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General Astronomy CO >- DO g K<OU_1 60556 >m Isj ^g OSMANIA UNIVERSITY LIBRARY Gall No. Accession No. * i Author Title U This book should be returned on or before the date last marked below. GENERAL ASTRONOMY BY H. SPENCER JONES, M.A., B.Sc. H.M. ASTRONOMER AT THE CAPE OF r,oon HOPS. LATE FELLOW OF JESUS COLLEGE, CAMBRIDGE. THIRD IMPRESSION LONDON EDWARD ARNOLD & CO. 1924 [All rights reserved] Made and Printed in Great Britain by Butler & Tanner Ltd , Frame and London PREFACE In this work the Author has endeavoured to cover as wide a field as possible and it has necessarily been somewhat difficult to decide upon what to include and what to omit, especially when dealing with the most recent developments. It is hoped, however, that the book will serve to give the reader a sufficiently complete view of the present state of Astronomy. Mathematics has been almost entirely excluded in order that the volume may appeal to the amateur, no less than the student. Where possible mathematical methods of reasoning have been followed, which it is hoped will prove of value to students of elementary mathematical astronomy in clothing the symbols with more descriptive matter than is given in the text-books on Mathematical Astronomy. Considerable trouble has been taken to procure a repre- sentative series of Astronomical photographs, and these have been obtained from various sources which arc indicated on the plates. The Author desires to express his thanks for permission to reproduce these photographs, and great pains have been taken to ensure accurate reproductions. The Author is greatly indebted to his friends, Mr. D. J. R. Edney and Mr. J. Jackson, both of the Royal Observatory, Greenwich to the former for in the of ; help preparation many of the line diagrams, and to the latter for reading through the proofs in his absence at Christmas Island while in charge of the Expedition to observe the total Eclipse of September 21, 1922. H. SPENCER JONES. CHRISTMAS ISLAND. 1922. CONTENTS CHAP. PAGE I THE CELESTIAL SPHERE ...... 1 II THE EARTH 11 III THE EARTH IN RELATION TO THE SUN ... 34 IV THE MOON 75 V THE SUN 108 VI ECLIPSES AND OCCULTATIONS . .141 VII ASTRONOMICAL INSTRUMENTS . .159 VIII ASTRONOMICAL OBSERVATIONS . .191 IX THE PLANETARY MOTIONS ...... 205 X THE PLANETS 228 XI COMETS AND METEORS ...... 265 XII THE STARS 284 XIII DOUBLE AND VARIABLE STARS ..... 323 XIV THE STELLAR UNIVERSE . .351 INDEX e ........ 387 vii LIST OF PLATES FACING NO. PAGE I a. The Moon. Aged 12 days 80 6. Copernicus II Portion of Moon, showing Plato, Copernicus and the Apennines . .100 III Portion of Moon round Mare Nubium . .106 IV a. Sun-Spot Group. 1920 March 21 . 116 6. Sun-Spot Vortices. 1908 October 7 V Solar and Sun-Spot Spectra with Iron Arc and Furnace Comparison Spectra . .122 VI a. Sun Photographed in K3 Light. 1910 April 11 .130 b. Sun Photographed in Ha Light. 1910 April 11 VII Large Solar Prominence of 1919 May 29 . .132 a. Sobral, Brazil. 1919 May 29 d. Oh. 2m. G.M.T. b. Principe. 1919 May 29 d. 2h. 13m. G.M.T. VIII a. Solar Corona, 1886 August 29 . .134 b. Solar Corona, 1901 May 18 IX Yerkes Observatory 40-inch Refractor . 160 X Mount Wilson Observatory 100- inch Reflector . .170 XI Dominion Astrophysical Observatory 72-inch Reflector . 186 XII a. Mars. 1899 January 30 and February 1 . 238 6. Jupiter. 1891 October 12 c. Saturn. 1894 July 2 XIII a. Mars. 1909 October 5 248 6. Jupiter. 1891 October 12 c. Saturn. 1911 November 19 XIV a. Three Minor Planet Trails 256 6. Halley's Comet and Venus, 1910 XV a. Comet Brookes (1911 c.) 266 6. Comet Delavan (1914) XVI Comet Morehouse 27 a. 1908 November 10 d. 6h. 14m. b. 1908 November 25 d. 5h. 55 rn. XVII a. Parallel Wire Diffraction Grating .... 294 b. Star Images Obtained with Grating XVIII Star Clouds in Sagittarius 330 XIX The Large Magellanic Cloud 352 XX a. The Pleiades 356 b. Tho Orion Nebula XXI a. Great Nebula in Andromeda .... 362 6. Cluster M13 Herculis XXII a. Spiral Nebula, M101. Ursa) Maj. .370 " b. Spiral Nebula, Whirlpool," Canum Venat XXIII Region round the Southern Cross . .376 XXIV Nebulous Region round p Ophiuchi . Froiriispiece viii CHAPTER I THE CELESTIAL SPHERE 1. The Celestial Sphere. Suppose an observer to be viewing the heavens on a clear night. He will see a large number of stars of different degrees of brightness, but he will have no reason to suppose that a given star is nearer to or farther from him than its neighbours. Although their distances may actually vary very considerably, they will appear to be at the same distance. The appearance will be just as though he were surrounded by a vast sphere, to the surface of which the stars are attached, the observer himself being at the