, is of a -brilliant white, with a steely glitter; Betelgeux, comparatively near -to it in the sky, is of a beautiful topaz tint, perhaps on the whole -the most exquisite single star in the sky, so far as regards colour; -Aldebaran is orange-yellow, while Vega is white with a bluish cast, -as is also Rigel. These diversities become much more apparent when -the telescope is employed. At the same time the observer may be warned -against expecting too much in the way of colour, for, as a matter of -fact, the colours of the stars, while perfectly manifest, are yet of -great delicacy, and it is difficult to describe them in ordinary terms -without some suspicion of exaggeration. Stars of a reddish tone, which -ranges from the merest shade of orange-yellow up to a fairly deep -orange, are not uncommon; several first-magnitude stars, as already -noted, have distinct orange tones. For anything approaching to real -blues and greens, we must go to the smaller stars, and the finest -examples of blue or green stars are found in the smaller members of -some of the double systems. Thus in the case of the double Beta Cygni -(Albireo), one of the most beautiful and easy telescopic objects in -the northern sky, the larger star is orange-yellow, and the smaller -blue; in that of Gamma Andromedæ the larger is yellow, and the smaller -bluish-green; while Gamma Leonis has a large yellow star, and a small -greenish-yellow one in connection. The student who desires to pursue -the subject of star colours should possess himself of the catalogue -published in the Memoirs of the British Astronomical Association, -which gives the colours of the lucid stars determined from the mean of -a very large number of observations made by different observers. - -In this connection it may be noticed that there is some suspicion that -the colours of certain stars have changed within historic times, or -at least that they have not the same colour now which they are said -to have had in former days. The evidence is not in any instance strong -enough to warrant the assertion that actual change has taken place; -but it is perfectly natural to suppose that it does, and indeed must -gradually progress. As the stars are intensely hot bodies, there must -have been periods when their heat was gradually rising to its maximum, -and there must be periods when they will gradually cool off to -extinction, and these stages must be represented by changes in the -colour of the particular star in question. In all probability, then, -the colour of a star gives some indication of the stage to which it -has advanced in its life-history; and as a matter of fact, this proves -to be so, the colour of a star being found to be generally a fair -indication of what its constitution, as revealed by the spectroscope, -will be. - -Another feature of the stars which cannot fail to be noticed is the -fact that they are not evenly distributed over the heavens, but are -grouped into a variety of configurations or constellations. In the -very dawn of human history these configurations woke the imaginations -of the earliest star-gazers, and fanciful shapes and titles were -attached to the star-groups, which have been handed down to the -present time, and are still in use. It must be confessed that in -some cases it takes a very lively imagination to find any resemblance -between the constellation and the figure which has been associated -with it. The anatomy of Pegasus, for example, would scarcely commend -itself to a horse-breeder, while the student will look in vain for any -resemblance to a human figure, heroic or unheroic, in the straggling -group of stars which bears the name of Hercules. At the same time a -few of the constellations do more or less resemble the objects from -which their titles are derived. Thus the figure of a man may without -any great difficulty be traced among the brilliant stars which form -the beautiful constellation Orion; while Delphinus presents at least -an approximation to a fish-like form, and Corona Borealis gives the -half of a diadem of sparkling jewels. - -A knowledge of the constellations, and, if possible, of the curious -old myths and legends attaching to them, should form part of the -equipment of every educated person; yet very few people can tell one -group from another, much less say what constellations are visible at -a given hour at any particular season of the year. People who are -content merely to gape at the heavens in 'a wonderful clear night -of stars' little know how much interest they are losing. When the -constellations and the chief stars are learned and kept in memory, the -face of the sky becomes instinct with interest, and each successive -season brings with it the return of some familiar group which is -hailed as one hails an old friend. Nor is the task of becoming -familiar with the constellations one of any difficulty. Indeed, there -are few pleasanter tasks than to trace out the figures of the old -heroes and heroines of mythology by the help of a simple star-map, -and once learned, they need never be forgotten. In this branch of the -subject there are many easily accessible helps. For a simple guide, -Peck's 'Constellations and how to Find Them' is both cheap and useful, -while Newcomb's 'Astronomy for Everybody' and Maunder's 'Astronomy -without a Telescope' also give careful and simple directions. -Maunder's volume is particularly useful for a beginner, combining, -as it does, most careful instructions as to the tracing of the -constellations with a set of clear and simple star-charts, and a most -interesting discussion of the origin of these ancient star-groups. -A list of the northern constellations with a few of the most notable -objects of interest in each will be found in Appendix II. - -Winding among the constellations, and forming a gigantic belt round -the whole star-sphere, lies that most wonderful feature of the heavens -familiar to all under the name of the Milky Way. This great luminous -girdle of the sky may be seen in some portion of its extent, and at -some hour of the night, at all seasons of the year, though in May it -is somewhat inconveniently placed for observation. Roughly speaking, -it presents the appearance of a broad arch or pathway of misty light, -'whose groundwork is of stars'; but the slightest attention will -reveal the fact that in reality its structure is of great complexity. -It throws out streamers on either side and at all angles, condenses at -various points into cloudy masses of much greater brilliancy than the -average, strangely pierced sometimes by dark gaps through which we -seem to look into infinite and almost tenantless space (Plate XXVII.), -while in other quarters it spreads away in considerable width, and -to such a degree of faintness that the eye can scarcely tell where it -ends. At a point in the constellation Cygnus, well seen during autumn -and the early months of winter, it splits up into two great branches -which run separate to the Southern horizon with a well-marked dark gap -dividing them. - -When examined with any telescopic power, the Milky Way reveals itself -as a wonderful collection of stars and star-clusters; and it will also -be found that there is a very remarkable tendency among the stars to -gather in the neighbourhood of this great starry belt. So much is this -case that, in the words of Professor Newcomb, 'Were the cloud-forms -which make up the Milky Way invisible to us, we should still be able -to mark out its course by the crowding of the lucid stars towards it.' -Not less remarkable is the fact that the distribution of the nebulæ -with regard to the Galaxy is precisely the opposite of that of the -stars. There are, of course, many nebulæ in the Galaxy; but, at the -same time, they are comparatively less numerous along its course, and -grow more and more numerous in proportion as we depart from it. It -seems impossible to avoid the conclusion that these twin facts are -intimately related to one another, though the explanation of them is -not yet forthcoming. - -In the year 1665 the famous astronomer Hooke wrote concerning the -small star Gamma Arietis: 'I took notice that it consisted of two -small stars very near together; a like instance of which I have not -else met with in all the heavens.' This is the first English record -of the observation of a double star, though Riccioli detected the -duplicity of Zeta Ursæ Majoris (Mizar), in 1650, and Huygens saw three -stars in Theta Orionis in 1656. These were the earliest beginnings -of double-star observation, which has since grown to such proportions -that double stars are now numbered in the heavens by thousands. Of -course, certain stars appear to be double even when viewed with the -unaided eye. Thus Mizar, a bright star in the handle of the Plough, -referred to above, has not far from it a fainter companion known as -Alcor, which the Arabs used to consider a test of vision. Either it -has brightened in modern times, or else the Arabs have received too -much credit for keenness of sight, for Mizar and Alcor now make a -pair that is quite easy to very ordinary sight even in our turbid -atmosphere. Alpha Capricorni, and Zeta Ceti, with Iota Orionis are -also instances of naked-eye doubles, while exceptionally keen sight -will detect that the star Epsilon Lyræ, which forms a little triangle -with the brilliant Vega and Zeta Lyræ, is double, or at least that it -is not single, but slightly elongated in form. Astronomers, however, -would not call such objects as these 'double stars' at all; they -reserve that title for stars which are very much closer together than -the components of a naked-eye double can ever be. The last-mentioned -star, Epsilon Lyræ, affords a very good example of the distinction. To -the naked eye it is, generally speaking, not to be distinguished -from a single star. Keen sight elongates it; exceptionally keen sight -divides it into two stars extremely close to one another. But on using -even a very moderate telescope, say a 2-1/2-inch with a power of -100 or upwards, the two stars which the keenest sight could barely -separate are seen widely apart in the field, while each of them has -in its turn split up into two little dots of light. Thus, to the -telescope, Epsilon Lyræ is really a quadruple star, while in addition -there is a faint star forming a triangle with the two pairs, and a -large instrument will reveal two very faint stars, the 'debilissima,' -one on either side of the line joining the larger stars. These I have -seen with 3-7/8-inch. - -What the telescope does with Epsilon Lyræ, it does with a great -multitude of other stars. There are thousands of doubles of all -degrees of easiness and difficulty--doubles wide apart, and doubles so -close that only the finest telescopes in the world can separate -them; doubles of every degree of likeness or of disparity in their -components, from Alpha Geminorum (Castor), with its two beautiful -stars of almost equal lustre, to Sirius, where the chief star is the -brightest in all the heavens, and the companion so small, or rather so -faint, that it takes a very fine glass to pick it out in the glare -of its great primary. The student will find in these double stars an -extremely good series of tests for the quality of his telescope. -They are, further, generally objects of great beauty, being often -characterized, as already mentioned, by diversity of colour in the -two components. Thus, in addition to the examples given above, -Eta Cassiopeiæ presents the beautiful picture of a yellow star in -conjunction with a red one, while Epsilon Boötis has been described as -'most beautiful yellow and superb blue,' and Alpha Herculis consists -of an orange star close to one which is emerald green. It has been -suggested that the colours in such instances are merely complementary, -the impression of orange or yellow in the one star producing a purely -subjective impression of blue or green when the other is viewed; but -it has been conclusively proved that the colours of very many of the -smaller stars in such cases are actual and inherent. - -Not only are there thousands of double stars in the heavens, but there -are also many multiple stars, where the telescope splits an apparently -single star up into three, four, or sometimes six or seven separate -stars. Of these multiples, one of the best known is Theta Orionis. It -is the middle star of the sword which hangs from the belt of Orion, -and is, of course, notable from its connection with the Great Nebula; -but it is also a very beautiful multiple star. A 2-1/2-inch telescope -will show that it consists of four stars in the form of a trapezium; -large instruments show two excessively faint stars in addition. Again, -in the same constellation lies Sigma Orionis, immediately below the -lowermost star of the giant's belt. In a 3-inch telescope this -star splits up into a beautiful multiple of six components, their -differences in size and tint making the little group a charming -object. - -Looking at the multitude of double and multiple stars, the question -can scarcely fail to suggest itself: Is there any real connection -between the stars which thus appear so close to one another? It can -be readily understood that the mere fact of their appearing close -together in the field of the telescope does not necessarily imply -real closeness. Two gas-lamps, for instance, may appear quite close -together to an observer who is at some distance from them, when in -reality they may be widely separated one from the other--the apparent -closeness being due to the fact that they are almost in the same -line of sight. No doubt many of the stars which appear double in the -telescope are of this class--'optical doubles,' as they are called, -and are in reality separated by vast distances from one another. -But the great majority have not only an apparent, but also a real -closeness; and in a number of cases this is proved by the fact that -observation shows the stars in question to be physically connected, -and to revolve around a common centre of gravity. Double stars which -are thus physically connected are known as 'binaries.' The discovery -of the existence of this real connection between some double stars is -due, like so many of the most interesting astronomical discoveries, -to Sir William Herschel. At present the number