Precession
Precession is the slow change in direction of the Earth's axis of rotation, like a spinning top; one full swing takes about 25,772 years. This heavy swing changes both the pole star and the alignment of the signs with the constellations over the ages.
| In brief | Precession is the change in direction of the Earth's axis of rotation, tracing one full cone about every 25,772 years. |
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| Astronomical account | The gravitational torque the Sun and the Moon exert on the Earth's equatorial bulge makes the axis swing like a spinning top. This swing slowly shifts the position of the poles in the sky and the position of the equinox points against the stars; for this reason the vernal point has moved from the constellation Aries to Pisces since antiquity. |
| Use in astrology | Because the tropical system of Western astrology stays tied to the seasonal points, it does not follow precession; the signs are fixed with respect to the equinox. The sidereal system of Indian astrology, by contrast, tries to preserve the alignment with the stars, and the difference between the two systems corresponds roughly to the accumulated shift of precession. |
| Evidence status | Precession has been known since the ancient Greek astronomer Hipparchus and is confirmed with high precision by modern measurement. The shift between signs and constellations can also be observed directly; what is disputed is what this shift means for systems of astrological interpretation, and that is a matter on which the schools of astrology themselves differ. |
| Common misconceptions |
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A planet like a spinning top
Just as the axis of a spinning top traces circles, the Earth's axis traces a similar cone under the gravitational effect the Sun and the Moon exert on the equatorial bulge. One full turn takes about 25,772 years. The amount of the axial tilt, about 23.4 degrees, hardly changes during this process; what changes is the direction the axis points in space.
Its concrete consequences
Today the northern end of our axis points close to Polaris; in about 12,000 years the bright star Vega will take over the role of pole star. The second consequence concerns the seasonal points: the vernal equinox slides back against the stars by a very small amount each year. That point, which lay in the constellation Aries in antiquity, is today in Pisces.
Why have the signs shifted?
The tropical zodiac of Western astrology ties its starting point not to the stars but to the vernal equinox. As the point has slid, the sign slices have moved away from the constellations of the same name; the shift accumulated over two thousand years is about the width of one sign. So the constellation of your sign is today in the neighbouring slice of the sky; this is the astronomical background to the debates about the Sun sign.
Related terms: Zodiac, The Ecliptic, Constellation, Sun Sign