An Etymological Dictionary of Astronomy and Astrophysics
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→ advance of perihelion, → relativistic precession.

→ apsidal; → precession.

1) A change in the orientation of the → rotation axis of a non-spherical, spinning body caused by → gravity. A rotating top will precess in a direction determined by the → torque exerted by its → weight. The precession → angular velocity is inversely proportional to the spin angular velocity, so that the precession is faster and more pronounced as the top slows down.
2) In particular, the gradual shift in the orientation of Earth's rotation axis which traces out a conical path once every ~ 26,000 years. The cause of the Earth's precession is the → equatorial bulge of the Earth, brought about by the → centrifugal force of the → rotation, which has changed the Earth from a perfect sphere to a slightly flattened one, thicker across the equator. The attraction of the Moon and Sun on the bulge is the reason which makes the Earth precess.

→ axial; → precession.

The secular motions of the → celestial equator and → ecliptic. In other words, the sum of → lunisolar precession, → planetary precession, and → geodesic precession.

→ general; → precession

The secular displacement of the → equinox on the → ecliptic of date.

→ general; → precession; → longitude.

The secular motion of the → equinox along the → celestial equator.

→ general; → precession; → right ascension.

→ geodesic precession.

→ geodesic; → precession

A → relativistic effect on the precession motion of a gravitational system due to the → curvature of the → space-time. When a body revolves around a primary, the → rotation axis of the orbiting body follows the curvature of spece-time. Over time the space-time warping causes the spin axis to precess. In the case of the Earth-Moon system, this means a small → direct motion of the → equinox along the → ecliptic, amounting to 1''.915 per century. The geodetic precession is given by: ψ_g = (3/2) k² (1 - e_⊕) n_⊕, where k is the → constant of aberration (in radians), e_⊕ the → eccentricity of the Earth and n_⊕ the mean angular orbital motion of the Earth (in arcsec/cy). Also called → Einstein-de Sitter effect and → geodesic precession.

→ geodetic; → precession

The process whereby the → Moon gradually moves out into a slightly larger orbit. The → gravitational attraction of the Moon on the → Earth creates two ocean → tidal bulges on the opposite sides of our planet. The Earth rotates faster than the Moon revolves about the Earth. Therefore, the tidal bulge facing the Moon advances the Moon with respect to the line joining the centers of the Earth and the Moon. The Moon's gravity pulls on the bulge and slows down the → Earth's rotation. As a result, the Earth loses → angular momentum and the days on Earth are gradually increasing by 2.3 milliseconds per century. Since the angular momentum in the → Earth-Moon system is conserved, the Earth must impart the loss in its own angular momentum to the Moon's orbit. Hence, the Moon is being forced into a slightly larger orbit which means it is receding from the Earth. However, eventually this process will come to an end. This is because the Earth's own rotation rate will match the Moon's orbital rate, and it will therefore no longer impart any angular momentum to it. In this case, the planet and the Moon are said to be tidally locked (→ tidal locking). This is a stable situation because it minimises the energy loss due to friction of the system. Long ago, the Moon's own rotation became equal to its orbital period about the Earth and so we only see one side of the Moon. This is known as → synchronous rotation and it is quite common in the solar system. The Moon's average distance from Earth in increasing by 3.8 cm per year. Such a precise value is possible due to the Apollo laser reflectors which the astronauts left behind during the lunar landing missions (Apollo 11, 14, and 15). Eventually, the Moon's distance will increase so much that it will be to far away to produce total eclipses of the Sun.

→ lunar; → recession.

→ precession of the equator.

From luni-, from → lunar, + → solar; → precession.

The → rotation of the Moon's → orbit within the → orbital plane, whereby the axes of the ellipse change direction. The Moon's → major axis makes one complete revolution every 8.85 Earth years, or 3,232.6054 days, as it rotates slowly in the same direction as the Moon itself (direct, or → prograde motion). The Moon's apsidal precession is a → relativistic effect, and should not be confused with its → axial procession.

→ Moon; → apsidal; → precession.

