Planets
The orbital period is the time a body takes to complete one orbit around another body.
An orbital period is the time it takes one object to travel all the way around the object it orbits. Earth's orbital period around the Sun is 365.25 days.
Kepler discovered that the closer a planet is to the Sun, the faster it moves and the shorter its orbital period. This relationship is captured in his third law: the square of the orbital period is proportional to the cube of the semi-major axis.
Orbital periods are measured against the background of fixed stars (sidereal) or relative to the Sun-Earth line (synodic). For planetary age, we use sidereal orbital periods expressed in Earth days.
T² ∝ a³ (Kepler's third law, with T the period and a the orbital radius)Mars sits 1.52 AU from the Sun. √(1.52³) ≈ 1.88 Earth years ≈ 687 days per Mars orbit.
They are extremely stable over human timescales, but gravitational tugs from other planets cause tiny variations over millions of years.