ASTRONOMY OF TO-DAY THE TOTAL ECLIPSE OF THE SUN OF AUGUST 30TH, 1905. The Corona; from a water-colour sketch, made at Burgos, in Spain, during the total phase, by the French Artist, Mdlle. ANDRÉE MOCH. ASTRONOMY OF TO-DAY A POPULAR INTRODUCTION IN NON-TECHNICAL LANGUAGE By CECIL G. DOLMAGE, M.A., LL.D., D.C.L. Fellow of the Royal Astronomical Society; Member of the British Astronomical Association; Member of the Astronomical Society of the Pacific; Membre de la Société Astronomique de France; Membre de la Société Belge d'Astronomie With a Frontispiece in Colour and 45 Illustrations & Diagrams THIRD EDITION LONDON SEELEY AND CO. LIMITED 38 GREAT RUSSELL STREET 1910 PREFACE THE object of this book is to give an account of the science of Astronomy, as it is known at the present day, in a manner acceptable to the general reader. It is too often supposed that it is impossible to acquire any useful knowledge of Astronomy without much laborious study, and without adventuring into quite a new world of thought. The reasoning applied to the study of the celestial orbs is, however, of no different order from that which is employed in the affairs of everyday life. The science of mathematics is perhaps responsible for the idea that some kind of difference does exist; but mathematical processes are, in effect, no more than ordinary logic in concentrated form, the shorthand of reasoning, so to speak. I have attempted in the following pages to take the main facts and theories of Astronomy out of those mathematical forms which repel the general reader, and to present them in the ordinary language of our workaday world. The few diagrams introduced are altogether supplementary, and are not connected with the text by any wearying cross-references. Each diagram is complete in itself, being intended to serve as a pictorial aid, in case the wording of the text should not have perfectly conveyed the desired meaning. The full page illustrations are also described as adequately as possible at the foot of each. As to the coloured frontispiece, this must be placed in a category by itself. It is the work of the artist as distinct from the scientist. The book itself contains incidentally a good deal of matter concerned with the Astronomy of the past, the introduction of which has been found necessary in order to make clearer the Astronomy of our time. It would be quite impossible for me to enumerate here the many sources from which information has been drawn. But I acknowledge my especial indebtedness to Professor F.R. Moulton's Introduction to Astronomy (Macmillan, 1906), to the works on Eclipses of the late Rev. S.J. Johnson and of Mr. W.T. Lynn, and to the excellent Journals of the British Astronomical Association. Further, for those grand questions concerned with the Stellar Universe at large, I owe a very deep debt to the writings of the famous American astronomer, Professor Simon Newcomb, and of our own countryman, Mr. John Ellard Gore; to the latter of whom I am under an additional obligation for much valuable information privately rendered. In my search for suitable illustrations, I have been greatly aided by the kindly advice of Mr. W. H. Wesley, the Assistant Secretary of the Royal Astronomical Society. To those who have been so good as to permit me to reproduce pictures and photographs, I desire to record my best thanks as follows:—To the French Artist, Mdlle. Andrée Moch; to the Astronomer Royal; to Sir David Gill, K.C.B., LL.D., F.R.S.; to the Council of the Royal Astronomical Society; to Professor E.B. Frost, Director of the Yerkes Observatory; to M.P. Puiseux, of the Paris Observatory; to Dr. Max Wolf, of Heidelberg; to Professor Percival Lowell; to the Rev. Theodore E.R. Phillips, M.A., F.R.A.S.; to Mr. W.H. Wesley; to the Warner and Swasey Co., of Cleveland, Ohio, U.S.A.; to the publishers of Knowledge, and to Messrs. Sampson, Low & Co. For permission to reproduce the beautiful photograph of the Spiral Nebula in Canes Venatici (Plate XXII.), I am indebted to the distinguished astronomer, the late Dr. W.E. Wilson, D.Sc., F.R.S., whose untimely death, I regret to state, occurred in the early part of this year. Finally, my best thanks are due to Mr. John Ellard Gore, F.R.A.S., M.R.I.A., to Mr. W.H. Wesley, and to Mr. John Butler Burke, M.A., of Cambridge, for their kindness in reading the proof-sheets. CECIL G. DOLMAGE. LONDON, S.W., August 4, 1908. PREFATORY NOTE TO THE SECOND EDITION THE author of this book lived only long enough to hear of the favour with which it had