PRECESSION: AND THE PYRAMIDS 45 1 



ciple which underlies it will be found well to repay the trouble it costs. 

 For to master it is to put one's self in possession of a celestial time-piece 

 one hour of which is 25,700 years and whose minute hand traces out the 

 lapse of centuries upon the dial of the sky. Not only as a clock is it a 

 possibility, but as we shall see it has been actually so used unconsciously 

 by man in days gone by, and his readings of it recorded in lithographs 

 still legible to-day. 



To understand its working we must in thought get off the earth and 

 see that body from without. We should then perceive our globe as a 

 mammoth top spinning in the sky as it moves along in its orbit. The 

 spin gives us our days, and as the equator is tilted to the path the pull 

 of the sun is all the time trying to bring the one to coincidence with the 

 other. If the earth were perfectly round there would be nothing for the 

 sun to grasp and the spin would remain unchanged both in amount and 

 in direction. The earth, however, is not round, but spheroidal, bulging 

 at the equator. There is thus a handle, two handles in fact, which may 

 be used for turning it. Now suppose the earth in the position of its 

 summer solstice when its axis is tilted away from the sun in a plane con- 

 taining the sun and a perpendicular to its orbit. The attraction of the 

 sun tends to rotate it in this plane. Meanwhile the earth is spinning 

 round its own polar axis at right angles to that direction. We have 

 then to compound two spins about axes perpendicular to one another. 

 Curious as it may seem, the result is not to pull the bulge down into 

 the orbit plane, but to make it back bodily along the ecliptic. It is as if 

 the earth resented the sun's attempt to right it and with almost human 

 perversity went the other way. Indeed one may feel the obstinacy of the 

 thing by appropriately pressing on a gyroscope, when instead of yield- 

 ing it will promptly buck against you with a force suggestive of intent. 

 The greater your shove the faster its opposing speed. Now with the 

 earth the pull of the sun is feeble compared with the great moment of 

 momentum of the earth, and in consequence the motion of the earth's 

 pole is most leisurely. The backing of the equinoxes to meet the sun is 

 but 50". 23 a year and the time it takes to complete the circuit of the 

 zodiac 25,700 years. 



The figure of the earth exposing it to such action, any body may set 

 it spinning and all do. The amount and direction of the spin depend 

 on the position, of the disturber. Because the greatest, by precession is 

 usually meant the luni-solar precession, caused by the combined action 

 of the sun and moon. In this the moon is about two and one eighth 

 times as effective as the sun in spite of its relative insignificance because 

 of being so much nearer. The amount of the precession depends among 

 other factors upon the cosine of the obliquity of the ecliptic. The greater 

 the obliquity, therefore, the less the precession. At the present moment 

 the obliquity is diminishing and the precession increasing. This will 

 continue to be the case for several thousand years. 



