. Astronomy for high schools and colleges . pound is less than half a kilogramme. CHAPTER I. THE EELATIOX OF THE EARTH TO THEHEAVENS. § 1. THE EARTH. The follo\ving are fundamental propositions of modernastronomy : I. The ea/rth is approximately a sphere,—Besides theproofs of this proposition familiarto the student of geography, weliave the fact that portions of theearths surface visible from ele-vated positions appear to bebounded by circles. This propertybelongs only to the surface of asphere. II. Tlie directions which we callup and down are not invariable^hut are always toward or from thece
. Astronomy for high schools and colleges . pound is less than half a kilogramme. CHAPTER I. THE EELATIOX OF THE EARTH TO THEHEAVENS. § 1. THE EARTH. The follo\ving are fundamental propositions of modernastronomy : I. The ea/rth is approximately a sphere,—Besides theproofs of this proposition familiarto the student of geography, weliave the fact that portions of theearths surface visible from ele-vated positions appear to bebounded by circles. This propertybelongs only to the surface of asphere. II. Tlie directions which we callup and down are not invariable^hut are always toward or from thecentre of the earth. — Therefore,they are different at different pointsof the earths surface. HI. The earth is completely isolated in space.—Themost obvious proof of this is that men have visited nearlyevery part of its surface without finding any communica-tion with other bodies. IV. The earth is one of a vast nuniber of globularbodies^ familiarly linown as stars and planets^ movingaccording to certain laws and separated by distances. Fig. 2. Illustrating the fact that theportions of the earth visiblefrom elevated positions, 5. S,S, etc., are bounded by circles. 10 ASTRONOMY. so immense^ that the raagnitudes of the bodies themselvesare insignificant in comparison. The first conception the student of astronomy has toform is that of living on the surface of a spherical earth,which, although it seems of immense size to him, is reallybut a point in comparison with the distances which sepa-rate him from the stars which he sees in the heavens. § 2. THE CELESTIAL SPHERE. The directions of the heavenly bodies are defined bytheir positions on an imaginary sphere called the celestialsphere. The celestial sphere, is an imaginary hollow sphere, hav-ing the earth in its centre, and of dimensions so great thatthe earth may be considered a point in comparison. One half of the celestial sphere is represented by thevault above our heads, commonly called the sky, in whichthe heavenly bodi
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