Same as → relativistic precession.

→ orbital; → precession.

→ advance of perihelion.

→ perihelion; → precession.

The motion of the → ecliptic plane caused by the gravitational influence of the other planets, mainly → Jupiter. The observational effect of planetary precession is similar to that of the → lunisolar precession. But planetary precession causes the → equinoxes to move along the ecliptic in the opposition direction (eastward) from that of luni-solar precession (westward) and at a much slower rate: 0''.12 per year. Same as → precession of ecliptic.

→ planetary; → precession.

The periodic motion of the → rotation axis of a body such as a → spinning top in which the axis of rotation gradually sweeps out a conical shape. In the case of the spinning Earth, it is due to the combined → gravitational attractions of the → Sun, the → Moon, and → planets on Earth's → equatorial bulge. Since the Earth's axis is tilted to its → orbital plane or → ecliptic, the gravitational force of the Sun and the Moon on the Earth's equatorial bulge tend to pull it back toward the plane of ecliptic. As a result, the axis → precesses. Earth's axis of rotation precesses with a period of about 25,770 years, describing one complete circle on the → celestial sphere (→ precession constant). This circle has a radius of approximately 23°.5, equal to the → inclination of the Earth's orbit. Since the → vernal equinox is the reference direction for the → equatorial coordinate system, the coordinates of "fixed" objects change with time and must therefore be referred to an → epoch at which they are correct. → sign of zodiac.
See also → general precession; → precession of the ecliptic; → precession of the equator; → precession of the equinoxes; → precessional circle; → precession time; → geodetic precession; → general precession in longitude; → general precession in right ascension; → orbital precession; → perihelion precession; → planetary precession; → precession period.

L.L. præcissionem "a coming before," from L. præcessus, p.p. of præcedere "to happen before," from the fact that the equinoxes occur earlier each year with respect to the preceding year, from præ- "before," → pre-, + cedere "to walk, to go, to happen."

Pišâyân, literally "coming before," from piš- "before" → pre- + ây- (present stem of âmadan "to come, arrive, become"), from Av. ay- "to go, to come," aēiti "goes;" O.Pers. aitiy "goes;" Skt. e- "to come near," eti "arrival;" L. ire "to go;" Goth. iddja "went," Lith. eiti "to go;" Rus. idti "to go;" + -ân suffix of space and time.

The amount by which the equinoctial points drift westward annually due to precession. Its value for epoch J2000.0 is 50''.26, resulting from the westward → precession of the equator (50".38), and the eastward → precession of the ecliptic (0".12).

→ precession; → constant.

The component of general precession caused by the gravitational attraction of the planets on the Earth's center of mass. It causes the equinox to move eastward by about 0''.12 per year in the opposite direction to the → precession of the equator. This terminology replaces → planetary precession, according to an IAU resolution adopted in August 2006.

→ precession; → ecliptic.

That component of general precession caused by the combined effect of the Moon, the Sun and the planets on the equatorial protuberance of the Earth, producing a westward motion of the equinoxes along the ecliptic about 50'' per year. According to an IAU resolution adopted in August 2006, the present terminology replaces lunisolar precession.

→ precession; → equator.

The slow motion of the equinoxes along the ecliptic, resulting from the combined motion of the equator (→ precession of the equator) and the ecliptic (→ precession of the ecliptic), or in other words the precession of the Earth's axis of rotation. Also know as → general precession. The First Point of Aries moves westward along the ecliptic at 50.38 arcseconds per year (1 degree every 71.6 years), causing the equinoxes to occur about twenty minutes earlier each sidereal year. See also → nutation.

→ precession; → equinox.

The gradual change in he orbital planes of a binary system.

→ precession; → node.

The interval with which a rotating body precesses. The precession period of the Earth is 25,770 years. For a → spinning top it is given by: T_p = (4π²I)/(mgrT_s), where I is the → moment of inertia, m the mass of the top, g gravity, r the distance between the center of mass and the contact point, and T_s is the spinning period of the top.

→ precession; → period.