. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. greatest. The result is that thepole of the heavens has a small oscillatory motion about itsmean place. This_motion is called nutation. If the effect ofnutation could be separated from that of precession, the polewould be found to move in a very minute ellipse, having amajor axis of , and a minor axis of 13. 7, the period of onerevolution in this ellipse being verynearly years. Since,how-ever, these two perturbations co-exist, the result is that the poleof the heavens revolves about the pole of
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. greatest. The result is that thepole of the heavens has a small oscillatory motion about itsmean place. This_motion is called nutation. If the effect ofnutation could be separated from that of precession, the polewould be found to move in a very minute ellipse, having amajor axis of , and a minor axis of 13. 7, the period of onerevolution in this ellipse being verynearly years. Since,how-ever, these two perturbations co-exist, the result is that the poleof the heavens revolves about the pole of the ecliptic in an undu-lating curve, as shown in Fig. 48. Practi-cally, nutation depends upon the moon only ;the period of years being that of therevolution of the moons nodes (Art. 128). 121. Change in the Obliquity of the Eclip-tic.—It was stated in Art. 118 that the lati-tudes of the stars were found to vary fromyear to year by a very minute amount. Thischange in the latitudes is due to a change in the position ofthe plane of the ecliptic, involving a change in the obliquity. 113 ABERRATION. of the ecliptic. The obliquity of the ecliptic in 1889 was23° 27 13: and the annual amount of diminution to which itis now subject is Mathematical investigations show thatafter certain moderate limits have been reached this diminutionwill cease, and the obliquity will begin to increase. - The arcthrough which the obliquity oscillates is about 1° 21, and thetime of one oscillation is about ten thousand years. 122. Advance of the Line of Apsides.—The line connectingthe earths perihelion and aphelion is called the line of line revolves from west to east, with an annual rate of *a perturbation due to the attraction exerted on the earth by thesuperior planets. The time in which the earth moves from peri-nelion to perihelion is called the anomalistie year (from anomaly,Art. 98). It is evidently longer than the sidereal year, and isfound to contain 365d. 6h.
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