Semi-Major Axis

The semi-major axis is half of the longest diameter of an elliptical orbit, and it is the standard way astronomers describe the overall size of any orbit, whether it belongs to a planet, comet, asteroid, moon, or artificial satellite. Because most real orbits are not perfect circles but ellipses, an orbit has both a longest axis, called the major axis, and a shortest axis, called the minor axis; the semi-major axis is simply half of that longer one, and it can be thought of as the orbit's average distance from the object it orbits. Earth's semi-major axis around the Sun, for example, is almost exactly one astronomical unit, the standard yardstick used for measuring distances within the solar system. The semi-major axis is directly connected to how long an orbit takes to complete: according to Kepler's third law of planetary motion, the square of an orbital period is proportional to the cube of the semi-major axis, meaning objects with larger orbits not only travel farther but also move more slowly, taking dramatically longer to complete one trip. Along with orbital eccentricity, which describes how elongated or stretched-out an ellipse is, the semi-major axis is one of the essential numbers that fully defines the size and shape of any orbit.