Astrophysics (Index)About

rotation period

(time it takes an astronomical body to revolve)

A rotating body's rotation period is the time it takes to revolve once, e.g., about a day or 24 hours for Earth. The body's rotation rate is the reciprocal of the rotation period, e.g., about 1 rotation per day or 1/24 of a rotation per hour for Earth.

For a non-solid body such as a star, the rotation period is not a single time period since portions rotate faster than other portions, termed differential rotation.

Regarding planetary rotation (or planet rotation), a planet's day consists of a rotation plus or minus a fraction of a rotation to the point where the same side faces the host star given their new positions in the course of their orbit. Thus the Earth's day is 86400 seconds, but its rotation period is about 86164 seconds. For Venus, which rotates in the opposite direction to its orbit, its rotation period is longer than its day. Determining the rotation of extra-solar planets is a challenge, so the number of planets for which rotation is known is limited. Rotation of rocky planets may be of interest in determining their habitability.

Pulsars are neutron stars, which are sufficiently compact to rotate fast, some have rotation periods of small fractions of a second (millisecond pulsars). Relativity places a limit on a rotation period of a large object because rotation cannot be so fast that any part of the object (e.g., its equator) moves faster than the speed of light in a vacuum.

If the rotation period matches the orbital period, the same side of the object faces its host, which is termed a synchronous rotation, indicating tidal locking. This is true of the Moon, facing Earth, and is true of most moons in the solar system.

Some rotation periods:

Sun 25-to-35 (Earth) days (differential).
Mercury 59 days.
Venus 243 days
Moon 27.3 days (synchronous).
Mars 1 day
Jupiter 0.41 days (interior).
Saturn 0.44 days (interior).
Uranus 0.72 days.
Neptune 0.67 days.
Pluto 6.39 days.
51 Pegasi b 4.2 days (synchronous).
Hulse-Taylor Binary 59 milliseconds.
J1713-0747 4.57 ms.
Black Widow Pulsar 1.61 ms.

Further reading:

Referenced by pages:
Ap star
Beta Pictoris b (β Pic b)
brown dwarf (BD)
color-period diagram
gravitational potential model
Hadley cell
Meier paradox
millisecond pulsar (MSP)
period derivative
planet formation
P-Pdot diagram
Hulse-Taylor Binary (PSR B1913+16)
pulsar (PSR)
pulsar characteristic age (τ)
rotating radio transient (RRAT)
stellar age determination
stellar parameter determination
synchronous orbit
tidal locking
tidal migration
T-Tauri star (TTS)