4 A Short History of Astronomy [CH. i 



For the purposes of this book it is not worth while to 

 make any attempt to disentangle from the mass of doubtful 

 tradition and conjectural interpretation of inscriptions, bear- 

 ing on this early astronomy, the few facts which lie embedded 

 therein ; and we may proceed at once to give some account 

 of the astronomical knowledge, other than that already dealt 

 with, which is discovered in the possession of the earliest 

 really historical astronomers the Greeks at the beginning 

 of their scientific history, leaving it an open question what 

 portions of it were derived from Egyptians, Chaldaeans, their 

 own ancestors, or other sources. 



7. If an observer looks at the- stars on any clear night 

 he sees an apparently innumerable * host of them, which 

 seem to lie on a portion of a spherical surface, of which he 

 is the centre. This spherical surface is commonly spoken 

 of as the sky, and is known to astronomy as the celestial 

 sphere. The visible part of this sphere is oounded by the 

 earth, so that only half can be seen at once ; but only the 

 slightest effort of the imagination is required to think of 

 the other half as lying below the earth, and containing other 

 stars, as well as the sun. This sphere -appears to the 

 observer to be very large, though he is incapable of forming 

 any precise estimate of its size, f 



Most of us at the present day have been taught in child- 

 hood that the stars are at different distances, and that this 

 sphere has in consequence no- real existence. The early 

 peoples had no knowledge of this, and for them the celestial 

 sphere really existed,; and was often thought to be a solid 

 sphere of crystal. 



Moreover modern astronomers, as well as ancient, find 

 it convenient for very many purposes to make use of this 

 sphere, though it has no material existence, as a means 

 of representing the directions in which the heavenly bodies 

 are seen and their motions. For all that direct observation 



* In our climate 2,000 is about the greatest number ever visible 

 at once, even to a keen-sighted person. 



f Owing to the greater brightness of the stars overhead they 

 usually seem a little nearer than those near the horizon, and con- 

 sequently the visible portion of the celestial sphere appears to be 

 rather less than a half of a complete sphere. This is, however, of r,o 

 importance, and will for the future be ignoredt 