of stars known to be -binary is somewhat under one thousand; but in the case of most -of these, the revolution round a common centre which proves their -physical connection is extremely slow, and consequently the majority -of binary stars have as yet been followed only through a small portion -of their orbits, and the change of position sufficient to enable -a satisfactory orbit to be computed has occurred in only a small -proportion of the total number. The first binary star to have its -orbit computed was Xi Ursæ Majoris, whose revolution of about sixty -years has been twice completed since, in 1780, Sir William Herschel -discovered it to be double. - -The star which has the shortest period at present known is the fourth -magnitude Delta Equulei, which has a fifth magnitude companion. The -pair complete their revolution, according to Hussey, in 5·7 years. -Kappa Pegasi comes next in speed of revolution, with a period of -eleven and a half years, while the star 85 of the same constellation -takes rather more than twice as long to complete its orbit. From such -swiftly circling pairs as these, the periods range up to hundreds of -years. Thus, for example, the well-known double star Castor, probably -the most beautiful double in the northern heavens, and certainly the -best object of its class for a small telescope, is held to have a -period of 347 years, which, though long enough, is a considerable -reduction upon the 1,000 once attributed to it. - -But the number of binary stars known is not confined to those which -have been discovered and measured by means of the telescope and -micrometer. One of the most wonderful results of modern astronomical -research has been the discovery of the fact that many stars have -revolving round them invisible companions, which are either dark -bodies, or else are so close to their primaries as for ever to defy -the separating powers of our telescopes. The discovery of these -dark, or at least invisible, companions is one of the most remarkable -triumphs of the spectroscope. It was in 1888 that Vogel first applied -the spectroscopic method to the well-known variable star, Beta -Persei--known as Algol, 'the Demon,' from its 'slowly-winking eye.' -The variation in the light of Algol is very large, from second to -fourth magnitude; Vogel therefore reasoned that if this variation were -caused by a dark companion partially eclipsing the bright star, the -companion must be sufficiently large to cause motion in Algol--that -is, to cause both stars to revolve round a common centre of gravity. -Should this be the case, then at one point of its orbit Algol must be -approaching, and at the opposite point receding from the earth; and -therefore the shift of the lines of its spectrum towards the violet -in the one instance and towards the red in the other would settle -the question of whether it had or had not an invisible companion. -The spectroscopic evidence proved quite conclusive. It was found that -before its eclipses, Algol was receding from the sun at the rate -of 26-1/3 miles per second, while after eclipse there was a similar -motion of approach; and therefore the hypothesis of an invisible -companion was proved to be fact. Vogel carried his researches further, -his inquiry into the questions of the size and distance apart of -the two bodies leading him to the conclusion that the bright star is -rather more, and its companion rather less than 1,000,000 miles in -diameter; while the distance which divides them is somewhat more than -3,000,000 miles. Though larger, both bodies prove to be less massive -than our sun, Algol being estimated at four-ninths and its companion -at two-ninths of the solar mass. - -The class of double star disclosed in this manner is known as the -'spectroscopic binary,' and has various other types differing from -the Algol type. Thus the type of which Xi Ursæ Majoris was the first -detected instance has two component bodies not differing greatly in -brightness from one another. In such a case the fact of the star being -binary is revealed through the consideration that in any binary system -the two components must necessarily always be moving in opposite -directions. Hence the shift of the lines of their spectrum will be -in opposite directions also, and when one of the stars (A) is moving -towards us, and the other (B) away from us, all the lines of the -spectrum which are common to the two will appear double, those of A -being displaced towards the violet and those of B towards the red. -After a quarter of a revolution, when the stars are momentarily in -a straight line with us, the lines will all appear single; but after -half a revolution they will again be displaced, those of A this time -towards the red and those of B towards the violet. - -There has thus been opened up an entirely new field of research, and -the idea, long cherished, that the stars might prove to have dark, or, -at all events, invisible, companions attendant on them, somewhat as -our own sun has its planets, has been proved to be perfectly sound. -So far, in the case of dark companions, only bodies of such vast size -have been detected as to render any comparison with the planets of -our system difficult; but the principle is established, and the -probability of great numbers of the stars having real planetary -systems attendant on them is so great as to become practically a -certainty. 'We naturally infer,' says Professor Newcomb, 'that ... -innumerable stars may have satellites, planets, or companion stars so -close or so faint as to elude our powers of observation.' - -From the consideration of spectroscopic binaries we naturally turn -to that of variable stars, the two classes being, to some extent at -least, coincident, as is evidenced by the case of Algol. While the -discovery of spectroscopic binaries is one of the latest results of -research, that of variability among stars dates from comparatively -far back in the history of astronomy. As early as the year 1596 David -Fabricius noted the star now known as Omicron Ceti, or Mira, 'the -Wonderful,' as being of the third magnitude, while in the following -year he found that it had vanished. A succession of appearances and -disappearances was witnessed in the middle of the next century by -Holwarda, and from that time the star has been kept under careful -observation, and its variations have been determined with some -exactness, though there are anomalies as yet unexplained. 'Once in -eleven months,' writes Miss Clerke, 'the star mounts up in about 125 -days from below the ninth to near the third, or even to the second -magnitude; then, after a pause of two or three weeks, drops again -to its former low level in once and a half times, on an average, the -duration of its rise.' This most extraordinary fluctuation means that -at a bright maximum Mira emits 1,500 times as much light as at a low -minimum. The star thus subject to such remarkable outbursts is, like -most variables, of a reddish colour, and at maximum its spectrum shows -the presence of glowing hydrogen. Its average period is about 331 -days; but this period is subject to various irregularities, and -the maximum has sometimes been as much as two months away from the -predicted time. Mira Ceti may be taken as the type of the numerous -class of stars known as 'long-period variables.' - -Not less interesting are those stars whose variations cover only short -periods, extending from less than thirty days down to a few hours. Of -these, perhaps the most easily observed, as it is also one of the most -remarkable, is Beta Lyræ. This star is one of the two bright stars of -nearly equal magnitude which form an obtuse-angled triangle with the -brilliant first-magnitude star Vega. The other star of the pair -is Gamma Lyræ, and between them lies the famous Ring Nebula, to be -referred to later. Ordinarily Beta Lyræ is of magnitude 3·4, but from -this it passes, in a period of rather less than thirteen days, through -two minima, in one of which it descends to magnitude 3·9 and in -the other to 4·5. This fluctuation seems trifling. It really means, -however, that at maximum the star is two and three-quarter times -brighter than when it sinks to magnitude 4·5; and the variation can be -easily recognised by the naked eye, owing to the fact of the nearness -of so convenient a comparison star as Gamma Lyræ. Beta Lyræ is a -member of the class of spectroscopic binaries, and belongs to that -type of the class in which the mutually eclipsing bodies are both -bright. In such cases the variation in brilliancy is caused by the -fact that when the two bodies are, so to speak, side by side, light -is received from both of them, and a maximum is observed; while, when -they are end on, both in line with ourselves, one cuts off more or -less of the other's light from us, thus causing a minimum. - -A third class, distinct from either of the preceding, is that of the -Algol Variables, so-called from the bright star Beta Persei, which has -already been mentioned as a spectroscopic binary. Than this star -there is no more notable variable in the heavens, and its situation -fortunately renders it peculiarly easy of observation to northern -students. Algol shines for about fifty-nine hours as a star of small -second magnitude, then suddenly begins to lose light, and in four and -a half hours has fallen to magnitude three and a half, losing in so -short a space two-thirds of its normal brilliancy. It remains in -this degraded condition for only fifteen minutes, and then begins to -recover, reaching its normal lustre in about five hours more. These -remarkable changes, due, as before mentioned, to the presence of -an invisible eclipsing companion, are gone through with the utmost -regularity, so much so that, as Gore says, the minima of Algol 'can -be predicted with as much certainty as an eclipse of the sun.' The -features of the type-star are more or less closely reproduced in the -other Algol Variables--a comparatively long period of steady light -emission, followed by a rapid fall to one or more minima, and a rapid -recovery of light. The class as yet is a small one, but new members -are gradually being added to it, the majority of them white, like the -type-star. - -The study of variable stars is one which should seem to be specially -reserved for the amateur observer. In general, it requires but little -instrumental equipment. Many variables can be seen at maximum, some -even at minimum, with the unaided eye; in other cases a good opera or -field glass is all that is required, and a 2-1/2 or 3-inch telescope -will enable the observer to command quite an extensive field of work. -Here, again, the beginner may be referred to the _Memoirs_ of the -British Astronomical Association for help and guidance, and may be -advised to connect himself with the Variable Star Section. - -With the exception of such variations in the lustre of certain stars -as have been described, the aspect of the heavens is, in general, -fixed and unchanging. There are, as we shall see, real changes of the -vastest importance continually going on; but the distances separating -us from the fixed stars are so enormous that these changes shrink into -nothingness, and the astronomers of forty centuries before our era -would find comparatively little change today in the aspect of the -constellations with which they were familiar. But occasionally a -very remarkable change does take place, in the apparition of a new or -temporary star. The accounts of the appearance of such objects are not -very numerous, but are of great interest. We pass over those recorded, -in more or less casual fashion, by the ancients, for the reason -that the descriptions given are in general more picturesque than -illuminative. It does not add much to one's knowledge, though it may -excite wonder, to find the Chinese annals recording the appearance, in -A.D. 173, of a new star 'resembling a large bamboo mat!' - -The first Nova, of which we have a really scientific record, was the -star which suddenly blazed out, in November, 1572, in the familiar -W of Cassiopeia. It was carefully observed by the great astronomer, -Tycho Brahé, and, according to him, was brighter than Sirius, Alpha -Lyræ, or Jupiter. Tycho followed it till March, 1574, by which time -it had sunk to the limit of unaided vision, and further observation -became impossible. There is at present a star of the eleventh -magnitude close to the place fixed for the Nova from Tycho's -observations. In 1604 and 1670, new stars were observed, the first by -Kepler and his assistants, the second by the monk Anthelme; but from -1670 there was a long break in the list of discoveries, which was -ended by Hind's observation of a new star in Ophiuchus (April, 1848). -This was never a very conspicuous object, rising only to somewhat less -than fourth magnitude, and soon fading to tenth or eleventh. We -can only mention the 'Blaze Star' of Corona Borealis, discovered by -Birmingham in 1866, the Nova discovered in 1876 by Schmidt of Athens, -near Rho Cygni--an object which seems to have faded out into a -planetary nebula, a fate apparently characteristic of this class of -star--and the star which appeared in 1885, close to the nucleus of the -Great Nebula in Andromeda. - -In 1892, Dr. Anderson of Edinburgh discovered in the constellation -Auriga a star which he estimated as of fifth magnitude. The discovery -was made on January 31, and the new star was found to have been -photographed at Harvard on plates taken from December 16, 1891, to -January 31, 1892. Apparently this Nova differed from other temporary -stars in the fact that it attained its full brightness only gradually. -By February 3 it rose to magnitude 3·5, then faded by April 1 to -fifteenth, but in August brightened up again to about ninth -magnitude. It is now visible as a small star. The great development -of spectroscopic resources brought this object, otherwise not a very -conspicuous one, under the closest scrutiny. Its spectrum showed many -bright lines, which were accompanied by dark ones on the side next the -blue. The idea was thus suggested that the outburst of brilliancy was -due to a collision between two bodies, one of which, that causing the -dark lines, was approaching the earth, while the other was receding -from it. Lockyer considered the conflagration to be