centre. After some time he will notice that a change has taken place : some of the stars will have disappeared from view beneath the horizon on one side and new ones will have appeared above the horizon on the opposite side, but he will not notice any change in the relative positions of the stars which remain visible. As a result of careful observation, he might conclude that the whole sphere was turning round an axis, carrying the stars with it, and he would be able to locate very approximately the direction of the axis by noticing which stars appeared not to change their positions. This imaginary sphere, at the centre of which the observer seems to be placed, is known as the Celestial Sphere. Although the sphere has no material existence, the conception is of fundamental importance in astrononry. This is due to the fact that astronomical measurements are not directly concerned with distances but with angles. Two stars are said to be at a distance of 5 apart when the directions to the two stars from the observer make an angle of 5 with one another. This angle can be measured without the observer having any knowledge of the actual linear distances of the two stars. 1 B 2 GENERAL ASTRONOMY The apparent position of any celestial body can therefore be regarded as the point in which the line drawn to it from the observer meets the sphere. It is convenient to suppose the radius of the sphere to be extremely large compared with the distance of the Earth from the Sun, so that wherever the observer may be he can always be regarded as being fixed at the centre of the sphere. The lines from all observers to any given star will then cut the celestial sphere in the same point. All straight lines which are fixed in direction with regard to the celestial sphere may therefore be considered also as being fixed in position, and as cutting the celestial sphere in the same point. 2. It has been mentioned that the whole sphere appears to an observer to be in rotation, carrying the stars with it as though fixed to it. As will be shown in the next chapter, this rotation is only apparent, actually being due to the rotation of the Earth on its axis, the observer being carried with it. It is nevertheless convenient for the present to suppose the observer to be at rest at the centre and the sphere to be in rotation. Since only relative motion is involved this assumption is permissible. The two ends of the axis about which the sphere rotates are called the Poles. They are also the points in which the axis of the Earth produced will cut the celestial sphere. If a star were situated at either of these points, its diurnal motion would be zero. The positions of the poles are not marked by any bright stars, but the bright star Polaris (the Pole-star) is at present only a little more than 1 distant from the northern pole. The soutnern pole is not marked by any conspicuous star. 3. The Zenith is the point on the celestial sphere vertically overhead. The Nadir is the diametrically opposite point. The positions of the zenith and nadir depend upon the position of the observer on the Earth's surface. For an observer at the north pole of the Earth's axis, the north pole of the heavens would be in the zenith. The direction to the nadir is, in all cases, determined by the direction of gravity at the point. THE CELESTIAL SPHERE The Horizon is the great circle of the celestial sphere which has the zenith and nadir as its two poles. The plane of the horizon is the plane passing through the observer at right angles to the direction of gravity. Vertical Circles are great circles passing through the zenith and nadir. They are therefore perpendicular to the horizon. The Meridian is the great circle passing through the zenith and the pole. It meets the horizon in the north and south points. The Prime Vertical is the vertical circle at right angles to the meridian. It meets the horizon in the east and west points. In Fig. 1 is the observer, Z is the zenith, Z' the nadir, P the (north) pole, NESW the horizon, SZPN the meridian, EZWZ' the prime vertical, N, S, E> W the north, south, east and west points respectively. If A denote the position of a star, the great circle ZAQZ' is the vertical circle through the star. FIG. 1. Altitude and Azimuth. 0, Observer Z, The Zenith N, 6', North and South Points Z' The Nadir NEtiW, Observer's Horizon NPZS, The Meridian P, P', The roles EZWZ' t Prime Vertical A, Star SQ or SWNQ, Star's Azimuth ZAQZ', Star's Vertical QA, Stai'a Altitude LAM, Star's Almucantur ZA, Star's Zenith Distance The position of the star can be fixed if its distance from the zenith be known and also the angle between the vertical circle through the star and the meridian, i,e.
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