Elements of astronomy ..with explanatory notes, and questions for examination . he moon andplanets. Explain nutation. Solar. 9* 102 THE EARTH VIEWED ASTRONOMICALLY. ing to nothing, when it is at the equator, namely, at thetime of the equinoxes. 168. A similar inequality likewise exists in the moonsaction, arising from a like cause. There is, however,this difference. The variations in the solar force alloccur in the space of one year, those of the lunar withinthe period of about eighteen and a half years. 169. This variation of force produces the 7iutation,and the pole of the equator, if free f
Elements of astronomy ..with explanatory notes, and questions for examination . he moon andplanets. Explain nutation. Solar. 9* 102 THE EARTH VIEWED ASTRONOMICALLY. ing to nothing, when it is at the equator, namely, at thetime of the equinoxes. 168. A similar inequality likewise exists in the moonsaction, arising from a like cause. There is, however,this difference. The variations in the solar force alloccur in the space of one year, those of the lunar withinthe period of about eighteen and a half years. 169. This variation of force produces the 7iutation,and the pole of the equator, if free from any other in-fluence, would, in virtue of this, describe among thestars a small ellipse in a period comprising about eighteenand a half years; the longer axis of the ellipse beingabout 18,5, and the shorter, 13,7. The centre of theellipse lies in the circumference of the circle which wouldbe described by the pole of the equator round the poleof the ecliptic, if the force producing the precessionnever varied. 170. This subject is illustrated in Fig. 33, where the FIG. NUTATION. circumference, B, of the large circle, represents the paththat the pole of the equator, Pl would describe aroundthe pole of the ecliptic, P, if precession alone existed; andA, is the small ellipse which P1 would describe if nutation Lunar. Within what period of time do the solnr variations occur? Within whatthe lunar? What kind of figure does the pole of the equator describe in consequenceof nutation ? What is the extent of this ellipse ? Where does its centre lie ? illustratethis subject by figures 33 and 34. NUTATION. 103 occurred without precession. Now, since these motionsco-exist, it is evident that neither a perfect circle nor acomplete ellipse will be described by the pole, P1; butat one time it will be outside the circumference, B, andat another within, revolving about P all the while. Itwill, therefore, actually describe a circular waving path,like that exhibited in Fig. 34, where P is t
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