equinoxes, precession of the


equinoxes, precession of the
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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      motion of the equinoxes along the ecliptic (the plane of the Earth's orbit) caused by the cyclic precession of the Earth's axis of rotation.

      In compiling his famous star catalog (completed in 129 BC), the Greek astronomer Hipparchus noticed that the positions of the stars were shifted in a systematic way from earlier Babylonian (Chaldean) measures. This indicated that it was not the stars that were moving but rather the observing platform—the Earth. Such a motion is called precession and consists of a cyclic wobbling in the orientation of the Earth's axis of rotation with a period of almost 26,000 years. Precession was the third-discovered motion of the Earth, after the far more obvious daily rotation and annual revolution. Precession is caused by the gravitational influence of the Sun and the Moon acting on the Earth's equatorial bulge. To a much lesser extent, the planets exert influence as well.

      The projection onto the sky of the Earth's axis of rotation results in two notable points at opposite directions: the north and south celestial poles. Because of precession, these points trace out circles on the sky. Today, the north celestial pole points to within just 1° of the arc of Polaris. It will point closest to Polaris in AD 2017. In 12,000 years the north celestial pole will point about 5° from Vega. Presently, the south celestial pole does not point in the vicinity of any bright star.

      Also moving with this wobble is the projection onto the sky of the Earth's equator. This projection, a great circle, is called the celestial equator. The celestial equator intersects another useful great circle, the ecliptic. As the Earth orbits the Sun, the constantly changing direction from which we view the Sun causes it to trace out the ecliptic. The celestial equator is inclined at a 23.5° angle to the ecliptic (the so-called obliquity of the ecliptic). The celestial equator and the ecliptic intersect at two points called the equinoxes (vernal and autumnal). During the course of the year, as the Earth orbits the Sun, the latter is seen crossing the equator twice, in March moving from the Southern Hemisphere into the Northern Hemisphere and in September moving in the opposite direction. The equinoxes drift westward along the ecliptic at the rate of 50.2 arc-seconds annually as the celestial equator moves with the Earth's precession.

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Universalium. 2010.

Look at other dictionaries:

  • Precession of the equinoxes — Precession Pre*ces sion, n. [L. praecedere, praecessum, to go before: cf. F. pr[ e]cession. See {Precede}.] The act of going before, or forward. [1913 Webster] {Lunisolar precession}. (Astron.) See under {Lunisolar}. {Planetary precession}, that… …   The Collaborative International Dictionary of English

  • precession of the equinoxes — n. Astron. the occurrence of the equinoxes earlier in each successive sidereal year because of a slow wobble in the earth s axial spin which shifts the equinoctial points slightly westward along the ecliptic: the wobble is caused by the pull of… …   English World dictionary

  • precession of the equinoxes — 1. the earlier occurrence of the equinoxes in each successive sidereal year because of the slow retrograde motion of the equinoctial points along the ecliptic, caused by the precession of the earth s axis of rotation; a complete precession of the …   Universalium

  • precession of the equinoxes — noun The slow westward shift of the equinoxes along the plane of the ecliptic, resulting from precession of the earths axis of rotation, and causing the equinoxes to occur earlier each year …   Wiktionary

  • precession of the equinoxes — noun a slow westward shift of the equinoxes along the plane of the ecliptic caused by precession of the Earth s axis of rotation • Hypernyms: ↑natural process, ↑natural action, ↑action, ↑activity …   Useful english dictionary

  • precession of the equinoxes — 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 are each year. A full cycle of precession occupies about 25,800 years. The term is used in… …   Aviation dictionary

  • precession of the equinoxes — noun the earlier occurrence of the equinoxes in each successive sidereal year because of a slow retrograde motion of the equinoctial points along the ecliptic, caused by the combined action of the sun and moon on the mass of matter accumulated… …   Australian English dictionary

  • precession of the equinoxes — Date: 1621 a slow westward motion of the equinoxes along the ecliptic caused by the gravitational action of sun and moon upon the protuberant matter about the earth s equator …   New Collegiate Dictionary

  • PRECESSION OF THE EQUINOXES —    name given to the gradual shifting of the equinoctial points along the ecliptic from east to west.    See EQUINOXES …   The Nuttall Encyclopaedia

  • precession of the equinoxes — preces′sion of the e′quinoxes n. astron. the earlier occurrence of the equinoxes in each successive sidereal year • Etymology: 1615–25 …   From formal English to slang


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