This action, were it not compensated by another force, would in time alter the angle of the ecliptic until the equatorial plane and the ecliptic coincided. There are few but have seen the philosophical toy called the Gyrascope. This toy, on a miniature scale, gives a fine illustration of the force brought in to correct the combined action of the sun and moon on the obliquity of the equator. The rotation of the earth is held in its own plane by its own revolution, the same as the gyrascope seems to overcome the laws of gravitation by its force of revolution. But not only do the sun and moon disturb the plane of the ecliptic, but the action of other planets on the earth and sun is to be taken into account. A very slow variation in the position of the plane of the ecliptic, in relation to the plane of the equator, is observed from these influences. It must be remembered that a very slight deviation in the angle can and would be detected by observation with modern instruments. We do find that this attraction affects the inclination of the ecliptic to the equator of 0´´.31 annually. This motion is entirely independent of the form of the earth. Now, if we assume that the sun and moon give the equinoctial points a retrograde motion on the ecliptic, we must deduct the influence of the planets. We may then calculate the mean disturbance by subtracting the latter from the former--the difference is settled by both theory and observation to be 50´´.1 annually. This motion of the equinoxes is called the precession of the equinoxes. Its consideration forms a very important element in the estimation of time, as the position of the various fixed stars, though so very distant, are all affected in longitude by this quantity of 50´´.1--being an increase of longitude. Therefore, if we were to calculate the position of any given star in order to get a transit for mean time, or true time, we must take this quantity into consideration. The increase is so great that the earliest astronomers, even with their imperfect modes of observation, detected it. Hipparchus, 128 years before Christ, compared his own observations with those of Timocharis, 153 years before. He found the solution of the problem the same as Diophantus found the solution of the squares and cubes, by analysis. In the time of Hipparchus, the sun was at a point 30° in advance of its present position, for it then entered into the constellation of Aries near the vernal equinox.