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precession (n.)

1.the act of preceding in time or order or rank (as in a ceremony)

2.the motion of a spinning body (as a top) in which it wobbles so that the axis of rotation sweeps out a cone

Merriam Webster

PrecessionPre*ces"sion (?), n. [L. praecedere, praecessum, to go before: cf. F. précession. See Precede.] The act of going before, or forward.

Lunisolar precession. (Astron.) See under Lunisolar. -- Planetary precession, that part of the precession of the equinoxes which depends on the action of the planets alone. -- Precession of the equinoxes (Astron.), the slow backward motion of the equinoctial points along the ecliptic, at the rate of 50.2″ annually, caused by the action of the sun, moon, and planets, upon the protuberant matter about the earth's equator, in connection with its diurnal rotation; -- so called because either equinox, owing to its westerly motion, comes to the meridian sooner each day than the point it would have occupied without the motion of precession, and thus precedes that point continually with reference to the time of transit and motion.

voir la définition de Wikipedia

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precession (n.)

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Apsidal precession • Axial precession (astronomy) • Equinoctial Precession • Free precession • Larmor precession • Larmor precession frequency • Lense–Thirring precession • Lunar precession • Precession (album) • Precession (disambiguation) • Precession (mechanical) • Precession of perihelion • Relativistic Precession • Steady-state free precession imaging • Thomas precession

action; busyness; activity; employment; occupation; pursuit[ClasseHyper.]

ensemble des phénomènes (fr)[Classe...]

être avant, selon l'ordre, la place (fr)[Classe]

être perçu avant l'arrivée de qqn, qqch (fr)[Classe]

precede[ClasseHyper.]

factotum[Domaine]

IntentionalProcess[Domaine]

Motion[Domaine]

successorInPosition[Domaine]

earlier[Domaine]

act, deed, human action, human activity - go, go along, locomote, move, travel - be, find o.s., lie, sit[Hyper.]

active, alive - active, participating - precedence, precedency, precession - forerunner, predecessor, trendsetter - precedent - precedent - antecedence, antecedency, anteriority, precedence, precedency, priority - case in point, precedent - antecedent, precedent[Dérivé]

preceding[Similaire]

inactivity - follow - come after, follow, follow on, follow upon, succeed - follow, postdate[Ant.]

action, activity, busyness, employment, occupation, pursuit[Hyper.]

forego, lead, lead on, lead the way, precede - come before - predate - antecede, antedate, forgo - precedent[Dérivé]

precession (n.)↕

factotum[Domaine]

Motion[Domaine]

state[Hyper.]

be active, move - in motion, moving, on the move - immobile, motionless, nonmoving, stationary, still, unmoving[Dérivé]

motion[Hyper.]

precession (n.)↕

mouvement dans l'espace sidéral (fr)[Thème]

physics[Domaine]

Process[Domaine]

Radiating[Domaine]

physical process - development, evolution[Hyper.]

mouvement dans l'espace sidéral (fr)[termes liés]

precession (n.)↕

Wikipedia

For other uses, see Precession (disambiguation).

**Precession** is a change in the orientation of the rotational axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle (nutation) is constant. In physics, there are two types of precession: torque-free and torque-induced.

In astronomy, "precession" refers to any of several slow changes in an astronomical body's rotational or orbital parameters, and especially to the Earth's precession of the equinoxes. See Precession (astronomy).

## Contents |

Torque-free precession occurs when the axis of rotation differs slightly from an axis about which the object can rotate stably: a maximum or minimum principal axis. Poinsot's construction is an elegant geometrical method for visualizing the torque-free motion of a rotating rigid body. For example, when a plate is thrown, the plate may have some rotation around an axis that is not its axis of symmetry. This occurs because the angular momentum (*L*) is constant in absence of torques. Therefore, it will have to be constant in the external reference frame, but the moment of inertia tensor (*I*) is non-constant in this frame because of the lack of symmetry. Therefore, the spin angular velocity vector () about the spin axis will have to evolve in time so that the matrix product remains constant.

When an object is not perfectly solid, internal vortices will tend to damp torque-free precession, and the rotation axis will align itself with one of the inertia axes of the body.

