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Precession of the Equinoxes

By , Wednesday 5th November 2008 in Astronomy Basics

The precession of the equinoxes refers to the change of the Earth's rotational axis with respect to the stars in the galaxy.

We all know that the North Star (or Pole Star) is Polaris, but the true North Celestial Pole is slightly above Polaris and it is moving.

It was Hipparchus who first discovered that the celestial pole was moving, and it was noted in his observations from between 147 BC to 127 BC. Back then the celestial pole was closer to Thurban than it was to Polaris.

The Blue disc represents the path of the equinox over its 26800 year journey.
The Blue disc represents the path of the equinox over its 26800 year journey.

The precession of the equinoxes is caused by the rotational axis of the Earth changing over a period of about 25,765 years, centred around the ecliptic north pole, with an angular radius of about 23.4°; the angle known as the obliquity of the ecliptic.

Precession of the Equinoxes over 26,800 year cycle
Precession of the Equinoxes over 26,800 year cycle

Currently this annual motion is about 50.3 arcseconds per year (1 degree every 71.6 years). The process is very slow, but cumulative. A complete precession cycle covers a period of approximately 25,765 years, the so called great Platonic year, during which time the equinox covers a full 360°.

Even in the last 50 years the North Celestial Pole has moved, albeit a very small amount.
Even in the last 50 years the North Celestial Pole has moved, albeit a very small amount.

The North Celestial Pole for the year 2000 is marked with a large capital X, while for the year 1950 it is marked with a small x, showing the effect of precession over a 50 year period. The ragged circle of faint stars down and to the left of Polaris is called the "Engagement Ring," with of course Polaris as its shining stone.

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Tim Trott

Tim is a professional software engineer, designer, photographer and astronomer from the United Kingdom. You can follow him on Twitter to get the latest updates.

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