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The orbital period is the time taken for a given object to make one complete orbit about another object.
When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.
There are several kinds of orbital periods for objects around the Sun (or other celestial objects):
Table of synodic periods in the Solar System, relative to Earth:
|Sidereal Period (a)||Synodic Period (a)||Synodic Period (d)|
|Solar surface||0.069 (25.3 days)||0.074||27.3|
|Mercury||0.240846 (87.9691 days)||0.317||115.88|
|Venus||0.615 (225 days)||1.599||583.9|
|Earth||1 (365.25636 solar days)||—||—|
|Apophis (Near Earth Asteroid)||0.886||7.769||2,837.6|
In the case of a planet's moon, the synodic period usually means the Sun-synodic period. That is to say, the time it takes the moon to complete its illumination phases, completing the solar phases for an astronomer on the planet's surface —the Earth's motion does not determine this value for other planets because an Earth observer is not orbited by the moons in question. For example, Deimos' synodic period is 1.2648 days, 0.18% longer than Deimos' sidereal period of 1.2624 d.
For all ellipses with a given semi-major axis the orbital period is the same, regardless of eccentricity.
When a very small body is in a circular orbit barely above the surface of a sphere of radius R and average density ρ in g/cm3, the above equation simplifies to:
So, for the Earth as central body (or for any other spherically symmetric body with the same average density) we get:
and for a body made of water
Thus, as an alternative for using a very small number like G, the strength of universal gravity can be described using some reference material, like water: the orbital period for an orbit just above the surface of a spherical body of water is 3 hours and 18 minutes. Conversely, this can be used as a kind of "universal" unit of time if we have a unit of mass, a unit of length and a unit of density.
In a parabolic or hyperbolic trajectory the motion is not periodic, and the duration of the full trajectory is infinite.
the Earth's surface
|speed||Orbital period||specific orbital energy|
|Earth's surface (for comparison)||6,400 km||0 km||7.89 km/s (17,650 mph)||85 minutes||-62.6 MJ/kg|
|Low Earth orbit||6,600 to 8,400 km||200 to 2,000 km||circular orbit: 7.8 to 6.9 km/s (17,450 mph to 15,430 mph) respectively
elliptic orbit: 8.2 to 6.5 km/s respectively
|89 to 128 min||-29.8 MJ/kg|
|Molniya orbit||6,900 to 46,300 km||500 to 39,900 km||10.0 to 1.5 km/s (22,370 mph to 3,335 mph) respectively||11 h 58 min||-4.7 MJ/kg|
|GEO||42,000 km||35,786 km||3.1 km/s (6,935 mph)||23 h 56 min||-4.6 MJ/kg|
|Orbit of the Moon||363,000 to 406,000 km||357,000 to 399,000 km||1.08 to 0.97 km/s (2,416 to 2,170 mph) respectively||27.3 days||-0.5 MJ/kg|
|Binary star||Orbital period|
|AM Canum Venaticorum||17.146 minutes|
|Beta Lyrae AB||12.9075 days|
|Alpha Centauri AB||79.91 years|
|Proxima Centauri - Alpha Centauri AB||500,000 years or more|
|Look up synodic in Wiktionary, the free dictionary.|
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