An Etymological Dictionary of Astronomy and Astrophysics
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Same as → rotation axis and → rotational axis.

→ axis; → rotation.

A system for counting rotations of the Sun based on the mean → synodic rotation period of the Sun. Initially, Lord Carrington determined the solar rotation rate by watching low-latitude → sunspots. He defined a fixed solar coordinate system that rotates in a sidereal frame exactly once every 25.38 days. This means that the solar rotation period, as viewed from the Earth, is assumed to be constant. However, the synodic rotation rate varies during the year because of the changing speed of the Earth in its orbit and the mean synodic period is about 27.2753 days. Carrington rotation number 1 began on November 9, 1853.

Named for Richard C. Harrington (1826-1875), British astronomer, who initiated the system; → rotation.

For any rotating planetary body, a thermal limit beyond which the → rotational velocity at the equator intersects the → Keplerian orbital velocity. Beyond this corotation limit, a hot planetary body forms a structure, called a → synestia, with a corotating inner region connected to a disk-like outer region. Beyond this limit a body cannot have a single → angular velocity. It can instead exhibit a range of morphologies with disk-like outer regions. The (CoRoL is a function that depends upon the composition, thermal state, → angular momentum and mass of a body (Simon J. Lock nd Sarah T. Stewart, 2017, arXiv:1705.07858v1).

→ co-; → rotational; → limit.

The act of corotating.

Verbal noun of → corotate.

1) In the → X-wind model of → accretion, the distance from the star where the → centrifugal force on a particle corotating with the star balances the → gravitational attraction; in other words, where the → accretion disk rotates at the same → angular velocity as the star.
2) In a → spiral galaxy, the place where the spiral → pattern speed has the same velocity as the → rotation curve of the → galactic disk. In the frame rotating with the wave, particles inside this radius will appear to revolve in the direction of the frame rotation (prograde) while outside this corotation radius, they will be retrograde.

→ corotation; → radius.

That condition of a → galactic disk at an orbital radius in which the → angular velocity of the disk equals the → pattern speed. It is significant that the spiral wave pattern rotates as a rigid body (Ω_P = const), whereas the galactic disk rotates differentially (Ω is a function of galactocentric distance r). The distance r_C at which the two angular velocities coincide (Ω(r_C) = Ω_P) is referred to as the → corotation radius. The corotation resonance and its position within the galaxy is one of the fundamental properties of a spiral galaxy.

→ corotation; → resonance.

The clockwise rotation of the → plane of polarization of light (as viewed by an observer looking straight in the incoming light) by certain substances. See also → levorotation.

→ dextro-; → rotation.

1) Of a single body (such as a star or a gaseous planet), the axial rotation of equatorial latitudes faster than polar latitudes.
2) Of a galaxy, the orbiting of stars nearer the center faster than those at the edge.

→ differential; → rotation.

The natural motion of the Earth around its own axis, which takes place once in a → sidereal day. The Earth rotates toward the → east, in the same direction as it revolves around the Sun. If viewed from the north celestial pole, the Earth turns → counterclockwise. The opposite is true when the Earth is viewed from the south celestial pole. The Earth's rotation is responsible for the diurnal cycles of day and night, and also causes the apparent movement of the Sun across the sky. The Earth's rotation velocity at the → equator is 1,673 km h^-1 or about 465 m s^-1. More generally, at the → latitude  φ it is given by: v_φ = v_eq cos φ, where v_eq is the rotation velocity at the equator. The Earth's rotation is gradually slowing down under the action of the → tides, which are generated by the → gravitational attraction of the → Moon. As the result of this → tidal friction, the day is becoming longer at a rate of about 2 milliseconds, or 0.002 seconds, per century (or one second every 50,000 years). Moreover, the loss of the Earth's → rotational angular momentum increases the Moon's → orbital angular momentum, because the angular momentum of the → Earth-Moon system is conserved. In consequence, the Moon slowly recedes from the Earth by about 4 cm per year, which leads to increasing its orbital period and the length of a month as well.

