Orbital Period Calculator
Calculate the orbital period of any object orbiting a central body using Kepler's Third Law of Planetary Motion.
Eccentricity does not affect the period — only the semi-major axis matters (Kepler's 3rd Law)
Formula
Kepler's Third Law (Newton's form):
T = 2π √( a³ / GM )
- T — Orbital period (seconds)
- a — Semi-major axis of the orbit (metres)
- G — Gravitational constant = 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻²
- M — Mass of the central body (kg)
Note: The orbital period depends only on the semi-major axis and the central mass — not on eccentricity. Two orbits with the same semi-major axis but different eccentricities have identical periods (Kepler's Third Law).
Mean orbital speed (circular approximation): v = 2πa / T
Assumptions & References
- Reference: Newton, I. (1687). Principia Mathematica; Kepler, J. (1619). Harmonices Mundi.