Page:Popular Astronomy - Airy - 1881.djvu/217

Rh that precession is not uniform. But in our calculations for the application of a correction to the places of the stars, as dependent on precession, it is convenient to begin in the first place by using a precession increasing uniformly with the time. And therefore, inasmuch as precession does not increase uniformly with the time, we are obliged to apply a correction to the precession computed as uniformly increasing, in order to take into account the inequality (both in the place of intersection of the equator with the ecliptic, and in the inclination of the equator to the ecliptic) with which precession goes on at different times; and that correction is the quantity called solar nutation.

There is, however, a much more important want of uniformity called lunar nutation, which I described in this way. The precession produced by the moon depends on the inclination of the moon's orbit to the earth's equator; and this inclination is not uniform. For the moon revolves in an orbit inclined to the ecliptic; and the sun attracts the earth and the moon, and disturbs the motion of the moon with regard to the earth when he acts unequally on the two, nearly in the same way as the sun disturbs the motion of the supposed mountain at the earth's equator; and the effect produced is similar, namely, that the intersection of the moon's orbit with the ecliptic travels backwards; and thus, at periods nearly ten years apart, it is alternately more inclined and less inclined to the earth's equator. And thus for nearly ten years the precession is going on too fast, and for an equal period it is going on too slowly; and thus a considerable inequality is produced in the motion of the intersection of the equator with the ecliptic. Moreover, for nearly ten years the moon's orbit is so inclined that the moon's action tends to diminish the inclination