The next thing which I have to mention, as one of the calculations which must be applied to observations in order to obtain an accurate result from them, is the nutation of the earth's axis. The nutation of the earth's axis may be described as arising in this way. I have explained that the action of the sun will produce a motion in the earth's equator and in the position of the earth's axis, principally at and near to the times of the winter solstice and the summer solstice, when either the North Pole or the South Pole of the earth is turned towards the sun. It matters not whether the North or South Pole is turned towards the sun, for the tendency is to produce equal motion of the axis, and in the same direction in both cases. But at the time of the equinoxes, when neither Pole is turned towards the sun, the sun's attraction has no tendency to produce precession. From this you may easily see that the precession of the equinoxes goes on more rapidly in summer and in winter than in the intermediate months; there is therefore a certain irregularity in the precession, and this irregularity is one part of the solar nutation. The other part arises from the circumstance that the inclination of the earth's axis to the ecliptic though unaltered on the whole, yet suffers slight changes from one instant to another; as clearly appears from the foregoing explanation.
I stated also that the moon produces a considerable part of the precession. There is a very small irregularity of precession produced by the moon in different parts of her monthly revolution, similar to that produced by the sun in different parts of his apparent yearly revolution. But the principal irregularity arises in this manner. The moon does not move in the plane of the ecliptic, but in an orbit inclined to the plane of the ecliptic; and this orbit (from an effect