→ Earth; → rotation.

The rotation of the plane of → polarization experienced by a beam of → linearly polarized radiation when the radiation passes through a material containing a magnetic field with a component in the direction of propagation. This effect occurs in → H II regions in which a magnetic field causes a change in the polarized waves passing through. Same as → Faraday effect.

→ farad; → rotation.

The effect of the Earth's rotation on the position of the image formed on the → focal plane of a telescope during long exposures. In the case of → equatorial mounting, the image remains fixed, whereas it turns continuously with an → altazimuth mounting. In the latter case the image motion must be compensated by an appropriate mechanism, → field rotator.

→ field; → rotation.

A galactic → rotation curve in which the → rotation velocity is constant in the outer parts. The flat component is preceded by a rising curve that shows solid body rotation in the very center of the → galaxy. A flat rotation curve implies that the mass is still increasing linearly with radius. See also → dark matter.

→ flat; → rotation; → curve.

The revolving of the gaseous and stellar content of a galaxy around its central nucleus. The rotation is not uniform, but differential. One revolution of the Sun within our own Galaxy takes about 220 million years, or one cosmic year.

→ galactic; → rotation.

The discrepancy between observed galaxy → rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material.

→ galactic; → rotation; → problem.

A → rotation curve in which the speed of the orbiting body is inversely proportional to the → square root of its distance from the mass concentrated at the center of the system.

→ Keplerian; → rotation; → curve.

The counterclockwise rotation of the → plane of polarization of light (as observed when looking straight through the incoming light) by certain substances.

Adj. related to → levorotation.

The Moon's motion around its axis, which takes place in 27.321 661 days (→ sidereal month). Since the Moon and the Earth are → tidally locked our satellite has a → synchronous rotation. This means that it rotates once on its axis in the same length of time it takes to revolve around Earth. That is why the Moon always shows the same face to us. However, over time we can see up to 59 percent of the lunar surface because the Moon does not orbit at a constant speed (→ libration in longitude) and its axis is not perpendicular to its orbit (→ libration in latitude). The Moon also creates tides in Earth oceans. As the Earth rotates, the rising and falling sea waters bring about friction within the liquid itself and between the water and solid Earth. This removes energy from Earth's rotation and causes it to spin more slowly. As a result, days are getting longer, at about 2 milliseconds per century. On the other hand, since the → angular momentum of the → Earth-Moon system must be conserved, the Moon gradually moves away from the Earth. This, in turn, requires its orbital period to increase and, because the Moon is tidally locked to Earth, to spin more slowly.

→ lunar; → rotation.

An instability that arises from the action of a weak → poloidal magnetic field in a → differentially rotating system, such as a → Keplerian disk. The MRI provides a mechanism to account for the additional outward → angular momentum transport. To put it simply, the → frozen magnetic field line acts as a spring connecting two radially neighboring fluid parcels. In a Keplerian disk the inner fluid parcel orbits more rapidly than the outer, causing the spring to stretch. The magnetic tension forces the inner parcel to slow down reducing its angular momentum by moving it to a lower orbit. The outer fluid parcel is forced by the spring to speed up, increase its angular momentum, and therefore move to a higher orbit. The spring tension increases as the two fluid parcels grow further apart, and eventually the process runs away. The MRI was first noted in a non-astrophysical context by E. Velikhov in 1959 when considering the stability of → Couette flow of an ideal hydromagnetic fluid. His result was later generalized by S. Chandrasekhar in 1960. The MRI was rediscovered by Balbus and Hawley 1991 (ApJ 376, 214) who demonstrated that this instability does indeed manifest itself in → accretion disks, and could account for the turbulent mixing needed to explain the observed mass → accretion rates.

→ magneto-; → rotational; → instability.

A → tumbling motion of an → asteroid. See also → tumbling asteroid.

→ non-; → principal axis; → rotational; → motion.

For a rotating object, the plane → perpendicular to the → rotation axis.

→ plane; → rotation.