THE PREDECESSORS OF COPERNICUS. 329 



calculations of phenomena could be made, not a physical explanation 

 based on mechanical laws. 



The problems relating to the motions of the heavenly bodies are 

 more complex and must be considered somewhat in detail. It is 

 necessary to describe the observed phenomena for each body separately, 

 and to adopt a system which will explain every phenomenon and 

 appearance in turn. 



The Moon. — The facts of observation, familiar to us all, are that 

 the new moon sets in the west about sunset, and that on every suc- 

 ceeding night the moon sets at a later hour. It, therefore, moves to 

 the east among the stars from night to night, which can readily be 

 verified by observation. If the moon is near the stars of Orion on 

 one night, it will be found many degrees to the east of them on the 

 night following. It sets later and later every night throughout the 

 month. If it is in the same longitude as the stars of Orion on any 

 one day, it will be again in that longitude about a month (27 days) 

 later (more exactly, 27<^ 7^ 43°^ 11^.5). It has moved through the 

 whole circuit of the heavens, 360°, in 27 days. Ptolemy explained 

 the phenomena, as we explain them to-day, by asserting that the moon 

 revolves in an orbit, about the earth as a center, making a complete 

 revolution among the stars (from one star back to the same star again) 

 in 27 days. Its motion among the stars is always forward — always 

 from west to east. 



The Sun. — The observed phenomena with regard to the sun are of 

 the same nature. If the sun rises at the same time as the bright 

 star Sirius on a particular day of the year, on the next day it will rise 

 later than Sirius. It has, therefore, moved a certain distance (about 

 one degree) eastwardly during that day. On the next following day 

 it will have moved about two degrees east of Sirius and will rise 

 correspondingly later; and so on for each succeeding day. After 180 

 days (six months) the sun will have moved about 180 degrees to the 

 east of Sirius. Sirius will be visible on the meridian at midnight, 

 (when the sun is 180° away from the meridian). At the end of 365 

 days (more exactly, 365.2564 days) the sun will have moved eastward 

 through 360° and will again rise at the same moment as Sirius. The 

 sun, then, appears to move eastwardly among the stars (from one star 

 back to the same star again) once in 36514 days. At different times 

 of the year it is among different groups of stars, and it is for this 

 reason, therefore, that we see different groups of stars at different 

 seasons of the year. Orion is visible in the winter skies, Scorpio in 

 the summer, because Orion and Scorpio are 180 degrees apart in 

 longitude. Ptolemy's explanation of all these phenomena is that 

 the sun moves about the earth in a circular orbit at such a rate as to 

 make a complete revolution in 3651,4 days. The explanation of 

 Copernicus is that the earth revolves about the sun in the same period. 