due to a collision -between two swarms of meteorites, Huggins that it was caused by the -near approach to one another of two gaseous bodies, while others -suggested that the rush of a star or of a swarm of meteorites through -a nebula would explain the facts observed. Subsequent observations of -the spectrum of Nova Aurigæ have revealed the fact that it has obeyed -the destiny which seems to wait on temporary stars, having become a -planetary nebula. - -Dr. Anderson followed up his first achievement by the discovery of a -brilliant Nova in the constellation Perseus. The discovery was made -on the night of February 21-22, 1901, the star being then of magnitude -2·7. Within two days it became about the third brightest star in the -sky, being a little more brilliant than Capella; but before the middle -of April it had sunk to fifth magnitude. The rapidity of its rise must -have been phenomenal! A plate exposed at Harvard on February 19, and -showing stars to the eleventh magnitude, bore no trace of the Nova. -'It must therefore,' says Newcomb, 'have risen from some magnitude -below the eleventh to the first within about three days. This -difference corresponds to an increase of the light ten thousandfold!' -Such a statement leaves the mind simply appalled before the spectacle -of a cataclysm so infinitely transcending the very wildest dreams of -fancy. Subsequent observations have shown the usual tendency towards -development into a nebula, and in August, 1901, photographs were -actually obtained of a nebulosity round the star, showing remarkable -condensations. These photographs, taken at Yerkes Observatory, when -compared with others taken at Mount Hamilton in November, revealed the -startling fact that the condensations of the nebula were apparently -in extraordinarily rapid motion. Now the Nova shows no appreciable -parallax, or in other words is so distant that its distance cannot be -measured; on what scale, therefore, must these motions have been to be -recorded plainly across a gulf measurable perhaps in hundreds of light -years! - -Nova Geminorum, discovered by Professor Turner, at Oxford, in March, -1903, had not the striking features which lent so much interest to -Nova Persei. It showed a crimson colour, and its spectrum indicated -the presence in its blaze of hydrogen and helium; but it faded so -rapidly as to show that the disturbance affected a comparatively small -body, and it has exhibited the familiar new star change into a nebula. - -One point with regard to the Novæ in Auriga and Perseus deserves -notice. These discoveries, so remarkable in themselves, and so -fruitful in the extension of our knowledge, were made by an amateur -observer with no greater equipment than a small pocket telescope and a -Klein's Star-Atlas. The thorough knowledge of the face of the heavens -which enabled Dr. Anderson to pick out the faint glimmer of Nova -Aurigæ and to be certain that the star was a new one is not in the -least unattainable by anyone who cares to give time and patience to -its acquisition; and even should the study never be rewarded by a -capture so dramatic as that of Nova Persei, the familiarity gained -in its course with the beauty and wonder of the star-sphere will in -itself be an ample reward. - - - - -CHAPTER XV - -CLUSTERS AND NEBULÆ - - -Even the most casual observer of the heavens cannot have failed to -notice that in certain instances the stars are grouped so closely -together as to form well-marked clusters. The most familiar example -is the well-known group of the Pleiades, in the constellation Taurus, -while quite close is the more scattered group of the Hyades. Another -somewhat coarsely scattered group is that known as Coma Berenices, the -Hair of Berenice, which lies beneath the handle of the Plough; and a -fainter group is the cluster Præsepe, which lies in the inconspicuous -constellation Cancer, between Gemini and Leo, appearing to the naked -eye like a fairly bright, hazy patch, which the smallest telescope -resolves into a cloud of faint stars. - -[Illustration: - - PLATE XXVIII. - - 1. - - 2. - -Irregular Star Clusters. Photographed by E. E. Barnard. - - 1. Messier 35 in Gemini. 2. Double Cluster in Perseus. -] - -The Pleiades form undoubtedly the most remarkable naked-eye group in -the heavens. The six stars which are visible to average eyesight are -Alcyone, 3rd magnitude; Maia, Electra, and Atlas, of the 4th; Merope, -4-1/3; and Taygeta, 4-1/2. While Celæno, 5-1/3; Pleione, 5-1/2; and -Asterope, 6, hang on the verge of visibility. With an opera-glass -about thirty more may be counted, while photographs show between -2,000 and 3,000. It is probable that the fainter stars have no real -connection with the cluster itself, which is merely seen upon a -background of more distant star-dust. Modern photographs have shown -that this cluster is involved in a great nebula, which stretches in -curious wisps and straight lines from star to star, and surrounds -the whole group. The Pleiades make a brilliant object for a small -telescope with a low magnifying power, but are too scattered for an -instrument of any size to be effective upon them. The finest of all -irregular star-clusters is that known as the Sword-handle of Perseus. -Midway between Perseus and the W of Cassiopeia, and directly in the -line of the Galaxy, the eye discerns a small, hazy patch of light, -of which even a 2 or 3 inch glass will make a beautiful object, while -with a large aperture its splendour is extraordinary. It consists -of two groups of stars which are both in the same field with a small -instrument and low powers. Towards the edge of the field the stars -are comparatively sparsely scattered; but towards the two centres of -condensation the thickness of grouping steadily increases. Altogether -there is no more impressive stellar object than this magnificent -double cluster (Plate XXVIII., 2). Another very fine example of -the irregular type of grouping is seen in M. 35, situated in the -constellation Gemini, and forming an obtuse-angled triangle with the -stars Mu and Eta Geminorum (Plate XXVIII., 1). There are many other -similar groups fairly well within the reach of comparatively small -instruments, and some of these are mentioned in the list of objects -(Appendix II.). - -Still more remarkable than the irregular clusters are those which -condense into a more or less globular form. There are not very -many objects of this class in the northern sky visible with a small -telescope, but the beauty of those which are visible is very notable. -The most splendid of all is the famous cluster M. 13 Herculis. (The -M. in these cases refers to the catalogue of such objects drawn up by -Messier, the French 'comet ferret,' to guide him in his labours.) M. -13 is situated almost on the line between Zeta and Eta Herculis, -and at about two-thirds of the distance from Zeta towards Eta. It is -faintly visible to the unaided eye when its place is known, and, -when viewed with sufficient telescopic power, is a very fine object. -Nichol's remark that 'perhaps no one ever saw it for the first time -through a large telescope without uttering a shout of wonder' seems -to be based on a somewhat extravagant estimate of the enthusiasm and -demonstrativeness of the average star-gazer; but the cluster is a very -noble object all the same, consisting, according to a count made on a -negative taken in 1899, of no fewer than 5,482 stars, which condense -towards the centre into a mass of great brilliancy. It takes a large -aperture to resolve the centre of the cluster into stars, but even a -3-inch will show a number of twinkling points of light in the outlying -streamers (Plate XXIX.). In the same constellation will also be found -the cluster M. 92, similar to, but somewhat fainter than M. 13; and -other globular clusters are noted in the Appendix. Most of these -objects, however, can only be seen after a fashion with small -instruments. Of the true nature and condition of these wonderful -aggregations we are so far profoundly ignorant. The question of -whether they are composed of small stars, situated at no very great -distance from the earth, or of large bodies, which are rendered faint -to our vision by immense distance, has been frequently discussed. Gore -concludes that they are 'composed of stars of average size and mass, -and that the faintness of the component stars is simply due to their -immense distance from the earth.' If so, the true proportions of some -of these clusters must be indeed phenomenal! A very remarkable feature -to be noticed in connection with some of them is the high proportion -of variable stars which they contain. Professor Bailey has found that -in such clusters as M. 3 and M. 5 the proportion of variables is one -in seven and one in eleven respectively, while several other groups -show proportions ranging from one in eighteen up to one in sixty. -As the general proportion of variables is somewhere about one in a -hundred, these ratios are remarkable. They only characterize a certain -number of clusters, however, and are absent in cases which seem -strictly parallel to others where they exist. - -[Illustration: - - PLATE XXIX. - -Cluster M. 13 Herculis. Photographed by Mr. W. E. Wilson.] - -We now pass from the star-clusters to the nebulæ properly so called. -Till after the middle of last century it was an open question whether -there was any real distinction between the two classes of bodies. -Herschel had suggested the existence of a 'shining fluid,' distributed -through space, whose condensations gave rise to those objects known -as nebulæ; but it was freely maintained by many that the objects which -could not be resolved into stars were irresolvable only because of -their vast distance, and that the increase of telescopic power would -result in the disclosure of their stellar nature. This view seemed to -be confirmed when it was confidently announced that the great Rosse -telescope had effected the resolution of the Orion Nebula, which was -looked upon as being in some sort a test case. But the supposed proof -of the stellar character of nebulæ did not hold its ground for long, -for in 1864 Sir William Huggins, on applying the spectroscope to the -planetary nebula in Draco, found that its spectrum consisted merely -of bright lines, one of which--the most conspicuous--was close to the -position of a nitrogen line, but has proved to be distinct from it; -while of the other two, one was unmistakably the F line of hydrogen -and the other remains still unidentified. Thus it became immediately -manifest that the nebula in Draco did not consist of distant stars, -but was of gaseous constitution; and Sir William Herschel's idea of -the existence of non-stellar matter in the universe was abundantly -justified. Subsequent research has proved that multitudes of nebulæ -yield a bright-line spectrum, and are therefore gaseous. Of these, by -far the most remarkable and interesting is the Great Nebula of Orion. -The observer will readily distinguish even with the unaided eye that -the middle star of the three that form the sword which hangs down from -Orion's belt has a somewhat hazy appearance. A small telescope reveals -the fact that the haziness is due to the presence of a great misty -cloud of light, in shape something like a fish-mouth, and of a -greenish colour. At the junction of the jaws lies the multiple star -Theta Orionis, which with a 2- or 3-inch glass appears to consist of -four stars--'the trapezium'--large instruments showing in addition two -very faint stars. - -With greater telescopic power additional features begin to reveal -themselves; the mist immediately above the trapezium assumes a roughly -triangular shape, and is evidently much denser than the rest of -the nebula, presenting a curdled appearance similar to that of the -stretches of small cloud in a 'mackerel' sky; while from the upper -jaw of the fish-mouth a great shadowy horn rises and stretches upward, -until it gradually loses itself in the darkness of the background. -This wonderful nebula appears to have been discovered in 1618, but was -first really described and sketched by Huygens in 1656, since when it -has been kept under the closest scrutiny, innumerable drawings of -it having been made and compared from time to time with the view of -detecting any traces of change. The finest drawings extant are those -of Sir John Herschel and Mr. Lassell, and the elaborate one made with -the help of the Rosse 6-foot mirror. - -Drawing, however, at no time a satisfactory method of representing the -shadowy and elusive forms of nebulæ, has now been entirely superseded -by the work of the sensitive plate. Common, Roberts, Pickering, and -others have succeeded admirably in photographing the Great Nebula with -exposures ranging from half an hour up to six hours. The extension -of nebulous matter revealed by these photographs is enormous (Plate -XXX.), so much so that many of the central features of the nebula -with which the eye is familiar are quite masked and overpowered in -the photographic print. The spectrum of the Orion Nebula exhibits -indications of the presence of hydrogen and helium, as well as the -characteristic green ray which marks the unknown substance named -'nebulium.' - -The appearance of this 'tumultuous cloud, instinct with fire and -nitre,' is always amazing. Sir Robert Ball considers it one of the -three most remarkable objects visible in the northern heavens, -the other two being Saturn and the Great Cluster in Hercules. But, -beautiful and wonderful as both of these may be, the Orion Nebula -conveys to the mind a sense of mystery which the others, in spite of -their extraordinary features, never suggest. Absolutely staggering -is the thought of the stupendous dimensions of the nebula. Professor -Pickering considers its parallax to be so small as to indicate a -distance of not less than 1,000 years light journey from our earth! It -is almost impossible to realize the meaning of such a statement. When -we look at this shining mist, we are seeing it, not as it is now, but -as it was more than a hundred years before the Norman Conquest; were -it blotted out of existence now, it would still shine to us and our -descendants for another ten centuries in virtue of the rays of light -which are already speeding across the vast gulf that separates our -world from its curdled clouds of fire-mist, and the astronomers of -A.D. 2906 might still be speculating on the nature and destiny of a -thing which for ages had been non-existent! That an object should be -visible at all at such a distance demands dimensions which are really -incomprehensible; but the Orion Nebula is not only visible, it is -conspicuous! - -[Illustration: - - PLATE XXX. - -Photograph of the Orion Nebula (W. H. Pickering).] - -The rival of this famous nebula in point of visibility is the -well-known spiral in the girdle of Andromeda. On a clear night it can -easily be seen with the naked eye near the star Nu Andromedæ, and may -readily be, as it has often been, mistaken for a comet. Its discovery -must, therefore, have been practically coincident with the beginnings -of human observation of the heavens; but special mention of it does -not occur before the tenth century of our era. A small telescope will -show it fairly well, but it must be admitted that the first view is -apt to produce a feeling of disappointment. The observer need not look -for anything like the whirling streams of light which are revealed on -modern long exposure photographs (Plate XXXI., 1). He will see what -Simon Marius so aptly described under the simile of 'the light -seen from a great distance through half-transparent horn plates'--a -lens-shaped misty light, brightening very rapidly towards a nucleus -which seems always on the point of coming to definition but is -never defined, and again fading away without traceable boundary into -obscurity on every side. The first step towards an explanation of the -structure of this curious object was made by Bond in the middle of -last century. With the 15-inch refractor of the Cambridge (U.S.A.) -Observatory, he detected two dark rifts running lengthwise through the -bright matter of the nebula; but it was not till 1887 and 1888 that -its true form was revealed by Roberts's photographs. It was then seen -to be a gigantic spiral or whirlpool, the rifts noticed by Bond being -the lines of separation between the huge whorls of the spiral. Of -course, small instruments are powerless to reveal anything of this -wonderful structure; still there is an interest in being able to see, -however imperfectly, an object which seems to present to our eyes the -embodiment of that process by which some assume that our own system -may have been shaped. So far as the powers of the best telescopes go, -the Andromeda Nebula presents no appearance of stellar constitution. -Its spectrum, according to Scheiner, is continuous, which would imply -that in spite of appearances it is in reality composed of stars; but -Sir William Huggins has seen also bright lines in it. Possibly it may -represent a stage intermediate between the stellar and the gaseous. - -[Illustration: - - PLATE XXXI. - - 1. [North.] - - 2. [North.] - -Photographs of Spiral Nebulæ. By Dr. Max Wolf. - - 1. Great Nebula in Andromeda. 2. Spiral in Triangulum (M. 33). -] - -Another remarkable example of a spiral nebula will be found in M. 51. -It is situated in the constellation Canes Venatici, and may be easily -picked up, being not far from the end star of the Plough-handle Eta -Ursæ Majoris. This strange object, 'gyre on gyre' of fire-mist, was -one of the first spirals to have its true character demonstrated by -the Rosse telescope. It is visible with moderate optical powers, but -displays to them none of that marvellous structure which the great -6-foot mirror revealed for the first time, and which has been amply -confirmed by subsequent photographic evidence (Plate XXXII.). - -[Illustration: - - PLATE XXXII. - -Photograph of Whirlpool Nebula (M. 51). Taken by Mr. W. E. Wilson, -March 6, 1897.] - -Among other classes of nebulæ we can only mention the ring and the -planetary. Of each of these, one good example can be seen, though, -it must be admitted, not much more than seen, with very modest -instrumental equipment. Midway between the two stars Beta and Gamma -Lyræ, already referred to in connection with the variability of the -former, the observer by a little fishing will find the famous Ring -Nebula of Lyra. With low powers it appears simply as a hazy oval spot; -but it bears magnifying moderately well, and its annular shape comes -out fairly with a power of eighty on a 2-1/2 inch, though it can -scarcely be called a brilliant object with that aperture, or indeed -with anything much under 8 inches. None the less, it is of great -interest, the curious symmetry of this gaseous ring making it an -almost unique object. It resembles nothing so much as those vortex -rings which an expert smoker will sometimes send quivering through the -air. Photographs show clearly a star within the ring, and this -star has a very curious history, having been frequently visible in -comparatively small telescopes, and again, within a year or two, -invisible in much larger ones. Photography seems to have succeeded -in persuading it to forgo these caprices, though it presents -peculiarities of light which are still unexplained. The actinic plate -reveals also very clearly that deficiency of light at the ends of the -longer diameter of the ring which can be detected, though with more -difficulty, by the eye. The class of annular nebulæ is not a large -one, and none of its other members come within the effective range of -small instruments. - - -Planetary nebulæ are so called because with ordinary powers they -present somewhat of the appearance of a planet seen very dimly and -considerably out of focus. The appearance of uniformity in their -boundaries vanishes under higher telescopic power, and they appear to -be generally decidedly elliptical; they yield a gaseous spectrum with -strong evidence of the presence of 'nebulium,' the unknown substance -which gives evidence of its presence in the spectrum of every true -nebula, and has, so far (with one doubtful exception) been found -nowhere else. The chief example of the class is that body in Draco -which first yielded to Huggins the secret of the gaseous nature of the -nebulæ. It lies nearly half-way between Polaris and Gamma Draconis, -and is described by Webb as a 'very luminous disc, much like a -considerable star out of focus.' It is by no means a striking object, -but has its own interest as the first witness to the true nature of -that great class of heavenly bodies to which it belongs. - -The multitude of nebulous bodies scattered over the heavens may be -judged from the fact that Professor Keeler, after partial surveys -carried out by means of photography with the Crossley reflector, came -to the conclusion that the number within the reach of that instrument -(36-inch aperture) might be put down at not less than 120,000. It is -a curious fact that the grouping of this great multitude seems to -be fundamentally different from that of the stars. Where stars are -densely scattered, nebulæ are comparatively scarce; where nebulæ -abound, the stars are less thickly sown. So much is this the case, -that, when Herschel in his historic 'sweeps' of the heavens came -across a notably starless region, he used to call out to his assistant -to 'prepare for nebulæ.' The idea of a physical connection between the -two classes of bodies is thus underlined in a manner which, as Herbert -Spencer saw so early as 1854, is quite unmistakable. - -There remain one or two questions of which the very shortest notice -must suffice--not because they are unimportant, but because their -importance is such that any attempt at adequate discussion of them is -impossible in our limited space. One of these inevitably rises to the -mind in presence of the myriads of the heavenly host--the familiar -question which was so pleasingly suggested to our growing minds by the -nursery rhyme of our childhood. To the question, What is a star? it -has now become possible to give an answer which is satisfactory so -far as it goes, though it is in a very rudimentary stage as regards -details. - -The spectroscope has taught us that the stars consist of incandescent -solid bodies, or of masses of incandescent gas so large and dense -as not to be transparent; and further, that they are surrounded by -atmospheres consisting of gases cooler than themselves. The nature of -the substances incandescent in the individual bodies has also to some -extent been learned. The result has been to show that, while there is -considerable variety in the chemical constitution and condition of the -stars, at least five different types being recognised, each capable -of more minute subdivision, the stars are, in the main, composed -of elements similar to those existing in the sun; and, in Professor -Newcomb's words, 'as the sun contains most of the elements found on -the earth and few or no others, we may say that earth and stars seem -to be all made out of like matter.' It is, of course, impossible to -say what unknown elements may exist in the stars; but at least it -is certain that many substances quite familiar to us, such as iron, -magnesium, calcium, hydrogen, oxygen, and carbon, are present in -their constitution. Indeed, our own sun, in spite of its overwhelming -importance to ourselves is to be regarded, relatively to the stellar -multitudes, as merely one star among many; nor, so far as can be -judged, can it be considered by any means a star of the first class. -There can be no doubt that, if removed to the average distance of -first magnitude stars--thirty-three years light journey--our sun would -be merely a common-place member of the heavenly host, far outshone by -many of its fellow-suns. In all probability it would shine as about a -fifth magnitude star, with suspicions of variability in its light. - -There remains to be noted the fact that the sun is not to be regarded -as a fixed centre, its fixity being only relative to the members -of its own system. With all its planets and comets it is sweeping -continually through space with a velocity of more than 1,000,000 miles -in the twenty-four hours. This remarkable fact was first suspected -by Sir William Herschel, who also, with that insight which was -characteristic of his wonderful genius, saw, and was able roughly -to apply, the method which would either confirm or disprove the -suspicion. - -The principle which lies at the bottom of the determination is in -itself simple enough, though its application is complicated in such a -manner as to render the investigation a very difficult one. A wayfarer -passing up the centre of a street lighted on both sides by lamps will -see that the lamps in front of him appear to open out and separate -from one another as he advances, while those that he is leaving behind -him have an opposite motion, appearing to close in upon one another. -Now, with regard to the solar motion, if the case were absolutely -simple, the same effect would be produced upon the stars among which -we are moving; that is to say, were the stars absolutely fixed, and -our system alone in motion among them, there would appear to be a -general thinning out or retreating of the stars from the point towards -which the sun is moving, and a corresponding crowding together of them -towards the point, directly opposite in the heavens, from which it is -receding. In actual fact the case is not by any means so simple, for -the stars are not fixed; they have motions of their own, some of -them enormously greater than the motion of the sun. Thus the apparent -motion caused by the advance of our system is masked to a great -extent by the real motion of the stars. It is plain, however, that the -perspective effect of the sun's motion must really be contained in the -total motion of each star, or, in other words, that each star, along -with its own real motion, must have an apparent motion which is common -to all, and results from our movement through space. If this common -element can be disentangled from the individual element, the proper -motion of each star, then the materials for the solution of the -problem will be secured. It has been found possible to effect this -disentanglement, and the results of all those who have attempted the -problem are, all things considered, in remarkably close agreement. - -Herschel's application of his principle led him to the conclusion -that there was a tendency among the stars to widen out from the -constellation Hercules, and to crowd together towards the opposite -constellation of Argo Navis in the southern hemisphere, and the point -which he fixed upon as the apex of the sun's path was near the star -Lambda Herculis. Subsequent discussions of the problem have confirmed, -to a great extent, his rough estimate, which was derived from a -comparatively small number of stars. So far as general direction was -concerned, he was entirely right; the conclusion which he reached as -to the exact point towards which the motion is directed has, however, -been slightly modified by the discussion of a much larger number of -stars, and it is now considered that the apex of the solar journey 'is -in the general direction of the constellation Lyra, and perhaps near -the star Vega, the brightest of that constellation' (Newcomb, 'The -Stars,' p. 91). There are but few stars more beautiful and interesting -than Vega; to its own intrinsic interest must now be added that -arising from the fact that each successive night we look upon it we -have swept more than 1,000,000 miles nearer to its brilliant globe, -and that with every year we have lessened, by some 400,000,000 miles, -the distance that divides us from it. There can surely be no thought -more amazing than this! It seems to gather up and bring to a focus -all the other impressions of the vastness of celestial distances and -periods. So swift and ceaseless a motion, and yet the gulfs that sever -us from our neighbours in space are so huge that a millennium of such -inconceivable travelling makes no perceptible change upon the face of -the heavens! There rise other thoughts to the mind. Towards what goal -may our world and its companions be voyaging under the sway of the -mighty ruler of the system, and at the irresistible summons of those -far-off orbs which distance reduces to the mere twinkling points of -light that in man's earliest childlike thought were but lamps hung -out by the Creator to brighten the midnight sky for his favourite -children? What