Precession of the Equinoxes
Collier's New Encyclopedia (1921)
in astronomy, the going forward of the equinoxes. The arrival of the sun at the point Aries a little earlier than he might be expected to reach it was first observed by Hipparchus about 150 b. c. Depending, as the phenomenon does, for its explanation, on the law of gravity, Hipparchus could not account for it. Sir Isaac Newton was the first who did so, and that his newly discovered law of gravitation explained the precession of 338. the equinoxes was a confirmation of the accuracy with which he had read the law itself. Excepting only at the two equinoxes, the the plane i in which the sun moves in his orbit and that in which the earth rotates do not coincide. By the law of gravitation one body does not attract another in mass, but acts on its separate particles. The sun then does not attract the earth as a whole, but tends to pull the parts nearest it away from those in proximity to the center, and the center again away from those on the other side. The bulged-out equatorial zone is specially liable to be thus acted upon, and, but for the rotation of the earth, would be so drawn down toward the ecliptic that it and the equator would ultimately be in one plane. The earth's rotation, however, modifies this action, and simply causes the points at which the earth's equator intersects the plane of the ecliptic to move slowly in a direction opposite to that in which the earth rotates. This is what is denominated the precession of the equinoxes. It is generally associated with the sun, but the moon is twice as potent in producing it; owing to her comparative nearness to the earth she is able to produce a greater differential effect on the nearer and more remote portions of our planet.
What Has Changed in the Last Century
The physics Collier's set out in 1921 is still the physics taught today: the Sun and Moon tug on the Earth's equatorial bulge, and the Moon, being nearer, contributes roughly twice the Sun's share. Modern measurement has simply sharpened the numbers — radio interferometry and the IAU's 2000/2006 precession-nutation models put the general precession at about 50.3 arcseconds a year, a full circuit in roughly 25,800 years, with a tiny extra contribution (about 19 milliarcseconds a year) from Einstein's general relativity that Newtonian gravity cannot supply. The largest genuinely new development is not in the mechanics but in the uses: since deep-sea sediment cores were read against orbital cycles in the 1970s, precession has been accepted as one of the Milankovitch rhythms pacing the ice ages. One historical footnote the entry glosses over is that Newton's own calculation of the effect, though right in principle, was quantitatively flawed; a rigorous treatment came from d'Alembert in 1749.
Written for this edition, 2026.