The torque-free precession rate of an object with an axis of symmetry, such as a disk, spinning about an axis not aligned with that axis of symmetry can be calculated as follows:

where is the precession rate, is the spin rate about the axis of symmetry, is the angle between the axis of symmetry and the axis about which it precesses, is the moment of inertia about the axis of symmetry, and is moment of inertia about either of the other two perpendicular principal axes. They should be the same, due to the symmetry of the disk.^{[1]}

For a generic solid object without any axis of symmetry, the evolution of the object's orientation, represented (for example) by a rotation matrix that transforms internal to external coordinates, may be numerically simulated. Given the object's fixed internal moment of inertia tensor and fixed external angular momentum , the instantaneous angular velocity is . Precession occurs by repeatedly recalculating and applying a small rotation vector for the short time , e.g. for the skew-symmetric matrix . The errors induced by finite time steps tend to increase the rotational kinetic energy, ; this unphysical tendency can be counter-acted by repeatedly applying a small rotation vector perpendicular to both and , noting that .

Another type of torque-free precession can occur when there are multiple reference frames at work. For example, the earth is subject to local torque induced precession due to the gravity of the sun and moon acting upon the earth’s axis, but at the same time the solar system is moving around the galactic center. As a consequence, an accurate measurement of the earth’s axial reorientation relative to objects outside the frame of the moving galaxy (such as distant quasars commonly used as precession measurement reference points) must account for a minor amount of non-local torque-free precession, due to the solar system’s motion.

Torque-induced precession (**gyroscopic precession**) is the phenomenon in which the axis of a spinning object (e.g., a part of a gyroscope) "wobbles" when a torque is applied to it, which causes a distribution of force around the acted axis. The phenomenon is commonly seen in a spinning toy top, but all rotating objects can undergo precession. If the speed of the rotation and the magnitude of the torque are constant, the axis will describe a cone, its movement at any instant being at right angles to the direction of the torque. In the case of a toy top, if the axis is not perfectly vertical, the torque is applied by the force of gravity tending to tip it over.

The device depicted on the right is gimbal mounted. From inside to outside there are three axes of rotation: the hub of the wheel, the gimbal axis, and the vertical pivot.

To distinguish between the two horizontal axes, rotation around the wheel hub will be called 'spinning', and rotation around the gimbal axis will be called 'pitching.' Rotation around the vertical pivot axis is called 'rotation'.

First, imagine that the entire device is rotating around the (vertical) pivot axis. Then, spinning of the wheel (around the wheelhub) is added. Imagine the gimbal axis to be locked, so that the wheel cannot pitch. The gimbal axis has sensors, that measure whether there is a torque around the gimbal axis.

In the picture, a section of the wheel has been named *dm _{1}*. At the depicted moment in time, section

The same reasoning applies for the bottom half of the wheel, but there the arrows point in the opposite direction to that of the top arrows. Combined over the entire wheel, there is a torque around the gimbal axis when some spinning is added to rotation around a vertical axis.

It is important to note that the torque around the gimbal axis arises without any delay; the response is instantaneous.

In the discussion above, the setup was kept unchanging by preventing pitching around the gimbal axis. In the case of a spinning toy top, when the spinning top starts tilting, gravity exerts a torque. However, instead of rolling over, the spinning top just pitches a little. This pitching motion reorients the spinning top with respect to the torque that is being exerted. The result is that the torque exerted by gravity - via the pitching motion - elicits gyroscopic precession (which in turn yields a counter torque against the gravity torque) rather than causing the spinning top to fall to its side.

Precession or gyroscopic considerations have an effect on bicycle performance at high speed. Precession is also the mechanism behind gyrocompasses.

Gyroscopic precession also plays a large role in the flight controls on helicopters. Since the driving force behind helicopters is the rotor disk (which rotates), gyroscopic precession comes into play. If the rotor disk is to be tilted forward (to gain forward velocity), its rotation requires that the downward net force on the blade be applied roughly 90 degrees (depending on blade configuration) before, or when the blade is to one side of the pilot and rotating forward.

To ensure the pilot's inputs are correct, the aircraft has corrective linkages that vary the blade pitch in advance of the blade's position relative to the swashplate. Although the swashplate moves in the intuitively correct direction, the blade pitch links are arranged to transmit the pitch in advance of the blade's position.