strange chances may be awaiting sun and planet alike in -those depths of space towards which we are rushing with such frightful -speed? Such questions remain unanswered and unanswerable. We are as -ignorant of the end of our journey, and of the haps that may attend -it, as we are helpless in the grasp of the forces that compel and -control it. - - - - -APPENDIX I - - -The following is a list of the Lunar Formations numbered as on the -Key-map, Plate XIX.: - - 1. Newton. | 38. Heinsius. | 75. Playfair. - 2. Short. | 39. Hainzel. | 76. Azophi. - 3. Simpelius. | 40. Bouvard. | 77. Sacrobosco. - 4. Manzinus. | 41. Piazzi. | 78. Fracastorius. - 5. Moretus. | 42. Ramsden. | 79. Santbech. - 6. Gruemberger. | 43. Capuanus. | 80. Petavius. - 7. Casatus. | 44. Cichus. | 81. Wilhelm Humboldt. - 8. Klaproth. | 45. Wurzelbauer. | 82. Polybius. - 9. Wilson. | 46. Gauricus. | 83. Geber. - 10. Kircher. | 47. Hell. | 84. Arzachel. - 11. Bettinus. | 48. Walter. | 85. Thebit. - 12. Blancanus. | 49. Nonius. | 86. Bullialdus. - 13. Clavius. | 50. Riccius. | 87. Hippalus. - 14. Scheiner. | 51. Rheita. | 88. Cavendish. - 15. Zuchius. | 52. Furnerius. | 89. Mersenius. - 16. Segner. | 53. Stevinus. | 90. Gassendi. - 17. Bacon. | 54. Hase. | 91. Lubiniezky. - 18. Nearchus. | 55. Snellius. | 92. Alpetragius. - 19. Vlacq. | 56. Borda. | 93. Airy. - 20. Hommel. | 57. Neander. | 94. Almanon. - 21. Licetus. | 58. Piccolomini. | 95. Catherina. - 22. Maginus. | 59. Pontanus. | 96. Cyrillus. - 23. Longomontanus. | 60. Poisson. | 97. Theophilus. - 24. Schiller. | 61. Aliacensis. | 98. Colombo. - 25. Phocylides. | 62. Werner. | 99. Vendelinus. - 26. Wargentin. | 63. Pitatus. | 100. Langrenus. - 27. Inghirami. | 64. Hesiodus. | 101. Goclenius. - 28. Schickard. | 65. Mercator. | 102. Guttemberg. - 29. Wilhelm I. | 66. Vitello. | 103. Isidorus. - 30. Tycho. | 67. Fourier. | 104. Capella. - 31. Saussure. | 68. Lagrange. | 105. Kant. - 32. Stöfler. | 69. Vieta. | 106. Descartes. - 33. Maurolycus. | 70. Doppelmayer. | 107. Abulfeda. - 34. Barocius. | 71. Campanus. | 108. Parrot. - 35. Fabricius. | 72. Kies. | 109. Albategnius. - 36. Metius. | 73. Purbach. | 110. Alphonsus. - 37. Fernelius. | 74. La Caille. | -----------------------+---------------------+------------------- - 111. Ptolemæus. | 151. Agrippa. | 191. Archimedes. - 112. Herschel. | 152. Arago. | 192. Timocharis. - 113. Davy. | 153. Taruntius. | 193. Lambert. - 114. Gueriké. | 154. Apollonius. | 194. Diophantus. - 115. Parry. | 155. Schubert. | 195. Delisle. - 116. Bonpland. | 156. Firmicus. | 196. Briggs. - 117. Lalande. | 157. Silberschlag. | 197. Lichtenberg. - 118. Réaumur. | 158. Hyginus. | 198. Theætetus. - 119. Hipparchus. | 159. Ukert. | 199. Calippus. - 120. Letronne. | 160. Boscovich. | 200. Cassini. - 121. Billy. | 161. Ross. | 201. Gauss. - 122. Fontana. | 162. Proclus. | 202. Messala. - 123. Hansteen. | 163. Picard. | 203. Struve. - 124. Damoiseau. | 164. Condorcet. | 204. Mason. - 125. Grimaldi. | 165. Plinius. | 205. Plana. - 126. Flamsteed. | 166. Menelaus. | 206. Burg. - 127. Landsberg. | 167. Manilius. | 207. Baily. - 128. Mösting. | 168. Eratosthenes. | 208. Eudoxus. - 129. Delambre. | 169. Gay Lussac. | 209. Aristoteles. - 130. Taylor. | 170. Tobias Mayer. | 210. Plato. - 131. Messier. | 171. Marius. | 211. Pico. - 132. Maskelyne. | 172. Olbers. | 212. Helicon. - 133. Sabine. | 173. Vasco de Gama. | 213. Maupertuis. - 134. Ritter. | 174. Seleucus. | 214. Condamine. - 135. Godin. | 175. Herodotus. | 215. Bianchini. - 136. Sömmering. | 176. Aristarchus. | 216. Sharp. - 137. Schröter. | 177. La Hire. | 217. Mairan. - 138. Gambart. | 178. Pytheas. | 218. Gérard. - 139. Reinhold. | 179. Bessel. | 219. Repsold. - 140. Encke. | 180. Vitruvius. | 220. Pythagoras. - 141. Hevelius. | 181. Maraldi. | 221. Fontenelle. - 142. Riccioli. | 182. Macrobius. | 222. Timæus. - 143. Lohrmann. | 183. Cleomedes. | 223. Epigenes. - 144. Cavalerius. | 184. Römer. | 224. Gärtner. - 145. Reiner. | 185. Littrow. | 225. Thales. - 146. Kepler. | 186. Posidonius. | 226. Strabo. - 147. Copernicus. | 187. Geminus. | 227. Endymion. - 148. Stadius. | 188. Linné. | 228. Atlas. - 149. Pallas. | 189. Autolycus. | 229. Hercules. - 150. Triesnecker. | 190. Aristillus. | - - - In the accompanying brief notes on a few important formations, - the diameter of each is given in miles, and the height of - the highest peak on wall in feet. The day of each lunation on - which it may be well seen is also added. - - - NO. - - 22. MAGINUS.--Great walled plain; 100 miles; 14,000 feet. - Central mountain 2,000 feet. Difficult in full, owing to rays - from Tycho. Plate XIV. Eighth and ninth days. - - 23. LONGOMONTANUS.--Walled plain; 90 miles; 13,314 feet. - Crossed by rays from Tycho. Plate XV. Ninth day. - - 26. WARGENTIN; 28. SCHICKARD.--Close together. 26. Curious - ring plain; 54 miles. Seemingly filled with lava. 'Resembles - a large thin cheese.' 28. Great walled plain; 134 miles; 9,000 - feet. Floor 13,000 square miles area, very varied in colour. - Walls would be invisible to spectator in centre of enclosure. - Plate XII. Thirteenth and fourteenth days. - - 30. TYCHO.--Splendid ring plain; 54 miles; 17,000 feet. - Central mountain 5,000 feet. Great system of streaks from - neighbourhood. Plates XII., XIII., XV. Ninth and tenth days. - - 32. STÖFLER.--Walled plain. Peak on N.E. wall 12,000 feet. - Floor very level. Beautiful steel-grey colour. Plate XVI. - Seventh day. - - 33. MAUROLYCUS.--Walled plain; 150 miles; 14,000 feet. In area - equal to about half of Ireland. Floor in full covered with - bright streaks. Plate XVI. Seventh day. - - 58. PICCOLOMINI.--Ring plain; 57 miles; 15,000 feet on E. Fine - central mountain. Very rugged neighbourhood. Plate XI. Fifth - and sixth days. - - 63. PITATUS.--58 miles. Wall massive on S., but breached on N. - side, facing Mare Nubium. Two clefts in interior shown Plate - XV. Ninth day. - - 78. FRACASTORIUS.--Another partially destroyed formation; 60 - miles. Wall breached on N., facing Mare Nectaris. Under low - sun traces of wall can be seen. Plate XI. Fifth and sixth - days. - - 80. PETAVIUS.--Fine object; 100 miles; 11,000 feet. Fine - central peak 6,000 feet. Great cleft from central mountain to - S.E. wall can be seen with 2-inch. Third and fourth days, but - best seen on waning moon a day or two after full. - - 90. GASSENDI.--Walled plain; 55 miles. Wall on N. broken by - intrusive ring-plain of Gassendi A. Fine central mountain - 4,000 feet. Between thirty and forty clefts in floor, more or - less difficult. Plates XII., XIII. Eleventh and twelfth days. - - 95. CATHERINA; 96. CYRILLUS; 97. THEOPHILUS.--Fine group - of three great walled plains. 95. Very irregular; 70 miles; - 16,000 feet. Connected by rough valley with 96. 96 has outline - approaching a square; walls much terraced, overlapped by 97, - and partially ruined on N.E. side. 97 is one of the finest - objects on moon; 64 miles; terraced wall, 18,000 feet. Fine - central mountain 6,000 feet. Plates XI., XVI. Sixth day. - - 84. ARZACHEL; 110. ALPHONSUS; 111. PTOLEMÆUS.--Another fine - group. 84 is southernmost; 66 miles; 13,000 feet. Fine central - mountain. 110. Walled plain; 83 miles; abutting on 111. Wall - rises to 7,000 feet. Bright central peak. Three peculiar dark - patches on floor, best seen towards full. 111 is largest of - three; 115 miles. Many large saucer-shaped hollows on floor - under low sun. Area 9,000 square miles. Plate XIII. Eighth and - ninth days. - - 125. GRIMALDI.--Darkest walled plain on moon; 148 miles - by 129; area 14,000 square miles; 9,000 feet. Plate XII. - Thirteenth and fourteenth days. - - 131. MESSIER AND MESSIER A.--Two bright craters; 9 miles. - Change suspected in relative sizes. From Messier A two - straight light rays like comet's tail extend across Mare - F[oe]cunditatis. Fourth and fifth days. - - 147. COPERNICUS.--Grand object; 56 miles; 10,000 to 12,000 - feet. Central mountain 2,400 feet. Centre of system of bright - rays. On W. a remarkable crater row; good test for definition. - Plates XII., XIII. Ninth and tenth days. - - 150. TRIESNECKER.--Small ring plain; 14 miles. Terraced wall - 5,000 feet. Remarkable cleft-system on W. Rather delicate for - small telescopes. Plate XIII. Seventh and eighth days. - - 158. HYGINUS.--Crater-pit 3·7 miles. Remarkable cleft runs - through it; visible with 2-inch: connected with Ariadæus rill - to W., which also an object for a 2-inch. Dark spot to N.W. - on Mare Vaporum named Hyginus N. Has been suspected to be new - formation. Plate XII. Seventh day. - - 168. ERATOSTHENES.--Fine ring plain at end of Apennines; 38 - miles. Terraced wall 16,000 feet above interior, which is - 8,000 feet below Mare Imbrium. Fine central mountain. Plate - XIII. Remarkable contrast to 148 Stadius, which has wall only - 200 feet, with numbers of craters on floor. Ninth and tenth - days. - - 175. HERODOTUS; 176. ARISTARCHUS.--Interesting pair. 175 is 23 - miles; 4,000 feet. Floor very dusky. Great serpentine valley; - most interesting object. Easy with 2-inch. 176 is most - brilliant crater on moon; 28 miles; 6,000 feet. Central peak - very bright. Readily seen on dark part of moon by earth-shine. - Plates XII., XIII. Twelfth day. - - 188. LINNÉ.--Small crater on M. Serenitatis near N.W. end of - Apennines. Suspected of change, but varies much in appearance - under different lights. Visible on Plate XVII. as whitish oval - patch to left of end of Apennines. Seventh day. - - 191. ARCHIMEDES.--Fifty miles; 7,000 feet. Floor very flat; - crossed by alternate bright and dark zones. Makes with 189 and - 190 fine group well shown Plate XVII. Eighth day. - - 208. EUDOXUS; 209. ARISTOTELES.--Beautiful pair of ring - plains. 208 is 40 miles. Walls much terraced; 10,000 to 11,000 - feet; 209 is 60 miles; 11,000 feet. Plate XVII. Sixth and - seventh days. - - 210. PLATO.--Great walled plain; 60 miles; 7,400 feet. Dark - grey floor, which exhibits curious changes of colour under - different lights, also spots and streaks too difficult for - small telescope. Landslip on E. side. Shadows very fine at - sunrise. Plates XII., XIII. Ninth day. - - 211. PICO.--Isolated mountain; 7,000 to 8,000 feet. S. of 210. - Casts fine shadow when near terminator. Ninth and tenth days. - - 228. ATLAS; 229. HERCULES.--Beautiful pair. 228 is 55 miles; - 11,000 feet. Small but distinct central mountain. 229 is - 46 miles. Wall reaches same height as 228, and is finely - terraced. Landslip on N. wall. Conspicuous crater on floor. - Plate XI. Fifth day. - - - - -APPENDIX II - - -The following list includes a number of double and multiple stars, -clusters, and nebulæ, which may be fairly well seen with instruments -up to 3 inches in aperture. A few objects have been added on account -of their intrinsic interest, which may prove pretty severe tests. The -places given are for 1900, and the position-angles and distances -are mainly derived from Mr. Lewis's revision of Struve's 'Mensuræ -Micrometricæ,' Royal Astronomical Society's Memoirs, vol. lvi., 1906. -For finding the various objects, Proctor's larger Star Atlas, though -constructed for 1880, is still, perhaps, the most generally useful. -Cottam's 'Charts of the Constellations' (Epoch 1890) are capital, but -somewhat expensive. A smaller set of charts will be found in Ball's -'Popular Guide to the Heavens,' while Peck has also published various -useful charts. The student who wishes fuller information than that -contained in the brief notes given below should turn to Gore's -exceedingly handy volume, 'The Stellar Heavens.' - -The brighter stars are generally known by the letters of the Greek -alphabet, prefixed to them by Bayer. When these are used up, recourse -is had either to the numbers in Flamsteed's Catalogue, or to those in -Struve's 'Mensuræ Micrometricæ.' The Struve numbers are preceded by -the Greek [Sigma]. A few of the more notable variable and red stars -are included; these are generally marked by capital letters, as V. -AQUILÆ. The order of the notes is as follows. First is given the -star's designation, then its place in hours and minutes of right -ascension and degrees and minutes of declination, N. and S. being -marked respectively by + and -; then follow the magnitudes; the -position-angles, which are measured in degrees from the north, or -bottom point of the field, round by east, south, and west to north -again; the distances of the components from one another in seconds of -arc; and, finally, short notes as to colour, etc. According to Dawes, -one inch aperture should separate the components of a 4·56″ double -star, two inches those of a 2·28″, three those of a 1·52″, and so -on. If the observer's glass can do this on good nights there is little -fault to find with it. Double stars may be difficult for other reasons -than the closeness of the components; thus, a faint companion to a -bright star is more difficult to detect than a companion which is not -far below its primary in brightness. Clusters and nebulæ, with a -few exceptions, are apt to prove more or less disappointing in small -instruments. The letters of the Greek alphabet are as follows: - - [alpha] Alpha. - [beta] Beta. - [gamma] Gamma. - [delta] Delta. - [epsilon] Epsilon. - [zeta] Zeta. - [eta] Eta. - [theta] Theta. - [iota] Iota. - [kappa] Kappa. - [lambda] Lambda. - [mu] Mu. - [nu] Nu. - [xi] Xi. - [omicron] Omicron. - [pi] Pi. - [rho] Rho. - [sigma] Sigma. - [tau] Tau. - [upsilon] Upsilon. - [phi] Phi. - [chi] Chi. - [psi] Psi. - [omega] Omega. - - ANDROMEDA. - - M. 31: 0 h. 37 m. + 40° 43′. Great Spiral Nebula. Visible to - naked eye near [nu] Andromedæ. Rather disappointing in small - glass. - - [Sigma] 205 or [gamma] : 1 h. 58 m. + 41° 51′ : 3-5 : 62′5° : - 10·2″. Yellow, bluish-green. 