194 The Story of the Constellations. [Sess. 



of Aries,' as we now call it, the point where the sun is at 

 the spring equinox, some 66°, corresponding to 4730 years, 

 since the celestial South pole was in the centre of that un- 

 mapped space." 



The phrase ' Precession of the Equinox ' sounds deeply 

 scientific, and though the phenomenon might be difficult to 

 explain readily by means of diagrams, I think it can be easily 

 demonstrated by the use of a small terrestrial globe. A spin- 

 ning body like the earth or a top tends to keep its axis in a 

 definite position in space, and, if no external forces are inter- 

 fering, the axis of spinning will keep parallel to itself however 

 the earth or the top are moved bodily about. When a top is 

 spinning in a perfectly steady manner on an upright axis, if 

 we apply any disturbing force, the top is not upset, but its 

 axis is moved from the perpendicular and turns round and 

 round at a definite angle and according to definite laws ; this 

 is precession. The earth is like the spinning top, and if it 

 were a perfect sphere its axis would always preserve one 

 definite position in space. As we all know, however, the 

 diameter of the earth at its equator is greater than that at 

 the poles, but if the axis were at right angles to the plane of 

 the ecliptic its position would still remain undisturbed ; but 

 because the axis of the earth is inclined 23° 28' from the 

 vertical, then the pull of the sun and the moon on the 

 equatorial protuberance of the earth tends to bring the axis 

 to an upright position ; but because the earth is a spinning 

 body the effect of the pulling force merely results in causing 

 the earth's axis to precess in the same manner as the disturbed 

 spinning top, and, moreover, because the disturbing force is 

 practically constant, precession is always going on. In con- 

 sequence of this movement of the earth's axis round the axis 

 of the ecliptic it follows that it must be successively pointing 

 to a different part of the heavens, and that in time the pole 

 star will recede from its present position and be replaced by 

 other stars which happen to lie near the circle of motion. In 

 about 26,000 years this circuit is accomplished, and meantime 

 the latitude and longitude of the stars in the celestial sphere 

 are constantly changing. But the apparent path of the sun 

 among the stars remains unchanged. The point where the 

 celestial equator cuts the plane of the ecliptic must, therefore, 



