Orbital period

An orbital period is the time a body takes to complete one orbit around another body or a shared center of mass. Its value depends on the orbit and the masses involved.

Kepler's third law: A law relating an orbit's period to its semimajor axis and the masses of the orbiting bodies. It gives the central relation between orbital size, mass, and period.
Sidereal period: The time an orbiting body takes to return to the same position relative to distant stars. It is the direct star-referenced measure of completing an orbit.
Anomalistic year: The interval between successive perihelion passages of Earth. It tracks return to the orbit's closest point, not alignment with distant stars.
Lunar month: A recurring interval in the Moon's motion, defined by a particular orbital reference point or phase. Its distinct definitions show why a period depends on the chosen reference.
Semimajor axis: Half the longest diameter of an ellipse; it sets the scale of an orbit. For a given central mass, a larger semimajor axis means a longer period.
Synodic period: The time between repeated alignments of two orbiting bodies as seen from a third body. It differs from an orbital period because the observing body also moves.
Tropical year: The interval between successive passages of the Sun through the vernal equinox. It is a seasonal cycle, distinct from Earth's sidereal orbital period.
Mercury (planet): The smallest planet in the Solar System and the closest to the Sun. Its short year illustrates how a small solar orbit produces a short period.
Gravitational parameter: The product of a system's gravitational constant and its relevant mass. It captures the mass dependence in the period of a two-body orbit.
Transit method: A method for detecting an exoplanet by measuring periodic dips in its star's brightness. Repeated transits reveal the planet's period.

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