Turn, and Turn Again

The constellations flow.

And the axis of the world beneath inscribes a circle in the sky a thousand

generations round, bestowing briefly as it passes by the honour of The

Pole on certain favoured stars.

Fragments of Berossus, Vega

Though the ancients conceived of the celestial sphere as eternal, unchangeable and perfect — and so were greatly disturbed by the appearances of "new stars" (novas and supernovas) and other "meteorological" phenomena (so called because they were "lofty" or "atmospheric") such as comets — that the heavens are not immutable is now common knowledge.

The so-called fixed stars move with respect to the solar system as they and we orbit the centre of the galaxy at different distances and varied speeds, and wander through space with our own peculiar, local motions; but such proper motions are not discernible by the human eye in the course of a single lifetime. What we may notice however is the precession of the equinoxes — the slow circling of the pole.

The precession of the equinoxes is the visible effect of the rotation of the Earth's axis — not the rotation of the Earth about its axis but the slow movement of the axis itself, as shown below. The cause is gyroscopic precession caused primarily by the moon's gravitational pull on a somewhat-less-than truly spherical Earth, but since there are other forces acting on the Earth (such as the Sun's gravity) and the Earth itself is not completely rigid, the motion of the pole illustrated above is neither a perfect circle nor perfectly periodic (every time a major earthquake redistributes the Earth's mass, the pole shifts by a microscopic but measurable amount).