# How does orbital altitude nomenclature work?

The orbital phase of many probes around a planet or moon is frequently described as N km x M km, with N and M being two numbers.

For example, China's Chang'e 5 lunar orbiter was "placed in a 200 by 5,300-Kilometer orbit".

Do the numbers refer to the periapsis and apoapsis, or the number of orbits times the average altitude?

• You had it right: altitude of periapsis by apoapsis. – Adam Wuerl Jun 3 '15 at 3:28

placed in a 200 by 5,300-Kilometer [lunar] orbit

The mean radius of the Moon, $R_☾ = 1,737.1 \text{ km}$.

$R_{perilune} = 200 \text{ km} + R_☾$ and $R_{apolune} = 5,300 \text{ km} + R_☾$ are the radii of an ellipse, perilune and apolune respectively, centered at the orbital focus, the Moon's center.