Precession of the Equinoxes

Why does the Precession of the Earth's Axis Cause the Equinoxes to Precess?

This is not a very easy thing to visualise. One way is to have a picture of the night skies in which all the stars are stuck to a large sphere surrounding the Earth. This, in fact, was how Classical astrologers thought the heavens behaved. It's the concept of the Celestial Sphere. That Sphere has an axis, which is the Earth's axis extended off to touch the stars at the Celestial Poles. Currently the north part of the axis touches the Sphere at Polaris, the Pole Star. However, the Axis is Precessing, and so it slowly moves away from our current Pole Star. As it does so it shifts the axis of the Celestial Sphere.

However, the Ecliptic, the Earth's orbit around the Sun, is completely unaffacted by Precession. So it stays in the same place whilst the axis of the Celestial Sphere moves. This has the effect of moving the points where the Celestial Equator and the Ecliptic meet around the Ecliptic. One of the points where they meet is the Vernal Equinox Point and hence the movement is the Movement of the Vernal Equinox Point.

Precession of the Equinoxes. One things remain the same in this diagram, the yellow circle. which represents the Ecliptic.

The vertical arrow represents the Earth's axis, as it is today pointing up towards the North Celestial Pole, and the Pole Star, Polaris. The other arrow represents the Earth's axis as it was thousands of years ago [or thousands of years in the future.] The Earth's axis has precessed to point at another point in the night sky. [The North Celestial Pole has moved and this Changes the Pole Star.] This changes where the Celestial Equator crosses the Ecliptic and hence moves the Vernal Equinox Point along the Ecliptic.

The History of the Idea of Precession

The famous Greek astrologer Hipparchos [c 190 - 120 BC] is thought to have been the first person to realise the Vernal Equinox Point was moving relative to the stars. However Hipparchos did not realise that this was due to the Precession of the Earth itself. At the time all astrologers still firmly believed in the geocentric theory of the Solar System, with a stationary Earth in the centre. They did not realise that much of what we see in the heavens is because the Earth is spinning on its axis, or orbiting around the Sun. Hipparchos followed a Celestial Sphere view of the Solar System, and it would not have occurred to him that the Movement of the Vernal Equinox Point had anything to do with the movement of the Earth itself.

This theory of the solar system held sway for nearly two millennia until it was swept away by Nicolas Copernicus "De revolutionibus" On the Revolutions of Heavenly Spheres. He showed that the Earth orbited the Sun and that the Earth spun on its axis. Copernicus [Book iii, chapters 1 to 12] also hypothesised that the Precession of the Equinoxes was due to a slight forward movement of the Earth in its orbit each calendar year. However, whilst this would account for the Precession of the Equinoxes that we see, Copernicus was unable to assign a first cause to the phenomenon - i.e. why it happened.

It was left to the great mathematician Pierre Laplace to show in Traité de Méchanique Céleste [1799–1825 AD] how the gravity of the Sun acting on the Earth accounted for the Precession of the Earth's Axis and hence the Precession of the Equinoxes.

Nicholas Copernicus (Mikolaj Kopernik) [1473 - 1543 AD] One of the greatest astrological thinkers of all time. "Of all discoveries and opinions, none may have exerted a greater effect on the human spirit than the doctrine of Copernicus." Gothe.