Precession is the result of the angular velocity of rotation and the angular velocity produced by the torque. It is an angular velocity about a line that makes an angle with the permanent rotation axis, and this angle lies in a plane at right angles to the plane of the couple producing the torque. The permanent axis must turn towards this line, since the body cannot continue to rotate about any line that is not a principal axis of maximum moment of inertia; that is, the permanent axis turns in a direction at right angles to that in which the torque might be expected to turn it. If the rotating body is symmetrical and its motion unconstrained, and, if the torque on the spin axis is at right angles to that axis, the axis of precession will be perpendicular to both the spin axis and torque axis.

Under these circumstances the angular velocity of precession is given by:

In which *I _{s}* is the moment of inertia, is the angular velocity of spin about the spin axis, and m*g and r are the force responsible for the torque and the perpendicular distance of the spin axis about the axis of precession. The torque vector originates at the center of mass. Using = , we find that the period of precession is given by:

In which *I _{s}* is the moment of inertia,

The special and general theories of relativity give three types of corrections to the Newtonian precession, of a gyroscope near a large mass such as the earth, described above. They are:

- Thomas precession a special relativistic correction accounting for the observer's being in a rotating non-inertial frame.
- de Sitter precession a general relativistic correction accounting for the Schwarzschild metric of curved space near a large non-rotating mass.
- Lense-Thirring precession a general relativistic correction accounting for the frame dragging by the Kerr metric of curved space near a large rotating mass.

In astronomy, precession refers to any of several gravity-induced, slow and continuous changes in an astronomical body's rotational axis or orbital path. Precession of the equinoxes, perihelion precession, and changes in the tilt of the Earth's axis to its orbit, and the eccentricity of its orbit over tens of thousands of years are all important parts of the astronomical theory of ice ages.

Main article: Axial precession (astronomy)

Axial precession is the movement of the rotational axis of an astronomical body, whereby the axis slowly traces out a cone. In the case of Earth, this type of precession is also known as the *precession of the equinoxes*, *lunisolar precession*, or *precession of the equator*. Earth goes through one such complete precessional cycle in a period of approximately 26,000 years or 1° every 72 years, during which the positions of stars will slowly change in both equatorial coordinates and ecliptic longitude. Over this cycle, Earth's north axial pole moves from where it is now, within 1° of Polaris, in a circle around the ecliptic pole, with an angular radius of about 23.5 degrees.

Hipparchus is the earliest known astronomer to recognize and assess the precession of the equinoxes at about 1º per century (which is not far from the actual value for antiquity, 1.38º).^{[2]} The precession of Earth's axis was later explained by Newtonian physics. Being an oblate spheroid, the Earth has a nonspherical shape, bulging outward at the equator. The gravitational tidal forces of the Moon and Sun apply torque to the equator, attempting to pull the equatorial bulge into the plane of the ecliptic, but instead causing it to precess.

Main article: Apsidal precession

The orbit of a planet around the Sun is not really an ellipse but a flower-petal shape because the major axis of each planet's elliptical orbit also precesses within its orbital plane, partly in response to perturbations in the form of the changing gravitational forces exerted by other planets. This is called perihelion precession or apsidal precession.

Discrepancies between the observed perihelion precession rate of the planet Mercury and that predicted by classical mechanics were prominent among the forms of experimental evidence leading to the acceptance of Einstein's Theory of Relativity (in particular, his General Theory of Relativity), which accurately predicted the anomalies.^{[3]}^{[4]}

See also nodal precession. For precession of the lunar orbit see lunar precession.

Wikimedia Commons has media related to: Precession |

- De Sitter precession
- Larmor precession
- Lense–Thirring precession
- Nutation
- Polar motion
- Precession (mechanical)
- Thomas precession
- Euler angles

**^**Boal, David (2001). "Lecture 26 - Torque-free rotation - body-fixed axes". http://www.sfu.ca/~boal/211lecs/211lec26.pdf. Retrieved 2008-09-17.**^**DIO 9.1 ‡3**^**Max Born (1924),*Einstein's Theory of Relativity*(The 1962 Dover edition, page 348 lists a table documenting the observed and calculated values for the precession of the perihelion of Mercury, Venus, and Earth.)**^**An even larger value for a precession has been found, for a black hole in orbit around a much more massive black hole, amounting to 39 degrees each orbit.

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