Precession

File:Gyroscope precession.gif

Precession of a gyroscopeTemplate:Clarify

File:Praezession.svg

  Rotation

  Precession

  Nutation in obliquity of a planet

Precession is a change in the orientation of the rotationalTemplate:Nbspaxis of a rotatingTemplate:Nbspbody. In an appropriate [[reference frame|referenceTemplate:Nbspframe]] it can be defined as a change in the first [[Euler angle|EulerTemplate:Nbspangle]], whereas the third EulerTemplate:Nbspangle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the secondTemplate:Nbspaxis. A motion in which the second EulerTemplate:Nbspangle changes is called nutation. In physics, there are two types of precession: Template:Nowr and Template:Nowr.

In astronomy, precession refers to any of several slow changes in an astronomicalTemplate:Nbspbody's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of [[rotation of the Earth|rotationTemplate:Nbspof the Earth]], known as the [[precession of the equinoxes|precessionTemplate:Nbspof the equinoxes]].

Torque-free or torque neglected

Torque-free precession implies that no external moment (torque) is applied to the body. In Template:Nowr precession, the angular momentum is a constant, but the angular velocity vector changes orientation with time. What makes this possible is a Template:Nowr [[moment of inertia|momentTemplate:Nbspof inertia]], or more precisely, a Template:Nowr inertia matrix. The inertia matrix is composed of the momentsTemplate:Nbspof inertia of a body calculated with respect to separate [[Basis (linear algebra)|coordinateTemplate:Nbspaxes]] (e.g.Template:NbspTemplate:Math, Template:Math, Template:Math). If an object is asymmetric about its principal axisTemplate:Nbspof rotation, the momentTemplate:Nbspof inertia with respect to each coordinate direction will change with time, while preserving angular momentum. The result is that the component of the angular velocities of the body about each axis will vary inversely with each axis' momentTemplate:Nbspof inertia.

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 axisTemplate:Nbspof symmetry can be calculated as follows: $${\mathit{\omega }}_{\mathrm{p}}=\frac{{\mathit{I}}_{\mathrm{s}}{\mathit{\omega }}_{\mathrm{s}}}{{\mathit{I}}_{\mathrm{p}}\cos (\mathit{\alpha })}$$ where Template:Math is the precession rate, Template:Math is the spin rate about the axis of symmetry, Template:Math is the moment of inertia about the axis of symmetry, Template:Math is moment of inertia about either of the other two equal perpendicular principal axes, and Template:Mvar is the angle between the moment of inertia direction and the symmetry axis.

When an object is not perfectly rigid, inelastic dissipation will tend to damp Template:Nowr precession, and the rotation axis will align itself with one of the inertia axes of the body.

For a generic solid object without any axis of symmetry, the evolution of the object's orientation, represented (for example) by a rotation matrix Template:Mvar that transforms internal to external coordinates, may be numerically simulated. Given the object's fixed internal moment of inertia tensor Template:Math and fixed external angular momentum Template:Mvar, the instantaneous angular velocity is $$\mathit{\omega }\left(\mathit{R}\right)=\mathit{R}{\mathit{I}}_0^{-1}{\mathit{R}}^T\mathit{L}$$ Precession occurs by repeatedly recalculating Template:Mvar and applying a small rotation vector Template:Math for the short time Template:Math; e.g.: $${\mathit{R}}_{\text{new}}=\exp \left({\left[\mathit{\omega }\left({\mathit{R}}_{\text{old}}\right)\right]}_{\times }dt\right){\mathit{R}}_{\text{old}}$$ for the skew-symmetric matrix Template:Math. The errors induced by finite time steps tend to increase the rotational kinetic energy: $$E\left(\mathit{R}\right)=\mathit{\omega }\left(\mathit{R}\right)\cdot \frac{\mathit{L}}{2}$$ this unphysical tendency can be counteracted by repeatedly applying a small rotation vector Template:Mvar perpendicular to both Template:Mvar and Template:Mvar, noting that $$E(\exp \left({\left[\mathit{v}\right]}_{\times }\right)\mathit{R})\approx E\left(\mathit{R}\right)+(\mathit{\omega }\left(\mathit{R}\right)\times \mathit{L})\cdot \mathit{v}$$

Torque-induced

Torque-induced precession (gyroscopic precession) is the phenomenon in which the axis of a spinning object (e.g., a gyroscope) describes a cone in space when an external 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 external torque are constant, the spin axis will move at right angles to the direction that would intuitively result from the external torque. In the case of a toy top, its weight is acting downwards from its [[center of mass|centerTemplate:Nbspof mass]] and the [[normal force|normalTemplate:Nbspforce]] (reaction) of the ground is pushing up on it at the point of contact with the support. These two opposite forces produce a torque which causes the top to precess.

File:Gyroscopic precession 256x256.png

The response of a rotating system to an applied torque. When the device swivels, and some roll is added, the wheel tends to pitch.

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 Template:Math. At the depicted moment in time, section Template:Math is at the perimeter of the rotating motion around the (vertical) pivot axis. Section Template:Math, therefore, has a lot of angular rotating velocity with respect to the rotation around the pivot axis, and as Template:Math is forced closer to the pivot axis of the rotation (by the wheel spinning further), because of the Coriolis effect, with respect to the vertical pivot axis, Template:Math tends to move in the direction of the top-left arrow in the diagram (shown at 45°) in the direction of rotation around the pivot axis. Section Template:Math of the wheel is moving away from the pivot axis, and so a force (again, a Coriolis force) acts in the same direction as in the case of Template:Math. Note that both arrows point in the same direction.

