MANUALS OF ELEMENTARY SCIENCE. ASTRONOMY. BY W. H. M. CHRISTIE, M.A., F.R.A.S., Fellow of Trin. Coll. Cambridge, and Chief Assistant PUBLISHED UNDER THE DIRECTION OF THE COMMITTEE OF GENERAL LITERATURE AND EDUCATION, CHRISTIAN KNOWLEDGE. LONDON: SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE; 77, GREAT QUEEN STREET, LINCOLN'S INN FIELDS; AND BY ALL BOOKSELLERS. PREFACE. ALTHOUGH this little Manual is quite elementary in its character, my aim has been to make it precise rather than popular in its language, and I have therefore entered on certain points which will probably present some difficulty to the beginner. I trust, however, that a reader of intelligence will be able, with a little attention, to follow the explanations given, brief though they are. In order to avoid circumlocution, I have found it necessary to introduce a few technical terms, which are explained in the text, and of which the reader will do well to grasp the meaning thoroughly, as any confusion on such points will add greatly to his difficulties afterwards. In every case I have taken the numerical data given in this little book from the latest original sources, putting them into the form which appeared to me to give the clearest idea of the relations of the Solar and Sidereal Systems. With this object I have avoided, as far as possible, the use of large numbers, which simply bewilder the imagination; and have endeavoured to express the proportions of the quantities involved, choosing the unit best adapted to the immediate object in view, and omitting all unnecessary figures. It is only by gradual steps that man can rise to the conception of the distances of the stars. Some of the questions discussed are still to a certain extent open, but they are among the most interesting in Astronomy; and my aim has been not only to give facts but also to suggest further enquiry on the part of the reader-and though some few results are not altogether free from doubt, I trust that he may have but little to unlearn when he enters more deeply into the subject. BLACKHEATH, May, 1875. W. H. M. CHRISTIE. CONTENTS. ELEMENTARY IDEAS. Apparent Motions of the Stars, p. 4-Rotation of the Earth, 6- Apparent Motion of the Sun among the Stars, p. 18-Right Ascen- sion and Declination, 22-Solar and Sidereal Time, 23-Equation of Time, 25-Real Movement of the Earth, 26-The Seasons, 27- Motion of the Moon, p. 39 Her Phases, 42-Eclipses, 45-Harvest Apparent Motions with reference to the Sun of Inferior Planets, p. 57-of Superior Planets, 60-Apparent Motions with reference to the Stars, 64-Transits of Mercury and Venus, 67-Their use for finding the distance of the Sun, 68-Other Methods, 72-Kepler's Laws, 75-Gravitation, 77-Tides, 79-Masses of the Sun and Moon compared with that of the Earth, 81-Mass of the Earth, 83- Bode's Law of Distances, 84-Appearance of the different Planets, Apparent and Real Motions, p. 91-Their Aspects, 94-Their Their Number, Magnitudes, p. 105-The Constellations, 107-The Milky Way, 112-Clusters and Nebulæ, 112-Laplace's Nebular Theory, 115-Distances of some of the Stars, 117-Proper Motions, 119-Motion of the Solar System, 119-Double Stars, 120-Physi- cal Constitution of Stars, 122-Temporary and Variable Stars, ELEMENTARY ASTRONOMY. CHAPTER I. THE most casual observation suffices to show us that the Sun rises in the east, gets gradually higher till it reaches its greatest elevation in the south,* when it begins to sink, finally setting in the west only to rise again in the east and perform the same round next day. If we turn to the stars at night we shall find something similar, but on examining more closely it will be seen that some of them are longer above the horizon than others, and that certain stars never set but remain constantly in view. If we watch these latter we shall find that there is one star which hardly changes its position, keeping constantly at nearly the same height, whilst the other stars appear to circulate round it at different distances. This star is called the Pole-star, for a reason which will presently be apparent. Now amidst all this diversity of movement there is one feature which is common to all, it is that every star returns to the same position after the lapse of twentyfour hours, and in this interval it will have described a circle. This latter fact may be rudely verified as follows:-Joint two rods together like a pair of compasses and rest one of them in forks so placed that it points to the Pole-star, then if the other rod be pointed to any other star it may be made to follow the star in its course by simply turning the first rod round in its forks, without altering the opening of the joint. * The reader is supposed to be in Europe, or other northern countries. For southern countries it will be sufficient to substitute south for north. |