24 A Short History of Astronomy [CH. n. 



Gregory XIII. introduced therefore, in 1582, a slight change; 

 ten days were omitted from that year, and it was arranged 

 to omit for the future three leap-years in four centuries 

 (viz. in 1700, 1800, 1900, 2100, etc., the years 1600, 2000, 

 2400, etc., remaining leap-years). The Gregorian Calendar, 

 or New Style, as it was commonly called, was not adopted 

 in England till 1752, when n days had to be omitted; 

 and has not yet been adopted in Russia and Greece, 

 the dates there being now 12 days behind those of 

 Western Europe. 



23. While their oriental predecessors had confined 

 themselves chiefly to astronomical observations, the earlier 

 Greek philosophers appear to have made next to no 

 observations of importance, and to have been far more 

 interested in inquiring into causes of phenomena. Thales, 

 the founder of the Ionian school, was credited by later 

 writers with the introduction of Egyptian astronomy into 

 Greece, at about the end of the 7th century B.C. ; but both 

 Thales and the majority of his immediate successors appear 

 to have added little or nothing to astronomy, except some 

 rather vague speculations as to the form of the earth 

 and its relation to the rest of the world. On the other 

 hand, some real progress seems to have been made by 

 ^L Pythagoras* and his followers. Pythagoras taught that 

 the earth, in common with the heavenly bodies, is a sphere, 

 and that it rests without requiring support in the middle 

 of the universe. Whether he had any real evidence in 

 support of these views is doubtful, but it is at any rate 

 a reasonable conjecture that he knew the moon to be 

 bright because the sun shines on it, and the phases to 

 be caused by the greater or less amount of the illuminated 

 half turned towards us ; and the curved form of the 

 boundary between the bright and dark portions of the 

 moon was correctly interpreted by him as evidence that 

 the moon was spherical, and not a flat disc, as it appears 

 at first sight. Analogy would then probably suggest that the 

 earth also was spherical. However this may be, the belief 

 in the spherical form of the earth never disappeared from 



* We have little definite knowledge of his life. He was born in 

 the earlier part of the 6th century B.C., and died at the end of the 

 e century or beginning of the next. 



