| precession |
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The uniform motion of the rotation axis of a freely rotating body when it is subject to turning forces (torque) due to external gravitational influences. Precession causes the Earth's rotation axis to sweep out, over a period of 25,800 years, a cone of angular radius about 23° 27' around the perpendicular to the plane of the Earth's orbit (the ecliptic). The main source of the torque is the combined effect of the gravitational pulls of the Sun and Moon on the Earth's equatorial bulge. (Precession would not occur if the Earth were a perfect sphere. Rotation, however, causes the equatorial radius to exceed the polar radius by about 0.3 per cent.) The combined effect of the Sun and Moon is called lunisolar precession. The Moon's contribution is about twice as large as the Sun's, because of its smaller distance. The gravitational force exerted on the Earth by the other planets causes small changes in the Earth's orbital elements, giving rise to planetary precession. The sum of planetary and lunisolar precession is called general precession. A consequence of the precession of the Earth's rotation axis is that the celestial poles trace out circles in the sky over 25,800 years. So, for example, about 13,000 years from the present, the nearest bright star to the north celestial pole will be Vega rather than Polaris. The zero point of right ascension, one of the equatorial coordinates, normally used to define the positions of celestial objects, is based on one of the points, known as "the First Point of Aries", where the celestial equator intersects the ecliptic. Because of precession, the equator is "sliding around" the ecliptic so that the intersection points are constantly changing. In fact, the First Point of Aries is no longer in the constellation Aries, but has moved into Pisces and will soon be in Aquarius. This phenomenon is known as the precession of the equinoxes. Its effect on the right ascension and declination of an object is noticeable from year to year with the positional accuracy attainable with many telescopes. Tabulated values of right ascension and declination are therefore referred to a particular epoch, at which they were precisely correct. |