5 is also double, a binary, but - a very difficult object at present. - - AQUARIUS. - - M. 2 : 21 h. 28 m. - 1° 16′. Globular cluster; forms flat - triangle with [alpha] and [beta]. - - [Sigma] 2909 or [zeta] : 22 h. 24 m. -0° 32′ : 4-4·1 : 319·1° - : 3·29″. Yellow, pale yellow. Binary. - - AQUILA. - - M. 11 : 18 h. 46 m. - 6° 23′. Fine fan-shaped cluster. Just - visible to naked eye. - - V : 18 h. 59 m. - 5° 50′. Red star, variable from 6·5 to 8·0. - - ARGO NAVIS. - - M. 46 : 7 h. 37 m. - 14° 35′. Cluster of small stars, about - 1/2° in diameter. - - ARIES. - - [Sigma] 180 or [gamma] : 1 h. 48 m. + 18° 49′ : 4·2-4·4 : - 359·4° : 8·02″. Both white. Easy and pretty. - - [lambda] 1 h. 52 m. + 23° 7′ : 4·7-6·7 : 47° : 36·5″. Yellow, - pointed to by [gamma] and [beta]. - - AURIGA. - - (Capella) [alpha] : 5 h. 9 m. + 45° 54′. Spectroscopic binary; - period 104 days. - - M. 37 : 5 h. 46 m. + 32° 31′. Fine cluster. M. 36 and M. 38 - also fine. All easily found close to straight line drawn from - [kappa] to [phi] Aurigæ. - - [beta] : 5 h. 52 m. + 44° 57′. Spectroscopic binary, period - 3·98 days. - - 41: 6 h. 4 m. + 48° 44′ : 5·2-6·4 : 353·7 : 7·90″. - Yellowish-white, bluish-white. - - BOÖTES. - - [Sigma] 1864 or [pi] : 14 h. 36 m. + 16° 51′ : 4·9-6 : 103·3° - : 5·83″. Both white. - - [Sigma] 1877 or [epsilon] : 14 h. 40 m. + 27° 30′ : 3-6·3 : - 326·4° : 2·86″. Yellow, blue. Fine object and good test. - - [Sigma] 1888 or [xi] : 14 h. 47 m. + 19° 31′ : 4·5-6·5 : - 180·4° : 2·70″. Yellow, purple, binary. - - [Sigma] 1909 or 44 : 15 h. 0 m. + 48° 2′ : 5·2-6·1 : 242° : - 4·32″. - - CAMELOPARDUS. - - V. : 3 h. 33 m. + 62° 19′. Variable, 7·3 to 8·8. Fiery red. - - CANCER. - - [Sigma] 1196 or [zeta] : 8 h. 6 m. + 17° 57′ : 5-5·7-6·5 : - 349·1°, 109·6° : 1·14″, 5·51″. Triple ; 5 and 5·7 binary, - period 60 years; 6·5 revolves round centre of gravity of all - in opposite direction. - - [Sigma] 1268 or [iota] : 8 h. 41 m. + 29° 7′ : 4·4-6·5 : 307° - : 30·59″. Yellow, blue. - - Præsepe: Cluster, too widely scattered for anything but lowest - powers. - - CANES VENATICI. - - [Sigma] 1622 or 2 : 12 h. 11 m. + 41° 13′ : 5-7·8 : 258° : - 11·4″. Gold, blue. - - [Sigma] 1645 : 12 h. 23 m. + 45° 21′ : 7-7·5 : 160·5° : - 10·42″. White. Pretty, though faint. - - [Sigma] 1692, 12, or [alpha] : 12 h. 51 m. + 38° 52′ : 3·1-5·7 - : 227° : 19·69″. Cor Caroli. White, violet. - - M. 51 : 13 h. 26 m. + 47° 43′. Great spiral. 3° S.W. of [eta] - Ursæ Majoris. - - M. 3 : 13 h. 38 m. + 28° 53′. Fine globular cluster; on line - between Cor Caroli and Arcturus, rather nearer the latter. - - CANIS MAJOR. - - M. 41 : 6 h. 43 m. - 20° 38′. Fine cluster, visible to naked - eye, 4° below Sirius. - - CANIS MINOR. - - (Procyon) [alpha] : 7 h. 34 m. + 5° 30′ : 0·5-14 : 5° 4·46″. - Binary, companion discovered, Lick, 1896, only visible in - great instruments. - - CAPRICORNUS. - - [alpha] : 20 h. 12 m. - 12° 50′ : 3·2-4·2. Naked eye double, - both yellow. - - M. 30 : 21 h. 35 m. - 23° 38′. Fairly bright cluster. - - CASSIOPEIA. - - [Sigma] 60 or [eta] : 0 h. 43 m. + 57° 18′ : 4-7 : 227·8° : - 5·64″. Binary; period about 200 years. - - [Sigma] 262 or [iota] : 2 h. 21 m. + 66° 58′ : 4·2-7·1-7·5 : - 250°, 112·6° : 1·93″, 7·48″. Triple. - - H. vi. 30 : 23 h. 52 m. + 56° 9′. Large cloud of small stars. - - [Sigma] 3049 or [sigma] : 23 h. 54 m. + 55° 12′ : 5-7·5 : - 325·9° : 3·05″. Pretty double, white, blue. - - CEPHEUS. - - [kappa] : 20 h. 12 m. + 77° 25′ : 4-8 : 123° : 7·37″. - Yellowish-green. - - [Sigma] 2806 or [beta] : 21 h. 27 m. + 70° 7′ : 3-8 : 250·6° : - 13·44″. White, blue. - - S : 21 h. 36 m. + 78° 10′. Variable, 7·4 to 12·3. Very deep - red. - - [Sigma] 2863 or [xi] : 22 h. 1 m. + 64° 8′ : 4·7-6·5 : 283·3°: - 6·87″. Yellow, blue. - - [delta] : 22 h. 25 m. + 57° 54′ : variable-5·3 : 192° : 40″. - Yellow, blue. Primary varies from 3·7 to 4·9. Period, 5·3 - days. Spectroscopic binary. - - [Sigma] 3001 or [omicron] : 23 h. 14 m. + 67° 34′ : 5·2-7·8 : - 197·3° : 2·97″. Yellow, yellowish-green. - - CETUS. - - (Mira) [omicron] : 2 h. 14 m. - 3° 26′. Variable. Period - about 331 days. Maxima, 1·7 to 5; minima, 8 to 9. Colour, deep - yellow to deep orange. - - [Sigma] 281 or [nu] : 2 h. 31 m. + 5° 10′ : 5-9·4 : 83·1°: - 7·74″. Yellow, ashy. - - [Sigma] 299 or [gamma] : 2 h. 38 m. + 2° 49′ : 3-6·8 : 291° : - 3·11″. Yellow, blue, slow binary. - - COMA BERENICES. - - [Sigma] 1657 or 24 : 12 h. 30 m. + 18° 56′ : 5·5-7 : 271·1° : - 20·23″. Orange, blue. - - M. 53 : 13 h. 8 m. + 18° 42′. Cluster of faint stars. - - CORONA BOREALIS. - - [Sigma] 1965 or [zeta] : 15 h. 36 m. + 36° 58′ : 4·1-5 : - 304·3° : 6·15″. White greenish. - - R : 15 h. 44 m. + 28° 28′. Irregularly variable, 5·5 to 10·1. - - [Sigma] 2032 or [sigma] : 16 h. 11 m. + 34° 6′ : 5-6·1 : - 216·3° : 4·80″. Yellow, bluish. Binary, period about 400 - years. - - CORVUS. - - [delta] : 12 h. 25 m. - 15° 57′ : 3-8·5 : 214° : 24·3″. - Yellow, lilac. - - CRATER. - - R. : 10 h. 56 m. - 17° 47′. Variable. About 8 magnitude. - Almost blood-colour. - - CYGNUS. - - [Sigma] 2486 : 19 h. 9 m. + 49° 39′ : 6-6·5 : 218·2° : 9·63″. - 'Singular and beautiful field' (Webb). - - (Albireo) [beta] : 19 h. 27 m. + 27° 45′ : 3-5·3 : 55° : - 34·2″. Orange-yellow, blue. Easy and beautiful. - - [Sigma] 2580 or [chi] : 19 h. 43 m. + 33° 30′ : 4·5-8·1 : - 71·6° : 25·50″. 4·5 is variable to 13·5. Period 406 days. - - Z : 19 h. 58 m. + 49° 45′. Variable, 7·1 to 12. Deep red. - - RS : 20 h. 10 m. + 38° 27′. Variable, 6 to 10. Deep red. - - U : 20 h. 16 m. + 47° 35′. Variable, 7 to 11·6. Very red. - - V : 20 h. 38 m. + 47° 47′. Variable, 6·8 to 13·5. Very red. - - [Sigma] 2758 or 61 : 21 h. 2 m. + 38° 13′ : 5·3-5·9 : 126·8° : - 22·52″. First star whose distance was measured. - - RV : 21 h. 39 m. + 37° 33′. Variable, 7·1 to 9·3. Splendid - red. - - [Sigma] 2822 or [mu] : 21 h. 40 m. + 28° 18′ : 4-5 : 122·2° : - 2·29″. Fine double; probably binary. - - DELPHINUS. - - [Sigma] 2727 or [gamma] : 20 h. 42 m. + 15° 46′ : 4-5 : 269·8° - : 10·99″. Yellow, bluish-green. - - V : 20 h. 43 m. + 18° 58′. Variable, 7·3 to 17·3. Period 540 - days. Widest range of magnitude known. - - DRACO. - - [Sigma] 2078 or 17 : 16 h. 34 m. + 53° 8′ : 5-6 : 109·5° : - 3·48″. White. - - [Sigma] 2130 or [mu] : 17 h. 3 m. + 54° 37′ : 5-5·2 : 144·2° : - 2·17″. White. - - H. iv. 37 : 17 h. 59 m. + 66° 38′. Planetary nebula, nearly - half-way between Polaris and [gamma] Draconis. Gaseous; first - nebula discovered to be so. - - [Sigma] 2323 or 39: 18 h. 22 m. + 58° 45′ : 4·7-7·7-7·1 : - 358·2°, 20·8° : 3·68″, 88·8″. Triple. - - [epsilon] : 19 h. 48 m. + 70° 1′ : 4-7·6 : 7·5° : 2·84″. - Yellow, blue. - - EQUULEUS. - - [Sigma] 2737 or [epsilon] : 20 h. 54 m. + 3° 55′ : 5·7-6·2-7·1 - : 285·9°, 73·8° : 0·53″, 10·43″. Triple with large - instruments. - - ERIDANUS. - - [Sigma] 518 or 40 or 0^2 : 4 h. 11 m. - 7° 47′ : 4-9-10·8 : - 106·3°, 73·6° : 82·4″, 2·39″. Triple, close pair binary. - - GEMINI. - - M. 35 : 6 h. 3 m. + 24° 21′. Magnificent cluster; forms obtuse - triangle with [mu] and [eta]. - - [Sigma] 982 or 38 : 6 h. 49 m. + 13° 19′ : 5·4-7·7 : 159·7° : - 6·63″. Yellow, blue. Probably binary. - - [zeta] : 6 h. 58 m. + 20° 43′. Variable, 3·8 to 4·3. Period - 10·2 days. Non-eclipsing binary. - - [Sigma] 1066 or [delta] : 7 h. 14 m. + 22° 10′ : 3·2-8·2 : - 207·3° : 6·72″. Pale yellow, reddish. - - (Castor) [alpha] : 7 h. 28 m + 32° 7′ : 2-2·8 : 224·3° : - 5·68″. White, yellowish-green. Finest double in Northern - Hemisphere. - - HERCULES. - - M. 13 : 16 h. 38 m. + 36° 37′. Great globular cluster, - two-thirds of way from [zeta] to [eta]. - - [Sigma] 2140 or [alpha] : 17 h. 10 m. + 14° 30′ : 3-6·1 : - 113·6° : 4·78″. Orange-yellow, bluish-green. Fine object. - - [Sigma] 2161 or [rho] : 17 h. 20 m. + 37° 14′ : 4-5·1 : 314·4° - : 3·80″. 'Gem of a beautiful coronet' (Webb). - - M. 92 : 17 h. 14 m. + 43° 15′. Globular cluster; fainter than - M. 13. - - [Sigma] 2264 or 95 : 17 h. 57 m. + 21° 36′ : 4·9-4·9 : 259·7° - : 6·44″. 'Apple-green, cherry-red' (Smyth), but now both pale - yellow. - - [Sigma] 2280 or 100 : 18 h. 4 m. + 26° 5′ : 5·9-5·9 : 181·7° : - 13·87″. Greenish-white. - - HYDRA. - - [Sigma] 1273 or [epsilon] : 8 h. 41 m. + 6° 48′ : 3·8-7·7 : - 231·6° : 3·33″. The brighter star is itself a close double. - - V : 10 h. 47 m. - 20° 43′. Variable, 6·7 to 9·5. Copper-red. - - W : 13 h. 44 m. - 27° 52′. Variable, 6·7 to 8·0. Deep red. - - LACERTA. - - LEO. - - [Sigma] 1424 or [gamma] : 10 h. 14 m. + 20° 21′ : 2-3·5 : - 116·5° : 3·70″. Fine double, yellow, greenish-yellow. - - [Sigma] 1487 or 54 : 10 h. 50 m. + 25° 17′ : 5-7 : 107·5° : - 6·38″. Greenish-white, blue. - - [Sigma] 1536 or [iota] : 11 h. 19 m. + 11° 5′ : 3·9-7·1 : - 55·0° : 2·36″. Yellow, blue. - - LEO MINOR. - - LEPUS. - - R : 4 h. 55 m. - 14° 57′. Variable, 6·7 to 8·5. Intense - crimson. - - LIBRA. - - M. 5 : 15 h. 13 m. + 2° 27′. Globular cluster, close to star 5 - Serpentis. Remarkable for high ratio of variables in it--1 in - 11. - - LYNX. - - [Sigma] 948 or 12 : 6 h. 37 m. + 59° 33′ : 5·2-6·1-7·4 : 116°, - 305·8° : 1·41″, 8·23″. Triple, greenish, white, bluish. - - [Sigma] 1334 or 38 : 9 h. 13 m. + 37° 14′ : 4-6·7 : 235·6° : - 2·88″. White blue. - - LYRA. - - T : 18 h. 29 m. + 36° 55′. Variable, 7·2 to 7·8. Crimson. - - (Vega) [alpha] : 18 h. 34 m. + 38° 41′ : 1-10·5 : 160° : - 50·77″. Very pale blue. The faint companion is a good test - for small telescopes. Vega is near the apex of the solar way. - - { [epsilon]^1 : 18 h. 41·1 m. + 39° 30′ : 4·6-6·3 : 12·4° : 2·85″. - [epsilon] { Pale yellow, pale orange yellow. - { [epsilon]^2 : 4·9-5·2 : 127·3° : 2·15″. Both pale yellow. - - [zeta] : 18 h. 41 m. + 37° 30′ : 4·2-5·5 : 150° : 43·7″. - Easy, both pale yellow. - - [beta] : 18 h. 46 m. + 33° 15′ : 3-6·7 : 149·8° : 45·3″. 3 - variable, 12·91 days. Spectroscopic binary. - - M. 57 : 18 h. 50 m. + 32° 54′. Ring Nebula, between [beta] and - [gamma]. Faint in small telescope. Gaseous. - - MONOCEROS. - - [Sigma] 919 or 11 : 6 h. 24 m. - 6° 57′ : A 5-B 5·5-C 6 : AB - 131·6° : 7·27″ : BC 105·7° : 2·65″. Fine triple. - - [Sigma] 950 or 15 : 6 h. 35 m. + 10°·0′ : 6-8·8-11·2 : 212·2°, - 17·9° : 2·69″, 16·54″. Triple, green, blue, orange. - - OPHIUCHUS. - - [rho] : 16 h. 19 m. - 23° 13′ : 6-6 : 355° : 3·4″. - - 39 : 17 h. 12 m. - 24° 11′ : 5·5-6 : 358° : 15″. Pale orange, - blue. - - [Sigma] 2202 or 61 : 17 h. 40 m. + 2° 37′ : 5·5-5·8 : 93·4° : - 20·68″. White. - - [Sigma] 2272 or 70 : 18 h. 1 m. + 2° 32′ : 4·5-6 : 178° : - 2·10″. Yellow, purple. Rather difficult. - - ORION. - - (Rigel) [beta] : 5 h. 10 m. -8° 19′ : 1-8 : 202·2° : 9·58″. - Bluish-white, dull bluish. Fair test for small glass. - - [delta] : 5 h. 27 m. - 0° 23′ : 2-6·8 : 359° : 52·7″. White, - very easy. - - [Sigma] 738 or [lambda] : 5 h. 30 m. + 9° 52′ : 4-6 : 43° 1′ : - 4·55″. Yellowish, purple. Pretty double. - - [theta] : 5 h. 30 m. - 5° 28′ : 6-7-7·5-8. The 'Trapezium' in - the Great Nebula. - - M. 42 : 5 h. 30 m. - 5° 28′ : 6-7-7·5-8. Great Nebula of - Orion. - - [Sigma] 752 or [iota] : 5 h. 30 m. - 5° 59′ : 3·2-7·3 : 141·7° - : 11·50″. White, fine field. - - [sigma] : 5 h. 34 m. - 2° 39′. Fine multiple, double triple in - small glass. - - [zeta] : 5 h. 36 m. - 2° 0′ : 2-6 : 156·3° : 2·43″. - Yellowish-green, blue. - - U : 5 h. 50 m. + 20° 10′. Variable, 5·8-12·3. Period 375 days. - - PEGASUS. - - M. 15 : 21 h. 25 m. + 11° 43′. Fine globular cluster, 4° N.E. - of [delta] Equulei. - - PERSEUS. - - H. VI. 33·34 : 2 h. 13 m. + 56° 40′. Sword-handle of Perseus. - Splendid field. - - M. 34 : 2 h. 36 m. + 42° 21′. Visible to naked eye. Fine - low-power field. - - [Sigma] 296 or [theta] : 2 h. 37 m. + 48° 48′ : 4·2-10-11 : - 299°, 225° : 17·4″, 80″. Triple. - - [Sigma] 307 or [eta] : 2 h. 43 m. + 55° 29′ : 4-8·5 : 300° : - 28″. Orange-yellow, blue. - - (Algol) [beta] : 3 h. 2 m. + 40° 34′. Variable, 2·1 to 3·2. - Period 2·8 days. Spectroscopic eclipsing binary. - - [Sigma] 464 or [zeta] : 3 h. 48 m: + 31° 35′ : 2·7-9·3 : - 206·7° : 12·65°. Greenish-white, ashy. Three other companions - more distant. - - [Sigma] 471 or [epsilon] : 3 h. 51 m. + 39° 43′ : 3·1-8·3 : - 7·8° : 8·8″. White, bluish-white. - - PISCES. - - [Sigma] 12 or 35 : 0 h. 10 m. + 8° 16′ : 6-8 : 150° : 12″. - White, purplish. - - [Sigma] 88 or [psi] : 1 h. 0·4 m. + 20° 56′ : 4·9-5 : 160° : - 29·96″. White. - - [Sigma] 100 or [zeta] : 1 h. 8 m. + 7° 3′ : 4·2-5·3 : 64° : - 23·68″. White, reddish-violet. - - [Sigma] 202 or [alpha] : 1 h. 57 m. + 2° 17′ : 2·8-3·9 : 318° - : 2·47″. Reddish, white. - - SAGITTA. - - SAGITTARIUS. - - M. 20 : 17 h. 56 m. - 23° 2′. The Trifid Nebula. - - SCORPIO. - - [beta] : 15 h. 59·6 m. - 19° 31′ : 2-5 : 25° : 13·6″. Orange, - pale yellow. - - (Antares) [alpha] : 16 h. 23 m. - 26° 13′ : 1-7 : 270° : 3″. - Difficult with small glass. - - SCUTUM SOBIESKII. - - M. 24 : 18 h. 12 m. - 18° 27′. Fine cluster of faint stars on - Galaxy. - - M. 17 : 18 h. 15 m. - 16° 14′. The Omega Nebula. Gaseous. - - R : 18 h. 42 m. - 5° 49′. Irregular, variable, 4·8 to 7·8. - - SERPENS. - - [Sigma] 1954 or [delta] : 15 h. 30 m. + 10° 53′ : 3·2-4·1 : - 189·3° : 3·94″. Yellow, yellowish-green, binary. - - [Sigma] 2417 or [theta] : 18 h. 51 m. + 4° 4′ : 4-4·2 : 103° : - 22″. Both pale yellow. - - SEXTANS. - - TAURUS. - - [Sigma] 528 or [chi] : 4 h. 16 m. + 25° 23′ : 5·7-7·8 : 24·2° - : 19·48″. White, lilac. - - [Sigma] 716 or 118 : 5 h. 23 m. + 25° 4′ : 5·8-6·6 : 201·8 : - 4·86″. White, bluish-white. - - M. 1 : 5 h. 28 m. + 21° 57′. The Crab Nebula. Faint in small - glass. - - TRIANGULUM. - - [Sigma] 227 or [iota] : 2 h. 7 m. + 29° 50′ : 5-6·4 : 74·6°: - 3·79″. Yellow, blue, beautiful. - - URSA MAJOR. - - [Sigma] 1523 or [xi] : 11 h. 13 m. + 32° 6′ : 4-4·9 : 137·2° : - 2·62″. Yellowish, binary. Period 60 years. - - [Sigma] 1543 or 57 : 11 h. 24 m. + 39° 54′ : 5·2-8·2 : 2·1° : - 5·40″. White, ashy. - - (Mizar) [zeta] : 13 h. 20 m. + 55° 27′ : 2·1-4·2 : 149·9° - : 14·53″. Fine pair, yellow and yellowish-green. Alcor, 5 - magnitude in same field with low power, also 8 magnitude star. - - URSA MINOR. - - (Polaris) [alpha] : 1 h. 22 m. + 88° 46′ : 2-9 : 215·6° : - 18·22″. Yellow, bluish, test for 2-inch. - - VIRGO. - - [Sigma] 1670 or [gamma] : 12 h. 37 m. - 0° 54′ : 3-3 : 328·3° - : 5·94″. Both pale yellow. Binary, 185 years. - - VULPECULA. - - M. 27 : 19 h. 55 m. + 22° 27′. The Dumb-bell Nebula. Just - visible with 1-1/4-inch. Gaseous. - - - - -INDEX - - - A - - Achromatic. See Telescope - - Adams, search for Neptune, 198-201 - - Aerolites, 227 - - Airy, search for Neptune, 197-201 - - Albireo, colour of, 236 - - Alcor, 241 - - Alcyone, 256 - - Aldebaran, 234; - colour of, 235 - - Algol, spectroscopic binary, 246; - diameter and mass of components, 246; - period of, 250; - variables, 250 - - Alps, lunar, 116; - valley of, 116, 117 - - Altai Mountains, 117 - - Altair, 234 - - Altazimuth, 25-28 - - Anderson discovers Nova Aurigæ, 253; - discovers Nova Persei, 254 - - Andromeda, great nebula of, 263, 264 - - Andromedæ [gamma], colour of, 236 - - Andromedes, 214, 215, 225, 226 - - Annular eclipse, 69, 70 - - Antares, 234 - - Anthelme observes new star, 252 - - Apennines, lunar, 116 - - Archimedes, 117 - - Arcturus, 234 - - Argelander, number of stars, 235 - - Ariadæus cleft, 119 - - Arided, 234 - - Arietis [gamma], observed by Hooke, 240 - - Aristillus, 117 - - Asteroids, number of, 150; - methods of discovery, 150, 151 - - Asterope, 256 - - Astræa, discovery of, 150 - - Atlas, 256 - - Atmosphere, solar, 75 - - Autolycus, 117 - - Auzout, aerial telescopes, 4 - - - B - - Bacon, Roger, 1 - - Bailey, cluster variables, 259 - - Ball, Sir R., 154, 262; - 'Popular Guide to the Heavens,' 278 - - Barnard, measures of Venus, 89; - markings on Venus, 95; - on Mars, 133; - measures of asteroids, 152; - discovers Jupiter's fifth satellite, 167; - measures of Saturn, 172; - drawing of Saturn, 172; - rotation of Saturn, 174; - on Saturnian markings, 184-185; - observation of Comet 1882 (iii.), 218 - - Bayer, lettering of stars, 278 - - Beer. See Mädler - - Bélopolsky, rotation of Venus, 96 - - Bessel, search for Neptune, 197 - - Betelgeux, 234; - colour of, 235 - - Biela's comet, 213, 214, 215, 224, 225 - - Birmingham observes Nova Coronæ, 252 - - Bode's law, 148, 149 - - Bond, G. P., discovers rifts in Andromeda nebula, 264 - - Bond, W. C., discovers Crape Ring, 178; - discovers Saturn's eighth satellite, 187; - verifies discovery of Neptune's satellite, 201 - - Boötis [epsilon], double star, 242 - - Bouvard, tables of Uranus, 197 - - Bradley uses aerial telescope, 4 - - Bremiker's star-charts, 200 - - Brooks' comet, 210; - observation of comet 1882 (iii.), 218 - - Brorsen's comet, 213 - - - C - - Calcium in chromosphere, 73 - - Campbell, atmosphere of Mars, 140; - bright projections on Mars, 141; - spectroscopic investigation of Saturn's rings, 180 - - Canals. See Mars - - Canes Venatici, great spiral nebula in, 265 - - Canopus, 234 - - Capella, 234 - - Capricorni [alpha], naked-eye double, 241 - - Carpathians, 117 - - Carrington, solar rotation, 59 - - Cassegrain. See Telescope, forms of - - Cassini uses aerial telescope, 4; - discovers four satellites of Saturn and division of ring, 4; - observations on Jupiter, 160; - discovers division in Saturn's ring, 177; - four satellites of Saturn, 184, 186, 187 - - Cassiopeiæ [eta], double star, 242; - Nova, 252 - - Castor, 234; - double star, 242; - binary, 245 - - Caucasus, lunar, 116 - - Cauchoix constructs 12-inch O.G., 6 - - Celaeno, 256 - - Celestial cycle, 18 - - Centauri [alpha], 231, 234 - - Ceres, discovery of, 149; - diameter of, 152; - reflective power, 152 - - Ceti [zeta], naked-eye double, 241; - Mira ([omicron]) variable star, 248; - period, 249 - - Challis, search for Neptune, 199 - - Chambers, G. F., on comets, 208-209; - number of comets, 209 - - Chromosphere, 71, 73, 76; - depth of, 73; - constitution of, 73 - - Clark, Alvan, constructs 18-1/2-inch, 8; - 26-inch, 8; - 30-inch Pulkowa telescope and 36-inch Lick, 8; - 40-inch Yerkes, 9 - - Clavius, lunar crater, 113, 114, 120 - - Clerke, Miss Agnes, 60, 73; - climate of Mercury, 85; - on Mars, 139; - albedo of asteroids, 152; - Jupiter's red spot, 161; - on comet 1882 (iii.), 218; - on Mira Ceti, 248 - - Clerk-Maxwell, constitution of Saturn's rings, 179 - - Cluster variables, 259 - - Clusters, irregular, 256; - globular, 258 - - Coggia's comet, 211 - - Coma Berenices, 256 - - Comas Solà, rotation of Saturn, 174 - - Comet of 1811, 206; - of 1843, 206, 215, 216; - of Encke, 207; - of Halley, 207, 213; - Brooks, 210; - Donati, 205, 210; - Tempel, 211; - 1866 (i.), 214, 224; - Winnecke, 211; - Coggia, 211; - Holmes, 211; - Biela, 213; - and Andromeda meteors, 214, 215, 224, 225; - great southern (1901), 211; - Wells, 213; - of 1882, 213, 216-219; - De Vico, 213; - Brorsen, 213; - of Swift 1862 (iii.), and Perseid meteors, 214, 224; - great southern (1880), 216; - of 1881, 216; - of 1807, 216 - - Comets, 203 _et seq._; - structure of, 205; - classes of, 206-208; - number of, 209; - spectra of, 211-213, 218; - constitution of, 212, 218; - connection with meteors, 214, 215, 224; - families of, 215-218; - observation of, 219-222 - - Common 5-foot reflector, 12; - photographs Orion nebula, 262 - - Constellations, formation of, 237, 238 - - Contraction of sun, 79 - - Cooke, T., and Sons, 25-inch Newall telescope, 8; - mounting of 6-inch refractor, 31 - - Copernicus, prediction of phases of Venus, 92; - lunar crater, 114; - ray system of, 120, 121 - - Corona, 71, 72, 76; - tenuity of, 71; - variations in structure, 71; - minimum type of, 71, 72; - maximum type of, 72; - constitution of, 72 - - Corona Borealis, 238; - Nova in, 252 - - Coronal streamers, analogy with Aurora, 71 - - Coronium, 72, 73 - - Cottam, charts of the constellations, 278 - - Crape ring of Saturn, 178 - - Craters, lunar, 109, 112; - ruined and 'ghost,' 111; - number and size, 112; - classification of, 112 - - Cygni, 61, 231; - [alpha], 234; - [beta], colour of, 236 - - - D - - Darwin, G. H., evolution of Saturnian system, 186 - - Dawes discovers crape ring, 178; - search for Neptune, 199, 200 - - Deimos, satellite of Mars, 143 - - Delphinus, 237 - - Denning, absence of colour in reflector, 22; - measuring sun-spots, 51, 53; - on naked-eye views of Mercury, 82; - abnormal features on Venus, 94; - on canals of Mars, 136; - observations of cloud on Mars, 139, 140; - changes on Jupiter, 159, 160; - rotation of Saturn, 174; - visibility of Cassini's division, 182; - number of meteor radiants, 225; - classification of sporadic meteors, 227; - meteoric observation, 227, 228; - stationary radiants, 229 - - Deslandres, calcium photographs of sun, 60; - on form of corona, 72; - photographs chromosphere and prominences, 74 - - De Vico's comet, 213 - - Dew-cap, 39 - - Digges, supposed use of telescopes, 1 - - Dollond, John, invention of achromatic, 5; - 5-foot achromatics, 6 - - Donati, comet of 1858, 205, 210; - spectrum of comet Tempel, 211 - - Doppler's principle, 180 - - Dorpat refractor, 6, 7, 31 - - Douglass, markings on Venus, 95 - - Draco, planetary nebula in, 266 - - Dunér, rotation of sun, 59 - - - E - - Earth-light on moon, 105 - - Eclipse, Indian, 1898, 70; - 1878, July 29, 72; - 1870, December 22, 74 - - Eclipses, solar, 68-70; - of moon, 105, 106 - - Electra, 256 - - Electrical influence of sun on earth, 63 - - Elger on lunar Maria, 111; - lunar clefts, 119; - lunar chart, 125 - - Elkin observes transit of comet 1882 (iii.), 212 - - Encke discovers division in ring of Saturn, 177; - search for Neptune, 200 - - Equatorial mountings, 29-31, 36 - - Equulei [delta], short-period binary, 245 - - Erck, Dr. Wentworth, satellites of Mars, 144 - - Eros, discovery of, distance of, 151; - variability of, 152 - - - F - - Fabricius observes Mira Ceti, 248 - - Faculæ, 59; - rotation period of, 59 - - Faculides, 60 - - Finder. See Telescope - - Finlay, transit of comet 1882 (iii.), 212 - - Flamsteed, catalogue of stars, 278 - - Fomalhaut, 234 - - Fowler, 'Telescopic Astronomy,' 17 - - Fracastorius, 111 - - - G - - Galaxy. See Milky Way - - Galilean telescope. See Telescope, forms of - - Galileo Galilei, invention of telescope, 2; - loss of sight, 47; - discovery of phases of Venus, 92; - on lunar craters, 112; - discovers four satellites of Jupiter, 166; - observations of Saturn, 175, 176 - - Galle discovers Neptune, 200 - - Gassendi observes transit of Mercury, 87; - lunar crater, 119 - - Geminorum [alpha]. See Castor - - George III. pensions Herschel, 193 - - Georgium Sidus, 194 - - Gore, period of Algol, 250; - globular clusters, 259; - 'The Stellar Heavens,' 278 - - Gregorian. See Telescope, forms of - - Grubb, 27-inch Vienna telescope, 8; - on telescopic powers, 41 - - Gruithuisen, changes on moon, 126 - - - H - - Hale, calcium photographs of sun, 60 - - Hall, Asaph, discovers satellites of Mars, 8, 143; - rotation of Saturn, 173, 174 - - Hall, Chester Moor, discovers principle of achromatic, 5 - - Halley's comet, 207, 213 - - Harding discovers Juno, 149 - - Hebe, discovery of, 150 - - Hegel proves that there are only seven planets, 149 - - Helium in chromosphere, 73 - - Helmholtz, speed of sensation, 48; - solar contraction, 79 - - Hencke discovers Astræa and Hebe, 150 - - Henry, 30-inch Nice telescope, 8 - - Heraclides promontory, 117 - - Hercules, 237 - - Herculis [alpha], double star, 242 - - Herodotus, valley of, 118, 119, 126 - - Herschel, Sir John, drawing of Orion nebula, 262 - - Herschel, Sir William, 4-foot telescope, 13; - impairs sight, 47; - misses satellites of Mars, 143, 144; - rotation of Saturn, 173; - discovers Saturn's sixth and seventh satellites, 186, 187; - early history, 190, 191; - discovers Uranus, 191; - discovers two satellites of Uranus, 196; - binary stars, 244; - gaseous constitution of nebulæ, 260; - distribution of nebulæ, 267; - translation of solar system, 269 - - Herschelian. See Telescope, forms of - - Hevelius, description of Saturn, 176 - - Hind discovers Nova Ophiuchi, 252 - - Hirst, colouring of Jupiter, 159 - - Hirst, Miss, colouring of Jupiter, 159 - - Holden on solar rotation, 59, 60 - - Holmes, Edwin, telescope-house, 38; - comet, 211 - - Holmes, Oliver Wendell, 'Poet at the Breakfast-table,' 13 - - Holwarda observes [omicron] Ceti, 248 - - Hooke, observation of Gamma Arietis, 240 - - Howlett, criticism of Wilsonian theory of sun-spots, 61 - - Huggins, atmosphere of Mars, 140; - gaseous nature of nebulæ, 210; - spectrum of Winnecke's comet, 211; - discovers nebula in Draco to be gaseous, 260; - spectrum of Andromeda nebula, 264 - - Humboldtianum, Mare, 111 - - Humboldt observes meteor-shower of 1799, 224 - - Hussey, search for Neptune, 197 - - Hussey, W. J., period of [delta] Equulei, 245 - - Huygens, improvement on telescopes, 3; - aerial telescopes, 4; - discovers nature of Saturn's ring and first - satellite of Saturn, 177, 186; - observation of [theta] Orionis, 240; - of great nebula in Orion, 261 - - Huygens, Mount, 116 - - Hydrogen in chromosphere, 73 - - Hyginus cleft, 119 - - - I - - Imbrium, Mare, 116 - - Iron in chromosphere, 73 - - - J - - Jansen, Zachariah, claim to invention of telescope, 1 - - Janssen, photographs of sun, 57 - - Journal of British Astronomical Association, 23, 38 - - Juno, discovery of, 149; - diameter of, 152 - - Jupiter, brilliancy compared with Venus, 90; - period of, 155; - distance of, 155; - diameter of, 155; - compression, volume, density, 155; - brilliancy, 156; - apparent diameter of, 156; - belts of, 157 _et seq._; - colouring, 158, 159; - changes on surface of, 159, 160; - great red spot, 160-164; - rotation period, 163-165; - resemblance to sun, 164-166; - satellites of, 166-169; - observation of, 169-171; - visibility of satellites, 166; - diameters of, 167; - occultations of, eclipses of, transits of, 167 - - - K - - Kaiser sea, Mars, 145 - - Keeler, report on Yerkes telescope, 9; - rotation of Saturn, 174; - constitution of Saturn's rings, 180; - photographic survey of nebulæ, 267 - - Kelvin, solar combustion, 78, 79 - - Kepler, suggestion for improved refractor, 3; - predicts transit of Mercury, 87; - lunar crater, ray-system of, 120, 121; - observes new star, 252 - - Kirchhoff, production of Fraunhofer lines, 75 - - Kirkwood, theory of asteroid formation, 153; - periodic meteors, 214 - - Kitchiner, visibility of Saturn's satellites, 188 - - Klein's Star Atlas, 255 - - - L - - Lampland, photographs of Mars, 137 - - Langley, heat of umbra of sun-spot, 50; - changes in sunspots, 55 - - Lassell, 4-foot reflector, 37; - discovers Saturn's eighth satellite, 187; - discovers satellite of Uranus, 196; - search for Neptune, 200; - discovers satellite of Neptune, 201; - drawing of Orion nebula, 262 - - Leibnitz, mountains, 117 - - Lemonnier, observations of Uranus, 193 - - Leonid, meteors, 214, 224, 225, 226 - - Leonis [gamma], colour of, 236 - - Leverrier, search for Neptune, 199-201 - - Lewis, revision of Struve's 'Mensuræ Micrometricæ,' 278 - - Lick, 36-inch telescope, 8 - - Light, speed of, 231 - - Light-year, 230 - - Lippershey, claim to invention of telescope, 1 - - Lohrmann, lunar chart of, 122 - - Lowell, rotation of Mercury, 85; - surface of Mercury, 86; - surface of Venus, 95; - rotation of Venus, 96; - 'oases' of Mars, 137, 138; - projections on Mars, 141 - - Lunar observation, 123-125 - - Lyræ [epsilon], double double, 241, 242; - [beta], variable star, 249; - spectroscopic binary, 250 - - Lyra, ring nebula in, 265; - photographs of, 266 - - Lyrid, meteors, 214, 224, 226 - - - M - - M. 35, cluster, 257; - M. 13, number of stars in, 258; - M. 92, 259; - M. 3 and M. 5, variables in, 259; - M. 51, 265 - - MacEwen, drawing of Venus, 94, 95 - - Mädler, heights of lunar mountains, 118; - lunar chart, 122, 124, 128 - - Maginus, 120 - - Magnesium in chromosphere, 73 - - Maia, 256 - - Maintenance of solar light and heat, 78, 79 - - Marius, Simon, description of Andromeda nebula, 264 - - Markwick, Colonel, 117 - - Mars, distance, diameter, rotation, year of, phase of, 130-132; - oppositions of, 130, 131; - polar caps, 132; - canals, 135-137; - dark areas, 133; - 'oases,' 137, 138; - atmosphere of, 139, 140; - projections on terminator, 141; - satellites of, 142-144; - visibility of details of, 144 - - Maunder, Mrs., photographs of coronal streamers, 70 - - Maunder, E. W., adjustment of equatorial, 22, 23; - electrical influence of sun on earth, 63; - 'Astronomy without a Telescope,' 238 - - Mee, Arthur, on amateur observation, 17; - visibility of Cassini's division, 183 - - Melbourne 4-foot reflector, 12 - - Mellor, lunar chart, 124 - - Mendenhall, illustration of sun's distance, 48 - - Mercury, elongations of, 81; - diameter of, 82; - orbit, 83; - bulk, weight, density, reflective power, 83; - phases, 84; - surface, 84; - rotation period, 85; - transits, 87, 88; - anomalous appearances in, 87 - - Merope, 256 - - Merz, Cambridge (U.S.A.), and Pulkowa refractors, 6 - - Messier, lunar crater, 126; - 'the comet ferret,' 219; - catalogue of nebulæ, 258 - - Meteors, 222 _et seq._; - shower of 1833, 223; - of 1866, 224; - Perseid, 214, 224, 225; - Leonid, 214, 224, 225; - Lyrid, 214, 224, 226; - Andromedes, 214, 215, 224, 225; - radiant point, 223, 224; - sporadic, 226; - observation of, 227-229 - - Metius's claim to invention of telescope, 1 - - Milky Way, 239; - clustering of stars towards, 240; - nebulæ in, 240 - - Mira, [omicron] Ceti, 248; - period of, 249 - - Mizar, 240, 241 - - Montaigne, 219 - - Month, lunar and sidereal, 103 - - Moon, size, orbit, area, volume, density, mass, force of gravity, 100; - lunar tides, 101, 102; - phases, 102; - synodic period, 103; - reflective power, 104; - 'old moon in young moon's arms,' 104; - earth's light on, 105; - lunar eclipses, 105, 106; - 'black eclipses,' 105; - Maria of, 109-111; - craters of, 109, 112-114; - mountain ranges, 109, 116-118; - clefts or rills, 109, 118, 119; - ray systems, 109, 120, 121; - atmosphere of, 126; - evidence of change, 127, 128 - - Mountings. See Telescope - - - N - - Nasmyth, willow-leaf structure of solar surface, 57; - lunar clefts, 119; - on lunar ray systems, 121; - and Carpenter, lunar chart, 125; - on powers for lunar observation, 127 - - Nebula of Orion, 261-263; - drawings of, 262; - photographs, 262; - distance of, 263; - of Andromeda, 263, 264; - photographs of, 264; - spectrum, 264 - - Nebulæ, few in neighbourhood of Galaxy, 240; - Messier's catalogue of, 258; - gaseous, 260 _et seq._; - spiral, 263-265; - ring, 265; - planetary, 266; - number of, 267 - - Neison on lunar walled plains, 115, 120; - lunar chart, 125 - - Neptune, 148, 196 _et seq._; - diameter, distance, period, spectrum, satellite of, 201 - - Newall, 25-inch refractor, 8 - - Newcomb on scale of solar operations, 77, 78; - on markings of Venus, 93; - phosphorescence of dark side of Venus, 97; - ratio of stellar increase, 235; - 'Astronomy for Everybody,' 238; - stars in galaxy, 240; - spectroscopic binaries, 248; - on Nova Persei, 254; - on constitution of stars, 268; - apex of solar path, 271 - - Newton, Sir Isaac, invents Newtonian reflector, 10 - - Nice, 30-inch refractor, 8 - - Nichol on M. 13, 258 - - Nilosyrtis, 145 - - Noble, method of observing sun, 67; - visibility of Saturn's satellites, 188 - - Nova Cassiopeiæ, 252; - Coronæ, 252; - Cygni, 253; - Andromedæ, 253; - Ophiuchi, 252; - Aurigæ, 253; - spectrum of, 253; - changes into planetary nebula, 254; - Persei, 254; - photographs of, 254; - nebulosity round, 254; - Geminorum, 255; - colour, spectrum of, 255 - - - O - - Object-glass, treatment of, 19, 20; - testing of, 20-23 - - Observation, methods of solar, 65-67 - - Olbers discovers Pallas and Vesta, 149; - theory of asteroid formation, 150, 152 - - Oppolzer, E. von, discovers variability of Eros, 152 - - Opposition, 130 (note); - of Mars, 130, 131 - - Orion, 237; - great nebula of, 261-263 - - Orionis [theta], observation of, 240; - [iota], naked-eye double, 241; - [theta], multiple star, 243; - [sigma], multiple star, 243 - - - P - - Palisa discovers asteroids, 151 - - Pallas, discovery of, 149; - diameter of, 152 - - Peck, 'Constellations and How to Find Them,' 238; - star-charts, 278 - - Pegasi [kappa], short-period binary, 245 - - Pegasus, 237 - - Perihelion of planets, 131 (note) - - Period, synodic, of moon, 103 - - Perrine discovers Jupiter's sixth and seventh satellites, 167 - - Perseid, meteors, 214, 224, 225 - - Perseus, sword-handle of, 257 - - Petavius cleft, 119 - - Peters discovers asteroids, 151 - - Phillips, Rev. T. E. R., polar cap of Mars, 134; - canals of Mars, 137; - clouds on Mars, 140 - - Phobos satellite of Mars, 143 - - Phosphorescence of dark side of Venus, 97 - - Photosphere, 75 - - Piazzi discovers Ceres, 149 - - Pickering, E. C., number of lucid stars in northern hemisphere, 233; - parallax of Orion nebula, 262 - - Pickering, W. H., on lunar ray systems, 120, 121; - changes on moon, 126; - on polar cap of Mars, 134, 135; - discovers Saturn's ninth and tenth satellites, 187; - photographs Orion nebula, 262 - - Planetary nebulæ, 266; - spectra of, 266; - nebula in Draco, 266 - - Plato, 117, 126 - - Pleiades, number of stars in, 233, 256, 257; - nebula of, 257 - - Pleione, 256 - - Polarizing eye-piece, 66 - - Pollux, 234 - - Præsepe, 256 - - Procellarum Oceanus, 111 - - Proctor, 2; - method of finding Mercury, 82; - on state of Jupiter, 166 - - Proctor on the Saturnian system, 181; - visibility of Cassini's division, 182; - on Challis's search for Neptune, 199; - Star Atlas, 278 - - Procyon, 234 - - Projecting sun's image, 67 - - Projections on terminator of Mars, 141 - - Prominences, 73, 74 - - Ptolemäus, 112 - - Pulkowa, 30-inch refractor, 8, 9 - - - R - - Radiant point of meteors, 223, 224; - number of, 225; - stationary, 229 - - Ranyard Cowper on parallax measures, 231 - - Regulus, 234 - - Reversing layer seen by Young, 74; - spectrum photographed by Shackleton, 75; - depth of, 75 - - Riccioli observes duplicity of [zeta] Ursæ Majoris, 240 - - Rigel, 232, 234; - colour of, 235 - - Ritchey, 5-foot reflector Yerkes Observatory, 12 - - Roche's limit, 186 - - Rosse, Earl of, 6-foot reflector, 12; - colouring of Jupiter, 158, 159; - telescope, resolution of Orion nebula, 260; - drawing of Orion nebula with, 262; - spiral character of M. 51, 265 - - Rotation period of Mercury, 85; - of Venus, 95, 96 - - - S - - Satellite of Venus, question of, 97, 98; - of Mars, 142-144; - of Jupiter, 166-169 - - Saturn, orbit of, sun-heat received by, period of, diameter of, - compression and density of, 172; - features of globe, rotation period, 173; - varying aspects of rings, 178; - measures of rings, 178; - constitution of rings, 179; - satellites of, 186-189; - satellites, transits of, 189 - - Scheiner, construction of refractors, 2 - - Scheiner, Julius, spectrum of Andromeda nebula, 264 - - Schiaparelli, rotation of Mercury, 85; - surface of Mercury, 86; - rotation of Venus, 96; - discovery of Martian canals, 135-137; - connection of comets and meteors, 214, 224 - - Schmidt, lunar map, 114; - observation of comet 1882 (iii.), 217, 218; - observes Nova Cygni, 253 - - Schröter, observations of Venus, 94; - lunar mountains, 118; - rills, 118; - lunar atmosphere, 126 - - Schwabe, discovery of sun-spot period, 61, 62 - - See, Dr., duration of sun's light and heat, 80 - - Serenitatis, Mare, serpentine ridge on, 110, 111; - crossed by ray from Tycho, 120 - - Shackleton photographs spectrum of reversing layer, 75 - - Sidereal month, 103 - - Siderites and siderolites, 227 - - Sinus Iridum, 117 - - Sirius, companion of, discovered, 8; - brightness, 234; - colour, 235; - brilliancy compared with Venus, 90; - with Jupiter, 156 - - Sirsalis cleft, 119 - - Smyth, Admiral, on amateur observers, 18, 19, 45 - - Sodium in chromosphere, 73 - - Solar system, translation of, 269-272 - - South, Sir James, 12-inch telescope, 6 - - Spectroscope, 73, 76 - - Spectroscopic observations of rotation of Venus, 96; - of Martian atmosphere, 140; - investigations of Saturn's rings, 180; - of Uranus, 195 - - Spectrum of reversing layer, 75; - of chromosphere, 73 - - Spencer, Herbert, relation of stars and nebulæ, 267 - - Spica Virginis, 234 - - Stars, distance of, 231; - number of, 232, 233; - magnitudes, 234; - numbers in different magnitudes, 235; - colours, 235-237; - change of colour in, 236, 237; - constellations, 237, 238; - double, 240; - multiple, 243; - binary, 244; - spectroscopic binaries, 245-248; - variable, 248-251; - new or temporary, 251-255; - constitution of, 268 - - Struve, F. G. W., 'Mensuræ Micrometricæ,' 278 - - Struve (Otto) discovers satellite of Uranus, 196; - verifies discovery of Neptune's satellite, 201 - - Sun, size, distance, 47, 48; - rotation period of, 57-59; - methods of observing, 65-67; - atmosphere of, 75; - light and heat of, 78 - - Sun-spots, 49, 50; - rapid changes in, 54, 55; - period of, 62; - zones and variation of latitude of, 62 - - Synodic period, 103 - - Syrtis Major, 145 - - Swift, Dean, satellites of Mars, 142 - - Swift's comet, 214, 224 - - - T - - Taygeta, 256 - - Telescope, invention of, 1, 2; - refracting, 3; - achromatic, 5; - reflecting, 10, 11; - forms of reflecting, Newtonian, Gregorian, Herschelian, - Cassegrain, 10, 11; - mirrors of reflecting, 11, 12; - finders, 23, 24; - mountings of, Altazimuth, 25-28; - equatorial, 30, 31; - house for, 37, 38; - management of, 39, 40; - powers of, 40, 41 - - Tempel's comet, 211 - - Terminator of moon, 107; - of Venus, 94 - - Titius, discovery of Bode's law, 148 - - Turner discovers Nova Geminorum, 255 - - Tycho, 114; - ray-system of, 108, 120, 121; - Brahé observes Nova Cassiopeiæ, 252 - - - U - - Uranus, 190; - distance from sun, period, diameter, visibility, 194; - spectrum and density, 195; - satellites, 196 - - Ursæ Majoris [zeta], duplicity of, 240; - [xi] binary, 244; - spectroscopic binary, 247 - - - V - - Variable stars, 248-251 - - Variation in sun-spot latitude, 62 - - Vega, 234; - colour of, 235; - apex of solar path, 271 - - Venus, diameter, 89; - orbit and elongations, 89; - visibility of, 89, 90; - brilliancy, 90; - reflective power, 90; - phases, 92; - as telescopic object, 93; - atmosphere, 93; - blunting of south horn, 94; - rotation period, 96; - 'phosphorescence' of dark side, 97; - question of satellite of, 97, 98; - transits, 98; - opportunities for observation, 98, 99 - - Vesta, discovery of 149; - diameter of, 152; - reflective power, 152 - - Vienna, 27-inch refractor, 8 - - Vogel, atmosphere of Mars, 140; - discovery of spectroscopic binaries, 245, 246 - - - W - - Washington, 26-inch refractor, 8 - - Watson, asteroid discoveries, 151, 153 - - Webb, Rev. J. W., remarks on telescope, 17; - on amateurs, 18; - on cleaning of eye pieces, 20; - visibility of Saturn's rings, 181; - lunar chart, 124; - 'Celestial Objects,' 124; - colouring of Jupiter, 158; - description of planetary nebula in Draco, 267 - - Williams, A. Stanley, seasonal variations in colour of Jupiter's - belts, 159; - periods of rotation (Jupiter), 163; - rotation of Saturn, 174 - - Wells's comet, 213 - - Wilson, theory of sun-spots, 60, 61 - - Winnecke's comet, 211 - - Wolf, asteroid discoveries, 151 - - - Y - - Yerkes observatory, 40-inch refractor, 8, 9; - 5-foot reflector, 12 - - Young, illustrations from 'The Sun,' 48; - electric influence of sun on earth, 63; - observations of prominences, 74; - of reversing layer, 74 - - - Z - - Zöllner, reflective power of Jupiter, 156 - - - - -THE END - - - - -BILLING AND SONS, LTD., PRINTERS, GUILDFORD. - - - - - Transcriber's Note - - - - - indicates italic print; - = = indicates bold print; - + + indicates Old English font; - ^ or ^{} indicates a superscript. - ° indicates hours (or degrees); - ′ indicates minutes (prime = minutes = feet); - ″ indicates seconds (double prime = seconds = inches). - - Sundry missing or damaged punctuation has been repaired. - - Illustrations (or Plates) which interrupted paragraphs have been - moved to more convenient positions between paragraphs. - - A few words appear in both hyphenatd and unhyphenated versions. - A couple have been corrected, for consistency; the others have - been retained. - - - Page x: 'XI' corrected to 'IX' - - "IX. THE ASTEROIDS 148" - - - Page 4: Corrected 'lengthwas' to 'length was'. - - "... with a glass whose focal length was 212-1/4 feet." - - Page 25: 'familar' corrected to 'familiar'. - - "... or, to use more familiar terms,..." - - Page 90: "... more especially if the - object casting the shadow have a sharply defined - edge,..." - - 'have' is correct, and has been retained (subjunctive after 'if'). - - Page 92: 'firstfruits' corrected to 'first-fruits'. - (OED, and matches 2 other occurrences.) - - "The actual proof of the - existence of these phases was one of the first-fruits - which Galileo gathered by means of his newly - invented telescope." - - Page 109: 'eyeryone' corrected to 'everyone'. - - "... --'the man in the moon'--with which everyone is familiar." - - Page 118: 'of' added - missing at page-turn. - - "They embrace some of the loftiest lunar peaks reaching...." - - Page 128: 'lnnar' corrected to 'lunar'. - - "The lunar night would be lit by our own earth,..." - - Page 157: 'imch' corrected to 'inch'. - - "[Illustration: FIG. 25. - - JUPITER, October 9, 1891, 9.30 p.m.; 3-7/8-inch, power 120.]" - - Page 158: 'eyepiece' corrected to 'eye-piece', to match all the rest. - - "... and a single lens eye-piece giving a power of 36." - - Page 205: removed extraneous 'of'. - - "The nucleus is the only part of [of] a comet's structure" - - Page 209: 'unconsidreed' corrected to 'unconsidered'. - - "... that some unconsidered little patch of haze...." - - Page 240: 'Ursae' corrected to 'Ursæ' to match entries in the Index, - and for consistency. - - "... though Riccioli detected the duplicity of Zeta Ursæ Majoris - (Mizar), in 1650,..." - - Page 248: 'in once and a half times,'. 'once' is as printed (and may - have been intended). As it is part of a quote, it has been retained. - - "'Once in eleven months,' writes Miss Clerke, - 'the star mounts up in about 125 days from below the ninth - to near the third, or even to the second magnitude; then, - after a pause of two or three weeks, drops again to its - former low level in once and a half times, on an average, - the duration of its rise.'" - - Page 256: Page 256: 'Celæno' appears here in the text; - 'Celaeno, 256' is the Index entry. Both are as printed. - - Page 281: '285·9″' corrected to '285·9°' - - "EQUULEUS. - - [Sigma] 2737 or [epsilon] : 20 h. 54 m. + 3° 55′ : 5·7-6·2-7·1 : - 285·9°, 73·8° : 0·53″, 10·43″. Triple with large instruments." - - This follows the pattern of preceding - - DRACO. - - [Sigma] 2323 or 39: 18 h. 22 m. + 58° 45′ : 4·7-7·7-7·1 : 358·2°, - 20·8° : 3·68″, 88·8″. Triple. - - Page 282: '3·80°' corrected to '3·80″' to match pattern. - - "[Sigma] 2161 or [rho] : 17 h. 20 m. + 37° 14′ : 4-5·1 : 314·4° : - 3·80″. 'Gem of a beautiful coronet' (Webb)." - - Page 288: 'Lyrae' corrected to 'Lyræ'. - - "Lyræ [epsilon], double double, 241, 242;" - - Page 291: 'obsering' corrected to 'observing'. - - "methods of observing, 65-67;" - - Page 292: 'elongagations' corrected to 'elongations'. - - "orbit and elongations, 89;" - - Page 292: 'GUIDFORD' corrected to 'GUILDFORD'. - - "BILLING AND SONS, LTD., PRINTERS, GUILDFORD." - - - - - -End of the Project Gutenberg EBook of Through the Telescope, by James Baikie - -*** END OF THIS PROJECT GUTENBERG EBOOK THROUGH THE TELESCOPE *** - -***** This file should be named 54378-8.txt or 54378-8.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/4/3/7/54378/ - -Produced by Chris Curnow, Lesley Halamek and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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