Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
The semimajor axis is the longest radius of an ellipse, representing half of the longest diameter. It is a crucial parameter in describing the size and shape of planetary orbits.
5 Must Know Facts For Your Next Test
The semimajor axis determines the orbital period of a planet according to Kepler's Third Law.
It is measured from the center of the ellipse to its furthest edge along the major axis.
In circular orbits, the semimajor axis equals the orbit's radius.
The Earth's semimajor axis is approximately 149.6 million kilometers, defining one Astronomical Unit (AU).
Changes in the semimajor axis can affect a planet’s distance from its star and thus its climate and habitability.