Definition
The semi-major axis a is half the length of the orbit's major axis — the longest diameter of the ellipse. For planets, a is commonly expressed in astronomical units (AU). Earth's a ≈ 1 AU.
Kepler's third law
The sidereal period P (years) and semi-major axis a (AU) are linked by P² = a³. Doubling the distance makes the period 2√2 ≈ 2.8 times longer.
