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BOOK IV.

MISCELLANEOUS ASTRONOMICALPHENOMENA.

CHAPTER I.

Variation in the Obliquity of the Ecliptic . — Precession . — Its Value .—Its Physical Cause. — Correction for Precession. — History of itsDiscovery. — Nutation. — Herschets Definition of it. — Connectionbetween Precession and Nutation.

Variation in the Obliquity of the Ecliptic . — Although it issufficiently near for all general purposes to consider the in-clination of the plane of the ecliptic as invariable; yet this isnot strictly the case, inasmuch as it is subject to a small but ap-preciable change of about 48" per century. This phenomenonhas long been known to astronomers, on account of the increaseit gives rise to, in the latitude of all stars in some situations,and corresponding decrease in the opposite regions. Itseffect at the present time is to diminish the inclination of thetwo planes of the equator and the ecliptic to each other ; butthis diminution will not go on beyond certain very moderatelimits, after which it will again increase, and thus oscillatebackwards and forwards through an arc of 1 0 21': the timeoccupied in one oscillation being about 10,000 years. Oneeffect of this variation of the plane of the ecliptic—that whichcauses its nodes on a fixed plane to change—is associated withthe phenomena of the precession of the equinoxes, and undis-tinguishable from it, except in theory. 1

1 The inclination of the ecliptic for the epoch of January 1, i860, is23 0 27' zy$ 6 ".