# precession

[pree-sesh-uh n] /priˈsɛʃ ən/
noun
1.
the act or fact of preceding; precedence.
2.
Mechanics. the motion of the rotation axis of a rigid body, as a spinning top, when a disturbing torque is applied while the body is rotating such that the rotation axis describes a cone, with the vertical through the vertex of the body as axis of the cone, and the motion of the rotating body is perpendicular to the direction of the torque.
3.
Astronomy.
1. the slow, conical motion of the earth's axis of rotation, caused by the gravitational attraction of the sun and moon, and, to a smaller extent, of the planets, on the equatorial bulge of the earth.
2. precession of the equinoxes.
Origin of precession
1300-1350
1300-50; < Late Latin praecessiōn- (stem of praecessiō) a going before, advance, equivalent to Latin praecess(us) (past participle of praecēdere to precede) + -iōn- -ion; see cession
Related forms
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2016.
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Examples from the Web for precessional
Historical Examples
• We know the rate at which the earth is spinning, and we have observed the precessional motion.

John Perry
• The effect of its fluctuation is inseparable from the precessional effect, and is related to it as a modifying condition.

• The search of the rocks for records of the ticks of the precessional clock is an out-of-door work.

• When the eccentricity is large the precessional rhythm is emphasized; when it is small the precessional effect is weak.

• So that the precessional motion changes its rate every quarter year from a maximum to nothing, or from nothing to a maximum.

John Perry
• But the precessional motion pulses steadily on through the ages, like the swing of a frictionless pendulum.

• If the precessional velocity is too small, the top will fall, and as it falls the precessional velocity increases.

John Perry
• For instance, the precessional motion of a top cannot be reversed unless we reverse the spin.

• The precessional movement is represented by a revolution around the pole of the ecliptic, as is shown in the figure.

Robert Stawell Ball
• This fundamental advance rendered inevitable the detection of precessional effects.

British Dictionary definitions for precessional

## precession

/prɪˈsɛʃən/
noun
1.
the act of preceding
3.
the motion of a spinning body, such as a top, gyroscope, or planet, in which it wobbles so that the axis of rotation sweeps out a cone
Derived Forms
Word Origin
C16: from Late Latin praecessiō a going in advance, from Latin praecēdere to precede
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
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Word Origin and History for precessional

## precession

n.

1590s, from Late Latin praecissionem (nominative praecissio) "a coming before," from past participle stem of Latin praecedere "to go before" (see precede). Originally used in reference to calculations of the equinoxes, which come slightly earlier each year.

Online Etymology Dictionary, © 2010 Douglas Harper
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precessional in Science
 precession   (prē-sěsh'ən)    The rotational motion of the axis of a spinning body, such as the wobbling of a spinning top, caused by torque applied to the body along its axis of rotation.The motion of this kind made by the Earth's axis, caused mainly by the gravitational pull of the Sun, Moon, and other planets. The precession of Earth's axis has a period of nearly 25,800 years, during which time the reference points on the equatorial coordinate system (the celestial poles and celestial equator) will gradually shift their positions on the celestial sphere. ◇ The precession of the equinoxes is the slow westward shift of the autumnal and vernal equinoxes along the ecliptic, resulting from precession of the Earth's axis. See also nutation.
The American Heritage® Science Dictionary
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