omløpstiden

Omløpstiden (the orbital period) is the time required for a body to complete one full orbit around another body. In astronomy, it describes how long a satellite takes to go once around a planet or how long a planet or other object takes to orbit the Sun. The symbol T is commonly used, and the period is expressed in seconds, days, or years.

In a two-body system, the orbital period is governed by Kepler's third law. If a is the orbit’s semi-major axis and M and m are the masses of the central and orbiting bodies, respectively, then T^2 = 4π^2 a^3 / [G(M + m)]. When the orbiting mass m is much smaller than M, this simplifies to T ≈ 2π sqrt(a^3 / GM). For orbits around the Sun, Kepler’s law can be written in convenient units as T^2 ≈ a^3 when T is in years and a is in astronomical units.

Examples illustrate the concept. The Earth’s orbital period around the Sun is about 365.25 days. The Moon’s sidereal orbital period around Earth is about 27.32 days; its synodic period, relative to the Sun-Earth line, is about 29.53 days. In practice, “orbital period” can refer to different definitions depending on reference points, such as sidereal versus synodic references.

Omløpstiden is central to orbital mechanics, satellite mission design, and the study of exoplanets, influencing navigation, timing, and the interpretation of periodic signals in astronomy.