precession of the equinoxes


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precession of the equinoxes,

westward motion of the equinoxesequinox
, either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The vernal equinox, also known as "the first point of Aries," is the point at which the sun appears to cross the celestial equator from south to north.
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 along the eclipticecliptic
, the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic). Because of the earth's yearly revolution around the sun, the sun appears to move in an annual journey through the heavens with the ecliptic as its
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. 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 systemequatorial coordinate system,
the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celestial sphere. The celestial sphere is an imaginary sphere with the observer at its center.
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), which lies in the plane of the earth's equator, moves on the celestial spherecelestial sphere,
imaginary sphere of infinite radius with the earth at its center. It is used for describing the positions and motions of stars and other objects. For these purposes, any astronomical object can be thought of as being located at the point where the line of sight
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, 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 PolarisPolaris
or North Star,
star nearest the north celestial pole (see equatorial coordinate system). It is in the constellation Ursa Minor (see Ursa Major and Ursa Minor; Bayer designation Alpha Ursae Minoris) and marks the end of the handle of the Little Dipper.
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). 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 nutationnutation,
in astronomy, a slight wobbling motion of the earth's axis. The causes of nutation are similar to those of the precession of the equinoxes, involving the varying attraction of the moon on the earth's equatorial bulge. However, the period of the motion is only 18.
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, 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 of the equinoxes

The slow continuous westward motion of the equinoxes around the ecliptic that results from the precession of the Earth's axis. It was first described by Hipparchus in the second century bc. As the axis precesses, the celestial equator, which lies in the plane perpendicular to the axis, moves relative to the ecliptic. The two points at which the planes intersect – the equinoxes – thus move round the ecliptic in the precessional period of about 25 800 years, the annual rate being about 50.27 arc seconds. Coordinates such as right ascension that refer to an equinox as their zero point thus change with time. Star catalogs therefore use a catalog equinox as their origin.

precession of the equinoxes

[prē′sesh·ən əvthə ′ē·kwə‚näk·səz]
(astronomy)
A slow conical motion of the earth's axis about the vertical to the plane of the ecliptic, having a period of 26,000 years, caused by the attractive force of the sun, moon, and other planets on the equatorial protuberance of the earth; it results in a gradual westward motion of the equinoxes.

precession of the equinoxes

precession of the equinoxesclick for a larger image
In the illustration, the equatorial plane changes its orientation in space as North Pole (P) rotates. The first point of Aries and Libra, alternatively known as the Spring and Autumnal Equinoxes, precess westerly at a mean rate of 5.26 s of arc. The true pole completes one circuit of the ecliptic in 25,800 years.
The slow westward motion of equinoctial points (the first point of Aries and the first point of Libra) along the ecliptic by about 50.26” of arc each year. A full cycle of precession occupies about 25,800 years. The term is used in astronavigation, which has been mostly replaced by satellite navigation, especially over the oceans.
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However, the identification of Spenser's Senecan sources raises further problems: not only are the passages conjoined on the basis of an Astraean eschatology that is nowhere made explicit, but Spenser has simultaneously redescribed the falling of the stars as an almost comic battle between the constellations, and reinterpreted it in astronomical terms as the precession of the equinoxes.
Classical scholars have a blind spot about the Precession of the Equinoxes.
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