rotation period

A rotating body's rotation period is the time it takes to revolve once, e.g., Earth's is essentially a day. The body's rotation rate is the reciprocal of the rotation period, e.g., Earth's is essentially 1 rotation per day. Its rotation speed is the speed of movement at the body's equator, on the order of 1,000 miles 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 (aka synodic rotation period) consists of a full 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 of 86400 seconds is longer than its actual rotation period of about 86164 seconds (sidereal rotation period). Venus, which rotates in the opposite direction to its orbit (retrograde rotation), has a day that is shorter than its rotation period. 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 rapidly, some having rotation periods of small fractions of a second (millisecond pulsars). Relativity places a limit on a rotation period of an object because rotation cannot be so rapid that any part of the object (e.g., its equator) moves faster than the speed of light in a vacuum, which places a limit on the possible rotation rate and period, based upon the size of the object. On the discovery of pulsars, their clock-like steadiness of the repeating pulses suggested a rotating source, and the implied rotation periods were far too small for a main sequence star, important evidence toward concluding they are neutron stars.

If the rotation period matches the orbital period and is prograde (i.e., not retrograde), the same side of the object constantly faces its host (if the orbit is circular; if not, i.e., the orbit is eccentric, there is some variation regarding exactly which part faces the host) which is termed a synchronous rotation, a sign of tidal locking. This is true of the Moon, facing Earth, and is true of most moons in the solar system. Some rotation periods: