io SCIENCE IN GRECO-ROMAN ANTIQUITY 



great religious festivals. The precision of these calcu- 

 lations may appear surprising at first sight, but they 

 can be made with very simple instruments. 1 



Whilst the Egyptians were chiefly interested in the 

 movement of the sun, the Chaldeans studied carefully 

 the movements of the planets, believing human destinies 

 to be bound up with these mysterious movements. 

 Favoured by exceptional atmospheric conditions, they 

 extended their observations very far. They quickly 

 recognized that the planets, the sun and the moon move 

 across practically the same region of the heavens, i.e. 

 the zodiac or plane of the* ecliptic. Therefore " as a 

 result of their astrological ideas, the Chaldeans, instead 

 of referring the positions of the stars or of the planets 

 to the equator, have referred them to the mean circle 

 of the zodiac, and this circumstance has been of great 

 historical importance, because, when the Greeks 

 inherited the Chaldean science, Hipparchus could 

 thereby discover the precession of the equinoxes. It is 

 clear that, if the system of co-ordinates by right 

 ascensions and declinations had then prevailed, the 

 complex law of the variations of these co-ordinates 

 could not have been discovered." 2 It is well known 

 that, by virtue of the precession, everything takes place 

 as if the axis of the earth described a cone of revolution, 

 and took a period of 26,000 years to describe it. The 

 result of this is that the North Pole slowly changes 

 its position in the sky, and that each year the plane of 

 the equator cuts that of the ecliptic at a point slightly 

 different from that at which it cut it the preceding 

 year at the same time of the year. 8 The Greeks, 



1 22 Milhaud, Nouvelles Etudes, p. 89. 



2 Paul Tannery, La Grande Encyclopedie, article : A s- 

 tronomie. 



3 The following fact illustrates this progressive displace- 

 ment. If the Ram occupies the foremost place in the nomen- 

 clature of our zodiacal signs, it is because, at the time when 

 these were depicted, the sun was entering the constellation 



