ÿþPrecession: Precession refers to a change in the direction of the axis of a rotating object. In physics, there are two types of precession, torque-free and torque-induced, the latter being discussed here in more detail. In certain contexts, "precession" may refer to the precession that the Earth experiences, the effects of this type of precession on astronomical observation, or to the precession of orbital objects. % Torque-free precession Only moving objects can be in torque-free precession. For example, when a plate is thrown, the plate may have some rotation around an axis that is not its axis of symmetry. When the object is not perfectly solid, internal vortices will tend to damp torque-free precession. % Torque-induced precession Torque-induced precession ( gyroscopic precession ) is the phenomenon by which the axis of a spinning object (e.g. a part of a gyroscope ) "wobbles" when a torque is applied to it. 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 trying to tip it over. A rolling wheel will tend to remain upright due to precession. When the wheel tilts to one side, the particles at the top are pushed to one side and the particles at the bottom are pushed the other way. However, since the wheel is rotating, these particles eventually switch places and cancel one another out. Precession or gyroscopic considerations have an effect on bicycle performance at high speed. Precession is also the mechanism behind gyrocompasses . This concept is easier to understand by examining the effects of inertia, which is often stated by the phrase "A body in motion tends to stay in motion." In this case the "motion" of a rotating body is in its rotation. If an external force pushes upon the rotating body, the body will resist the force by pushing back against it, but the reaction is delayed. Gyroscopic precession also plays a large role in the flight controls on helicopters. Since the driving force behind helicopters is the rotor head (which rotates), gyroscopic precession comes into play. If the rotor head is tilted to the right, its counter-clockwise movement forces the aircraft to fly forward. To ensure the pilot's inputs are correct the aircraft has corrective linkages which tilt the rotor head to the right when the pilots push the "cyclic stick" forward, or to the left when the stick is pulled to the back. % The physics of precession Precession is the resultant of the angular velocity of rotation and the angular velocity produced by the torque. It is an angular velocity about a line which 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 which 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 period of precession is given by: In which I s is the moment of inertia , T s is the period of spin about the spin axis, and Q is the torque. In general the problem is more complicated than this, however. For a layman s explanation of Precession: we will have to imagine the wheel of a gyroscope as a group of particles that are being forced to move in circle. Remember the particles want to move in a straight line. In order for the particles to move in a curved line there must be a force. This force is provided by the structure of the wheel holding the particles within the wheel. Now let s see what happens to our accelerating particles when a torque is applied to the spinning wheel. Assume the axis of rotation created by the torque is through the center of the wheel at 90 degrees to the primary rotation of the wheel. Let s look at a particle that is on this axis of rotation. Since the particle is on the axis of rotation there is no direct motion applied to the particle at the instant of the applied torque. But let s look at what will need to happen at the next moment in time. The particle is now going to be forced to curve again. This time in the direction of the curve so as to accommodate the tilt of the wheel. Now we have a particle that is already moving and it wants to keep moving in a straight line. So the particle will exert a force on the wheel. If you look at a particle on the other side of the wheel you will see that the force of the second particle is in the opposite direction of the first particle. That pair of unmatched forced is what causes the precession torque that is 90 degrees to the applied torque. % Precession of the equinoxes The Earth goes through one complete precession cycle in a period of approximately 25,800 years, during which the positions of stars as measured in the equatorial coordinate system will slowly change; the change is actually due to the change of the coordinates. Over this cycle the 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 23 degrees 27 arcminutes, or about 23.5 degrees. The shift is 1 degree in 180 years, with the angle is taken from the observer, not from the center of the circle. This precession was noted by ancient astronomers, and was explained by Newtonian physics. The Earth has a nonspherical shape, being oblate spheroid, bulging outward at the equator. The gravitational tidal forces of the Moon and Sun apply torque as they attempt to pull the equatorial bulge into the plane of the ecliptic. The portion of the precession due to the combined action of the Sun and the Moon is called lunisolar precession . % Precession of planetary orbits Precession of the perihelion (very exaggerated) The revolution of a planet in its orbit around the Sun is also a form of rotary motion. (In this case, the combined system of Earth and Sun is rotating.) So the axis of a planet's orbital plane will also precess over time. The major axis of each planet's elliptical orbit also precesses within its orbital plane, in response to perturbations in the form of the changing gravitational forces exerted by other planets. This is called perihelion precession or apsidal precession (see apsis). 