Jax. 13, 1882.] 



• KNOWLEDGE • 



219 



cuts the plane of her motion (the ecliptic) is constantly 

 shifting in position. Suppose ABDF a view of tlie earth 

 from a point on the northern side of the plane of her 

 motion, tlie North Pole being at P. Then, again, let 



ABDF be a section taken through the earth's centre, C, 

 and let AD be the line in which the plane of the equator, 

 AEDE' (E' being on the farther or southern hemisphere), 



Fig. 2. 



cuts ABDF. Then, if the arrow outside ABDF shows 

 the direction in which the earth moves round her orbit, the 

 small arrows inside show the direction in which this line of 



intersection is constantly travelling. In some thousand 

 years, for instance, it would pass from the position ACD, 

 to the position aCd : tlie equator would have passed to the 

 position acil (one h;df only is shown), the northern pole 

 from the position P to the position p. 



Now, if we consider this motion carefully, paying 

 especial attention to the mov«>ment of the pole, we see 

 that it precisely resembles the reeling of a top. P might 

 be the middle of the top's upper surface, C the peg, CP 

 and Cp two positions of tlie axis of the top as it reeled 

 around C. 



Or, instead of a top, the middle of the body of which is 

 not the centre round which the reeling occurs, imajcine a 

 globe as GEPEH ha\-ing a section such as is shown by the 

 dotted lines in Fig. 2, GCH indicatuig where a conical hole 

 has been cut into the globe, right to the centre. Such a 

 globe set rapidly spimiing in an inclined position (as in the 

 fig.) on the top of a vertical spike like KCL, would be 

 found to reel in a direction contrary to that of its rotation 

 (just as in the case of the earth's precessional reeling), and 

 if a circle EE' were drawn to represent the equator, and a 

 point P marked exactly opposite the small circle GH, 

 this point (or preferably a point p at the end of a polar 

 spike P/<), watched from above, would be seen to move 

 round just as the pole of the earth supposed to be watched 

 from the direction indicated in Fig. 1. 



The motion of the earth, then, bj' which precession is 

 caused, resembles that of a reeling body, like a great 

 spherical top. The period of this reeling motion is, how- 

 ever, so much longer than that of the revolution, that its 

 eflects seem, by comparison, \ery slight. The whole Ufe- 

 tinie of a man may pass, and only tlie astronomer would 

 notice the effects which it had in reality produced. 



Let us see what these effects must be. 



The poles of the heavens are those points on the celestial 

 sphere towards which the axis of the earth is directed, 

 while the celestial equator is the great cii-cle of the celestial 

 sphere lying midway between the poles. The celestial 

 equator, in fact, corresponds with the eartli's equator, inso- 

 much that, if we imagine a straight line drawn from the 

 earth's centre to any point of the terrestrial equator, and 

 prolonged indefinitely outwards, then this line as it was 

 carried round by the earth's rotation, would sweep out the 

 circle which we call the celestial equator. 



Now, since the earth is reeling like a gigantic top, its 

 axis moves — ^just as the axis of such a top moves — over the 

 surface of a gigantic cone. Neglecting — as relatively insig- 

 nificant — the range of the earth round her orbit, we may 

 regard the earth's centre as the apex of tills gigantic cone. 

 The points in wliich a line perpendicular to the plane of 

 the earths motion meet the imaginary celestial sphere, or 

 the poles of the equator, are those towards which the axis 

 of the great precessional cone is directed. Around these 

 points the poles of the heavens revolve in two small circles, 

 the distance of each pole from the pole of the ecliptic round 

 which it rotates corresponding to the inclination of the 

 earth's axis, or in arc to about 23|°. The equator moves 

 coiTespondingly, and a complete circuit is accomplished by 

 each pole in rather less than 2.'), 900 years. 



Next week I propose to consider more particularly the 

 efiects of this reeling motion, so far as it affects the appa- 

 rent position of the heavenly bodies — showing in particular 

 how it has affected the positions of certain constellations. 

 Then we have to consider how the seasons are affected by 

 the change. Lastly, it will be well to consider how the 

 reeling motion is produced, and how certain peculiarities in 

 its progi'ess are brought about. 



In the meantime, I would invite the student to notice 

 that very interesting illustrations of the earth's precessional 



