From his great demonstration of the law of gravitation, Newton went on to apply it in many directions. He showed that the Earth could not be truly spherical in shape, but that there must be a flattening of its poles. He showed also that the Moon, which is exposed to the attractions both of the Earth and of the Sun, and, to a sensible extent, of some of the other planets, must show irregularities in her motion, which at that time had not been noticed. The Moon's orbit is inclined to that of the Earth, cutting its plane in two opposite points, called the "+nodes+." It had long been observed that the position of the nodes travelled round the ecliptic once in about nineteen years. Newton was able to show that this was a consequence of the Sun's attraction upon the Moon. And he further made a particular application of the principle thus brought out, for, the Earth not being a true sphere, but flattened at the poles and bulging at the equator, the equatorial belt might be regarded as a compact ring of satellites revolving round the Earth's equator. This, therefore, would tend to retrograde precisely as the nodes of a single satellite would, so that the axis of the equatorial belt of the Earth--in other words, the axis of the Earth--must revolve round the pole of the ecliptic. {36} Consequently the pole of the heavens appears to move amongst the stars, and the point where the celestial equator crosses the equator necessarily moves with it. This is what we know as the "+Precession of the Equinoxes+," and it is from our knowledge of the fact and the amount of precession that we are able to determine roughly the date when the first great work of astronomical observation was accomplished, namely, the grouping of the stars into constellations by the astronomers of the prehistoric age.