precession of the equinoxes, westward motion of the equinoxes along the ecliptic. This motion was first noted by Hipparchus c.120 B.C. The precession is due to the gravitational attraction of the moon and sun on the equatorial bulge of the earth, which causes the earth's axis to describe a cone in somewhat the same fashion as a spinning top. As a result, the celestial equator (see equatorial coordinate system), which lies in the plane of the earth's equator, moves on the celestial sphere, while the ecliptic, which lies in the plane of the earth's orbit around the sun, is not affected by this motion. The equinoxes, which lie at the intersections of the celestial equator and the ecliptic, thus move on the celestial sphere. Similarly, the celestial poles move in circles on the celestial sphere, so that there is a continual change in the star at or near one of these poles (see Polaris). After a period of about 26,000 years the equinoxes and poles lie once again at nearly the same points on the celestial sphere. Because the gravitational effects of the sun and moon are not always the same, there is some wobble in the motion of the earth's axis; this wobble, called nutation, causes the celestial poles to move, not in perfect circles, but in a series of S-shaped curves with a period of 18.6 years. There is some further precession caused by the gravitational influences of the other planets; this precession affects the earth's orbit around the sun and thus causes a shift of the ecliptic on the celestial sphere. The precession of the earth's orbital plane is sometimes called planetary precession, and that of the earth's equatorial plane (caused by the sun and moon) is called luni-solar precession; the combined effect of the moon, the sun, and the planets is called general precession. Planetary precession is much less than luni-solar precession. The precession of the equinoxes was first explained by Isaac Newton in 1687.

precession: see gyroscope.

Motion of the points where the Sun crosses the celestial equator, caused by precession of Earth's axis. Hipparchus noticed that the stars' positions were shifted consistently from earlier measures, indicating that Earth, not the stars, was moving. This precession, a wobbling in the orientation of Earth's axis with a cycle of almost 26,000 years, is caused by the gravity of the Sun and the Moon acting on Earth's equatorial bulge. The planets also have a small influence on precession. Projecting Earth's axis onto the celestial sphere locates the northern and southern celestial poles. Precession makes these points trace out circles on the sky and also makes the celestial equator wobble, changing its points of intersection (equinoxes) with the ecliptic.

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Phenomenon associated with the action of a gyroscope or a spinning top and consisting of a comparatively slow rotation of the axis of rotation of a spinning body about a line intersecting the spin axis. It arises as a result of external torque acting on the body. One example of precession is the smooth, slow circling of a spinning top (the uneven wobbling is called nutation). Precession of the earth's axis of rotation is the reason that the positions of celestial bodies appear to drift systematically with the passage of time. See also precession of the equinoxes.

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