16 OUR PHYSICAL WORLD 



cuts the ecliptic a trifle before it would if this pull of the moon 

 were not acting on the bulge. 



It is evident, then, that the equinoctial point is ever occurring 

 a trifle earlier than it would occur if it were not for this action of 

 moon and sun. This phenomenon is known as the precession 

 of the equinoxes. As a result, the North Pole of the earth's 

 axis does not point continually to the same spot in the celestial 

 sphere, but makes a rotation once in 25,868 years. As a matter 

 of fact, the motion is not as simple as described, for the moon, sun, 

 and earth are constantly changing their relative positions, so it 

 is quite irregular, though entirely predictable when the move- 

 ments and consequent relations of the three bodies are known. 

 It is a matter of relatively simple calculation to determine the 

 point in the sky to which the Pole pointed thousands of years 

 ago or will point in the future. 



Some authorities claim that when the great pyramid at 

 Cheops, Egypt, was built it was so oriented that a narrow pas- 

 sageway over 300 feet long pointed to the star that was then the 

 polestar, alpha of the constellation Draco. This pyramid was 

 located quite exactly on 30 north latitude. Certain of its 

 dimensions apparently record the length of the year, the period 

 of the precession of the equinoxes, and other astronomical data, 

 so that it really is a record of quite wonderful astronomical 

 knowledge on the part of its builders. 



As one looks up into the starry skies on any clear night, it 

 seems as if the stars were as numerous as the sand grains on 

 the seashore. Yet, as a matter of fact, there are only about 

 2,000 visible to the average eye at any one time. And even if 

 you should watch the heavens year in and year out from points 

 both north and south of the Equator, you would see only 4,000 

 to 6,000. These stars differ in brilliancy from the brightest 

 one, Sirius, down to those that are just visible to the naked eye. 

 They are consequently said to differ in magnitude, sixth- 

 magnitude stars being those that are only just visible, first- 

 magnitude stars those that are most brilliant. This latter group 



