# Orbital period

The orbital period is the time taken for a given object to make one complete orbit around 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):

• The sidereal period is the temporal cycle that it takes an object to make a full orbit, relative to the stars. This is the orbital period in an inertial (non-rotating) frame of reference.
• The synodic period is the temporal interval that it takes for an object to reappear at the same point in relation to two or more other objects, e.g. when the Moon relative to the Sun as observed from Earth returns to the same illumination phase. The synodic period is the time that elapses between two successive conjunctions with the Sun–Earth line in the same linear order. The synodic period differs from the sidereal period due to the Earth's orbiting around the Sun.
• The draconitic period, or draconic period, is the time that elapses between two passages of the object through its ascending node, the point of its orbit where it crosses the ecliptic from the southern to the northern hemisphere. This period differs from the sidereal period because both the orbital plane of the object and the plane of the ecliptic precess with respect to the fixed stars, so their intersection, the line of nodes, also precesses with respect to the fixed stars. Although the plane of the ecliptic is often held fixed at the position it occupied at a specific epoch, the orbital plane of the object still precesses causing the draconitic period to differ from the sidereal period.
• The anomalistic period is the time that elapses between two passages of an object at its periapsis (in the case of the planets in the solar system, called the perihelion), the point of its closest approach to the attracting body. It differs from the sidereal period because the object's semimajor axis typically advances slowly.
• Also, the Earth's tropical period (or simply its "year") is the time that elapses between two alignments of its axis of rotation with the Sun, also viewed as two passages of the object at right ascension zero. One Earth year has a slightly shorter interval than the solar orbit (sidereal period) because the inclined axis and equatorial plane slowly precesses (rotates in sidereal terms), realigning before orbit completes with an interval equal to the inverse of the precession cycle (about 25,770 years).

## Relation between the sidereal and synodic periods

Table of synodic periods in the Solar System, relative to Earth:[citation needed]

Object Sidereal period (yr) Synodic period (yr) Synodic period (d)
Solar surface       0.069[1] (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)     —     —
Moon       0.0748   (27.32 days)   0.0809   29.5306
99942 Apophis (near-Earth asteroid)       0.886   7.769   2,837.6
Mars       1.881   2.135   779.9
4 Vesta       3.629   1.380   504.0
1 Ceres       4.600   1.278   466.7
10 Hygiea       5.557   1.219   445.4
Jupiter       11.86   1.092   398.9
Saturn       29.46   1.035   378.1
Uranus       84.01   1.012   369.7
Neptune       164.8   1.006   367.5
134340 Pluto       248.1   1.004   366.7
136199 Eris       557   1.002   365.9
90377 Sedna       12050   1.00001   365.1[citation needed]

In the case of a planet's moon, the synodic period usually means the Sun-synodic period, namely, 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's synodic period is 1.2648 days, 0.18% longer than Deimos's sidereal period of 1.2624 d.[citation needed]

## Calculation

### Small body orbiting a central body

According to Kepler's Third Law, the orbital period $T\,$ (in seconds) of two bodies orbiting each other in a circular or elliptic orbit is:

$T = 2\pi\sqrt{a^3/\mu}$

where:

• $a\,$ is the orbit's semi-major axis in meters
• $\mu = GM \,$ is the standard gravitational parameter in $m^3/s^2$
• $G \,$ is the gravitational constant,
• $M \,$ is the mass of the more massive body.

For all ellipses with a given semi-major axis the orbital period is the same, regardless of eccentricity.

### Orbital period as a function of central body's density

When a very small body is in a circular orbit barely above the surface of a sphere of any radius and mean density ρ (in kg/m3), the above equation simplifies to (since $M = \rho V = \rho {\frac {4}{3}} \pi a^3$):[citation needed]

$T = \sqrt{ \frac {3\pi}{G \rho} }$

So, for the Earth as the central body (or any other spherically symmetric body with the same mean density, about 5515 kg/m3)[2] we get:

$T = 1.41$ hours

and for a body made of water (ρ≈1000 kg/m3)[3]

$T = 3.30$ hours

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.

### Two bodies orbiting each other

In celestial mechanics, when both orbiting bodies' masses have to be taken into account, the orbital period $T\,$ can be calculated as follows:[4]

$T= 2\pi\sqrt{\frac{a^3}{G \left(M_1 + M_2\right)}}$

where:

• $a\,$ is the sum of the semi-major axes of the ellipses in which the centers of the bodies move, or equivalently, the semi-major axis of the ellipse in which one body moves, in the frame of reference with the other body at the origin (which is equal to their constant separation for circular orbits),
• $M_1+M_2\,$ is the sum of the masses of the two bodies,
• $G\,$ is the gravitational constant.

Note that the orbital period is independent of size: for a scale model it would be the same, when densities are the same (see also Orbit#Scaling in gravity).[citation needed]

In a parabolic or hyperbolic trajectory, the motion is not periodic, and the duration of the full trajectory is infinite.[citation needed]

### Synodic period

When two bodies orbit a third body in different orbits, and thus different orbital periods, their respective, synodic period can be found. If the orbital periods of the two bodies around the third are called $P_1$ and $P_2$, so that $P_1 < P_2$, their synodic period is given by

$\frac{1}{P_{syn}}=\frac{1}{P_1}-\frac{1}{P_2}$

## Binary stars

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