noun
Technical term used in astronomy and orbital mechanics to describe the longest radius of an elliptical orbit.
地球の軌道長半径は約1天文単位です。
The semi-major axis of Earth's orbit is about 1 astronomical unit.
軌道長半径が大きいほど、惑星の公転周期は長くなります。
The larger the semi-major axis, the longer the planet's orbital period.
Compound of 軌道 (orbit), 長 (long), and 半径 (radius), directly translating the concept of semi-major axis.