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, which predicted the anomalies accurately. It is generally understood that the gravitational pulls of the Sun and the Moon cause the precession of the Earth's orbit which operate on cycles of 23,000 and 19,000 years. These periodic changes of the orbital parameters, as well as that of the inclination of the Earth's axis on its orbit, are an important part of the astronomical theory of ice ages . An analogous phenomenon to apsidal precession is nodal precession (see orbital node), which affects the orientation of the orbital plane. Precession is also an important consideration in the dynamics of atoms and molecules . % Discovery of precession Precession of the equinoxes is caused by a polar motion, a change in the orientation of the Earth's axis. As a result of this motion, the stars as seen from Earth appear to change position slowly. The equinoctial points, where the ecliptic crosses the celestial equator, appear to "precess" through the zodiac. Also, the celestial poles appear to trace small circles in the sky. In actuality, the geographical poles change their orientation. The Earth goes through one complete precession cycle in a period of approximately 25,800 years, so the discovery of precession requires astronomical observations over many centuries. % Geometric precession Geometric precession is the term used to describe the Earth's changing orientation to inertial space not caused by local forces . Local forces, principally the gravity of the Sun and moon acting upon the oblate Earth, are thought to be the principal factors reorienting the Earth s axis relative to the Sun, moon and objects within the solar system (gyroscopic precession). At the same time the solar system is orbiting the galactic center with a period of about 200 to 240 million years. Consequently, the Earth s orientation relative to objects outside the galaxy , which is typically used to measure changes in the Earth s orientation, will also be affected by this motion without any additional local force acting directly upon the Earth. This movement of the larger reference frame, a solar system that curves through space, results in a geometric reorientation of the Earth relative to inertial space as measured from Earth. It is a component in the total amount of observable precession, but not included in the calculation of local dynamics. % Larmor precession In physics , Larmor precession (also "Lamour") refers to the precession of the magnetic moments of electrons or atomic nuclei in atoms around the direction of an external magnetic field. The magnetic field exerts a torque on the magnetic moment, producing a gyroscopic motion, much like the spinning of a top. The angular velocity of the precession is, where ³ is the gyromagnetic ratio and is the external magnetic field. The gyromagnetic ratio is different for each type of atomic nucleus, but is typically given by, where g is the Lande g-factor , ¼ B is the Bohr magneton , and is Dirac's constant. For an electron, the gyromagnetic ratio is approximately 17.61 Mhz / Oe. A famous 1935 paper published by Lev Landau and Evgeny Lifshitz predicted the existence of ferromagnetic resonance of the Larmor precession, which was verified experimentally by J. H. E. Griffiths in 1946. The concept of Larmor precession is used in nuclear magnetic resonance . % Polar motion Polar motion is the movement of Earth 's rotation axis across its surface. The axis of the Earth's rotation tends, like the axis of a gyroscope, to maintain its orientation in inertial space. However, due to external forces acting upon the Earth, this axis nevertheless exhibits a slow, large-scale motion, known as precession and nutation. Nutation is divided into a predictable part from a "nutation theory," a long series of trigonometric terms depending on time, and derived from motions of the Moon and models of the Earth, plus corrections called the Celestial Pole Offset , which are the measured departures (two-dimensional on the celestial sphere) of the celestial rotation pole from the theory [1] These corrections, of order 0.03 arcseconds, are of no importance to most users, but by monitoring them, a better theory of nutation may be developed in the future. When using, instead of an inertial reference frame, a frame attached to the body of the solid Earth (a so-called Earth-centred, Earth-fixed or ECEF reference frame), the rotation axis also varies slightly. This variation, which is only a few metres measured on the Earth's surface, is called Polar motion. It consists of two quasi-periodic components and a gradual drift, mostly westward, of the Earth's instantaneous rotational axis or North pole, from a conventionally defined reference axis, the CIO (Conventional International Origin), being the pole's average location over the year 1900. The two periodic parts are a more or less circular motion called Chandler wobble with a period of about 435 days, and a yearly circular motion. There is also a slow drift which is less well known. These motions are illustrated on International Earth Rotation and Reference Systems Service [2]. One can see that the mean displacement far exceeds in magnitude the wobbles. This can lead to errors in software for Earth observing spacecraft, since analysts may read of a 5 meter circular motion and ignore it, while a 20 meter offset sits there, fouling the accuracy of the calculated latitude and longitude. The latter are determined based on the International Terrestrial Reference System, which follows the polar motion. The slow westward drift, about 20 m since 1900, is partly due to motions in the Earth's core and mantle, and partly to the redistribution of water mass as the Greenland ice sheet melts, as well as to isostatic rebound, i.e. the slow rise of land that was formerly burdened with ice sheets or glaciers [3]. The drift is roughly along the 80th meridian west. % Thomas precession Thomas precession , named after Llewellyn Thomas , is a correction to the spin-orbit interaction in Quantum Mechanics, which takes into account the relativistic time dilation between the electron and the nucleus in hydrogenic atoms. Basically, it states that spinning objects precess when they accelerate in special relativity because rotations do not commute with boosts . Previous chapter:Next chapter

0: Odd, but interesting: 1: 10 Confounding Cosmic Questions 2: Top 10 Cool Moon Facts. 3: Top 10 Star Mysteries 4: Top 10 strangest things in space. 5: The Wildest Weather in the Galaxy 6: Space Station Assembly. 7: ISS: Home, Space Home. 8: Impressive New Tricks of Light, All Within the Laws of Physics 9: Earth-Moon size and distance 10: Dictionary Results for magnetism : 11: Exploring Mars: Basic Mars Facts:- 12: THE MOON 13: What's New on the Moon? 14: Precession: 15: Sedna: A Clue to Nibiru

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