precession of the equinoxes

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Slow wobble of the Earth on its axis, like that of a spinning top. The gravitational pulls of the Sun and Moon on the Earth's equatorial bulge cause the Earth's axis to trace out a circle on the sky every 25,800 years. The position of the celestial poles (see celestial sphere) is constantly changing owing to precession, as are the positions of the equinoxes (the points at which the celestial equator intersects the Sun's path around the sky). The precession of the equinoxes means that there is a gradual westward drift in the ecliptic – the path that the Sun appears to follow – and in the coordinates of objects on the celestial sphere.

This is why the dates of the astrological signs of the zodiac no longer correspond to the times of year when the Sun actually passes through the constellations. For example, the Sun passes through Leo from mid-August to mid-September, but the astrological dates for Leo are between about 23 July and 22 August.

The Earthseverals precession is caused by the gravitational pull of the Sun and Moon on the equatorial bulge of our planet. Precession also occurs in other planets. Uranus has the Solar System's fastest known precession (264 days). Planetary precession is a particular example of the phenomenon of precession shown by rotating objects in general – the wobbling of a spinning top is another instance.

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