Page:Popular Astronomy - Airy - 1881.djvu/201

Rh of the sun's attraction, exactly similar to the motion of the earth's equator produced by the sun's attraction) moves so as to change the place of its intersection with the ecliptic, performing a complete revolution in 19 years. Therefore, during nearly half of this time, the moon's orbit is inclined to the ecliptic, in the same way as the earth's equator, (but not so much,) as at m n, Figure 51; and therefore the moon's path is



little inclined to the earth's equator. And during nearly the other half of the time, the moon's orbit is inclined to the ecliptic in the way opposite to the earth's equator, as at p q, Figure 51, and the moon's path is much inclined to the earth's equator. In the former state, the moon's force to tilt the earth is small, and the precession goes on slowly; in the latter, the moon's force to tilt the earth is great, and the precession goes on rapidly. The consequence of this is, that there is a very sensible irregularity in the motion of the earth's axis every 19 years, and the place of the Pole is irregularly shifted in different ways through more than 18 seconds. This is lunar nutation. As the right ascensions and North Polar distances of all stars and other objects are affected by this irregular disturbance of the Pole, it is necessary to take it into account in comparing observations made at one time with observations made at another time.

These corrections, then, (precession and nutation,)