Semi-major Axis
Half the longest diameter of an elliptical orbit, defining the average distance between a celestial body and the object it orbits.
The semi-major axis is the longest radius of an ellipse, extending from the center to the farthest point on the curve. In astronomy, it represents half the longest diameter of an elliptical orbit and is a fundamental parameter for defining the size and energy of a celestial body's path around another object.
What this means in real life
Earth's semi-major axis is about 93 million miles from the Sun. This single number tells astronomers how long Earth's year lasts and how much energy it takes to reach it from another planet.
What it isn’t
It is not the distance from the Sun to Earth at any given moment—that varies because orbits are elliptical, not circular. The semi-major axis is an average measure, not a snapshot.
Commonly misused online
People often confuse it with the actual distance to a planet, tweeting 'Mars is 228 million km away' when they really mean its semi-major axis. The real distance changes constantly as Mars orbits.