been received, and to make a few corrections in view of the second edition which it has so soon reached. December 1908. CONTENTS CHAPTER I PAGE THE ANCIENT VIEW 17 CHAPTER II THE MODERN VIEW 20 CHAPTER III THE SOLAR SYSTEM 29 CHAPTER IV CELESTIAL MECHANISM 38 CHAPTER V CELESTIAL DISTANCES 46 CHAPTER VI CELESTIAL MEASUREMENT 55 CHAPTER VII ECLIPSES AND KINDRED PHENOMENA 61 CHAPTER VIII FAMOUS ECLIPSES OF THE SUN 83 CHAPTER IX FAMOUS ECLIPSES OF THE MOON 101 CHAPTER X THE GROWTH OF OBSERVATION 105 CHAPTER XI SPECTRUM ANALYSIS 121 CHAPTER XII THE SUN 127 CHAPTER XIII THE SUN—continued 134 CHAPTER XIV THE INFERIOR PLANETS 146 CHAPTER XV THE EARTH 158 CHAPTER XVI THE MOON 183 CHAPTER XVII THE SUPERIOR PLANETS 209 CHAPTER XVIII THE SUPERIOR PLANETS—continued 229 CHAPTER XIX COMETS 247 CHAPTER XX REMARKABLE COMETS 259 CHAPTER XXI METEORS OR SHOOTING STARS 266 CHAPTER XXII THE STARS 278 CHAPTER XXIII THE STARS—continued 287 CHAPTER XXIV SYSTEMS OF STARS 300 CHAPTER XXV THE STELLAR UNIVERSE 319 CHAPTER XXVI THE STELLAR UNIVERSE—continued 329 CHAPTER XXVII THE BEGINNING OF THINGS 333 CHAPTER XXVIII THE END OF THINGS 342 INDEX 351 LIST OF ILLUSTRATIONS LIST OF PLATES PLATE THE TOTAL ECLIPSE OF THE SUN OF AUGUST 30, 1905 Frontispiece I. THE TOTAL ECLIPSE OF THE SUN OF MAY 17, 1882 To face 96 page II. GREAT TELESCOPE OF HEVELIUS " " 110 III. A TUBELESS, OR "AERIAL" TELESCOPE " " 112 IV. THE GREAT YERKES TELESCOPE " " 118 V. THE SUN, SHOWING SEVERAL GROUPS OF SPOTS " " 134 VI. PHOTOGRAPH OF A SUNSPOT " " 136 VII. FORMS OF THE SOLAR CORONA AT THE EPOCHS OF SUNSPOT " " 142 MAXIMUM AND SUNSPOT MINIMUM RESPECTIVELY. (A) THE TOTAL ECLIPSE OF THE SUN OF DECEMBER 22, 1870. (B) THE TOTAL ECLIPSE OF THE SUN OF MAY 28, 1900 VIII. THE MOON " " 196 IX. MAP OF THE MOON, SHOWING THE PRINCIPAL "CRATERS," " " 198 MOUNTAIN RANGES AND "SEAS" X. ONE OF THE MOST INTERESTING REGIONS ON THE MOON " " 200 XI. THE MOON (SHOWING SYSTEMS OF "RAYS") " " 204 XII. A MAP OF THE PLANET MARS " " 216 XIII. MINOR PLANET TRAILS " " 226 XIV. THE PLANET JUPITER " " 230 XV. THE PLANET SATURN " " 236 XVI. EARLY REPRESENTATIONS OF SATURN " " 242 XVII. DONATI'S COMET " " 256 XVIII. DANIEL'S COMET OF 1907 " " 258 XIX. THE SKY AROUND THE NORTH POLE " " 292 XX. ORION AND HIS NEIGHBOURS " " 296 XXI. THE GREAT GLOBULAR CLUSTER IN THE SOUTHERN CONSTELLATION " " 306 OF CENTAURUS XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANES VENATICI " " 314 XXIII. THE GREAT NEBULA IN THE CONSTELLATION OF ANDROMEDA " " 316 XXIV. THE GREAT NEBULA IN THE CONSTELLATION OF ORION " " 318 LIST OF DIAGRAMS FIG. PAGE 1. THE PTOLEMAIC IDEA OF THE UNIVERSE 19 2. THE COPERNICAN THEORY OF THE SOLAR SYSTEM 21 3. TOTAL AND PARTIAL ECLIPSES OF THE MOON 64 4. TOTAL AND PARTIAL ECLIPSES OF THE SUN 67 5. "BAILY'S BEADS" 70 6. MAP OF THE WORLD ON MERCATOR'S PROJECTION, SHOWING A PORTION OF THE PROGRESS OF 81 THE TOTAL SOLAR ECLIPSE OF AUGUST 30, 1905, ACROSS THE SURFACE OF THE EARTH 7. THE "RING WITH WINGS" 87 8. THE VARIOUS TYPES OF TELESCOPE 113 9. THE SOLAR SPECTRUM 123 10. A SECTION THROUGH THE SUN, SHOWING HOW THE PROMINENCES RISE FROM THE 131 CHROMOSPHERE 11. ORBIT AND PHASES OF AN INFERIOR PLANET 148 12. THE "BLACK DROP" 153 13. SUMMER AND WINTER 176 14. ORBIT AND PHASES OF THE MOON 184 15. THE ROTATION OF THE MOON ON HER AXIS 187 16. LAPLACE'S "PERENNIAL FULL MOON" 191 17. ILLUSTRATING THE AUTHOR'S EXPLANATION OF THE APPARENT ENLARGEMENT OF CELESTIAL 195 OBJECTS 18. SHOWING HOW THE TAIL OF A COMET IS DIRECTED AWAY FROM THE SUN 248 19. THE COMET OF 1066, AS REPRESENTED IN THE BAYEUX TAPESTRY 263 20. PASSAGE OF THE EARTH THROUGH THE THICKEST PORTION OF A METEOR SWARM 269 ASTRONOMY OF TO-DAY CHAPTER I THE ANCIENT VIEW IT is never safe, as we know, to judge by appearances, and this is perhaps more true of astronomy than of anything else. For instance, the idea which one would most naturally form of the earth and heaven is that the solid earth on which we live and move extends to a great distance in every direction, and that the heaven is an immense dome upon the inner surface of which the stars are fixed. Such must needs have been the idea of the universe held by men in the earliest times. In their view the earth was of paramount importance. The sun and moon were mere lamps for the day and for the night; and these, if not gods themselves, were at any rate under the charge of special deities, whose task it was to guide their motions across the vaulted sky. Little by little, however, this simple estimate of nature began to be overturned.