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 gravityTemplate:Snd via the pitching motionTemplate:Snd 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.

Classical (Newtonian)

File:PrecessionOfATop.svg

The torque caused by the normal force – Template:Math and the weight of the top causes a change in the angular momentum Template:Math in the direction of that torque. This causes the top to precess.

Precession is the change of angular velocity and angular momentum produced by a torque. The general equation that relates the torque to the rate of change of angular momentum is: $$\mathit{\tau }=\frac{\mathrm{d}\mathbf{𝐋}}{\mathrm{d}t}$$ where $\mathit{\tau }$ and $\mathbf{𝐋}$ are the torque and angular momentum vectors respectively.

Due to the way the torque vectors are defined, it is a vector that is perpendicular to the plane of the forces that create it. Thus it may be seen that the angular momentum vector will change perpendicular to those forces. Depending on how the forces are created, they will often rotate with the angular momentum vector, and then circular precession is created.

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

${\mathit{\omega }}_{\mathrm{p}}=\frac{mgr}{I_{\mathrm{s}}{\mathit{\omega }}_{\mathrm{s}}}=\frac{\tau }{I_{\mathrm{s}}{\mathit{\omega }}_{\mathrm{s}}\sin (\theta )}$

where Template:Math is the moment of inertia, Template:Math is the angular velocity of spin about the spin axis, Template:Mvar is the mass, Template:Math is the acceleration due to gravity, Template:Mvar is the angle between the spin axis and the axis of precession and Template:Math is the distance between the center of mass and the pivot. The torque vector originates at the center of mass. Using Template:Math, we find that the period of precession is given by: $$T_{\mathrm{p}}=\frac{4{\pi }^2{I}_{\mathrm{s}}}{mgr{T}_{\mathrm{s}}}=\frac{4{\pi }^2{I}_{\mathrm{s}}\sin (\theta )}{\tau T_{\mathrm{s}}}$$

Where Template:Math is the moment of inertia, Template:Math is the period of spin about the spin axis, and Template:Mvar is the torque. In general, the problem is more complicated than this, however.

Relativistic (Einsteinian)

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 Earth, described above. They are:

• Thomas precession, a special-relativistic correction accounting for an object (such as a gyroscope) being accelerated along a curved path.
• de Sitter precession, a general-relativistic correction accounting for the Template:Langr metric of curved space near a large non-rotating mass.
• Lense–Thirring precession, a general-relativistic correction accounting for the frame dragging by the KerrTemplate:Nbspmetric of curved space near a large rotating mass.

The [[Schwarzschild geodesics|Template:Langr geodesics]] (sometimes Template:Langr precession) are used in the prediction of the anomalous perihelion precession of the planets, most notably for the accurate prediction of the apsidal precession of Mercury.

Astronomy

In astronomy, precession refers to any of several gravity-induced, slow and continuous changes in an astronomical body's rotational axis or orbital path. PrecessionTemplate:Nbspof the equinoxes, perihelion precession, changes in the [[tilt of Earth's axis|tiltTemplate:Nbspof Earth's axis]] to its orbit, and the eccentricity of its orbit over tensTemplate:Nbspof thousands of years are all important parts of the astronomical theory of [[ice age|iceTemplate:Nbspage]]s. Template:Xref

Axial precession (precession of the equinoxes)

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 precessionTemplate:Nbspof the equinoxes, lunisolar precession, or precessionTemplate:Nbspof the equator. Earth goes through one such complete precessional cycle in a period of approximately 26,000Template:Nbspyears or 1° every 72Template:Nbspyears, 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|eclipticTemplate:Nbsppole]], with an angular radius of aboutTemplate:Nbsp23.5°.

The ancient Greek astronomer Template:LangrTemplate:Nbsp(Template:C.) is generally accepted to be the earliest known astronomer to recognize and assess the precessionTemplate:Nbspof the equinoxes at about 1° perTemplate:Nbspcentury (which is not far from the actual value for antiquity, 1.38°), although there is some minor dispute about whether he was. In ancient China, the Jin dynasty scholar-official Template:LangrTemplate:Nbsp(Template:Fl.) made a similar discovery centuries later, noting that the position of the Sun during the winter solstice had drifted roughly one degree over the course of fifty years relative to the position of the stars. The precession of Earth's axis was later explained by Newtonian physics. Being an oblate spheroid, Earth has a Template:Nowr 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. The torque exerted by the planets, particularly Jupiter, also plays a role.

Apsidal precession

File:Precessing Kepler orbit 280frames e0.6 smaller.gif

Apsidal precession—the orbit rotates gradually over time.

The orbits of planets around the Sun do not really follow an identical ellipse each time, but actually trace out 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.

In the adjunct image, Earth's apsidal precession is illustrated. As the Earth travels around the Sun, its elliptical orbit rotates gradually over time. The eccentricity of its ellipse and the precession rate of its orbit are exaggerated for visualization. Most orbits in the Solar System have a much smaller eccentricity and precess at a much slower rate, making them nearly circular and nearly stationary.

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. Deviating from Newton's law, Einstein's theory of gravitation predicts an extra term of Template:Math, which accurately gives the observed excess turning rate of Template:Val perTemplate:Nbspcentury.

Nodal precession

Orbital nodes also precess over time.

See also

• Template:Anl
• Template:Anl
• Template:Anl
• Template:Anl
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External links

• Template:Commonscat-inline
• Explanation and derivation of formula for precession of a top
• Precession and the Milankovich theory From Stargazers to Starships