One curious irregularity in the motion of the earth's poles must be mentioned in connection with the precession of the equinoxes. This is a kind of “nodding,” known as nutation. It arises from variation in the effect of the attraction of the sun and the moon, due to the varying directions in which the attraction is exercised. As far as the sun is concerned, the precession is slower near the time of the equinoxes than in other parts of the year; in other words, it is most rapid in mid-summer and mid-winter when one or the other of the poles is turned sunward. A similar, but much larger, change takes place in the effect of the moon's attraction owing to the inclination of her orbit to the ecliptic. During about nine and a half years, or half the period of revolution of her nodes (see Part III, Section 4), the moon tends to hasten the precession, and during the next nine and a half years to retard it. The general effect of the combination of these irregularities is to cause the earth's poles to describe a slightly waving curve instead of a smooth circle round the poles of the ecliptic. There are about 1400 of these “waves,” or “nods,” in the motion of the poles in the course of their 26,000-year circuit. In accurate observation the astronomer is compelled to take account of the effects of nutation upon the apparent places of the stars. A very remarkable general consequence of the change in the direction of the earth's axis will be mentioned when we come to deal with the seasons. [Illustration: =The Great Southern Star-Cluster ω Centauri= Photographed by S. I. Bailey at the South American Station of Harvard Observatory. Note the streaming of small stars around the cluster. The cluster itself is globular and its stars are too numerous to be counted, or even to be separately distinguished in the central part. ] ------------------------------------------------------------------------ PART II. THE EARTH. ------------------------------------------------------------------------ PART II. THE EARTH. =1. Nature, Shape, and Size of the Earth.= The situation of the earth in the universe has been briefly described in Part I; it remains now to see what the earth is in itself, and what are some of the principal phenomena connected with it as a celestial body inhabited by observant and reasoning beings.