Ignoring all motions but the one we are speaking of, let the points of the knitting needle (Earth’s axis of rotation) trace out small circles in space, and the equator of the orange will be seen to alter the direction of its tilt, but without turning round ([Fig. 26]). Stick a pin in the equator, and others in north or south latitudes, between equator and pole; these will always remain facing you, but while the pole makes its small circle, the equatorial pin will be seen to move up and down, while the tropical and temperate pins trace out ellipses. These are the movements which we see reflected in the stars; and if Earth’s diurnal rotation were suddenly to cease, while her revolution in her orbit and the movement of “precession” continued, we should see Spica, for instance, sink slowly lower in the southern sky and after ages rise again northwards, but there would be very little preceptible movement east or west.
The movement observed by Hipparchus, then, was not a movement of Spica and other stars, but a movement of the equinox. For the celestial equator is simply a reflection of Earth’s equator in the skies, and as it keeps changing the direction of its tilt in the way described, it changes the point at which it cuts the ecliptic. This may best be seen by taking two rings or hoops (two large curtain rings, for instance), one of which just fits inside the other. Tilt the inner ring, so that half of it is above and half below the other ring, and they touch at two points, 1 and 2 ([Fig. 27]). The outer ring is the Ecliptic, the inner the Equator, and where they touch each other are the Equinoxes. Now move the inner ring, not sliding it round, nor making any difference in the angle between the two, but simply so that they touch at fresh points, 1′ and 2′. In this way you may make the points of contact revolve entirely round. This is what the real equinoxes are doing: while the equator opposite the group of stars in [figure 25] rises and falls, the equinox travels on, and finally returns to the same place.
Fig. 26. The movement of Earth’s axis,
which is the true cause of Precession.
V is the vernal equinox, at the intersection of the equator and plane of the ecliptic: APX the earth’s axis, which always preserves the same inclination (23½%) to the plane of the ecliptic. As APX slowly revolves round T in the direction of the arrow, the vernal equinox is gradually shifted to V′, and so on.
(From Young’s “Manual of Astronomy,” 1902.)
The phenomenon is called “precession of the equinoxes,” because they thus move on to meet the sun in his yearly course.