Semi-Major Axis

By making a series of careful measurements, astronomers can determine the orbit for a newly discovered Solar System object. Three properties used to describe an orbit are used throughout this investigation.

Orbital size is related to the orbit's semi-major axis (which is defined as half of the longest diameter of the object's orbit). It is measured in astronomical units (au). You can also think of the orbit's semi-major axis as its average distance from the Sun.

An image indicating the semi-major and semi-minor axis of an ellipse.
An image indicating the semi-major and semi-minor axis of an ellipse. Credit: Sae1962, Creative Commons 4.0 international.

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