Moon glossary

Every term you will meet while exploring the Moon, in plain English. They come to life on the interactive Moon globe, where the terminator, libration, and the orbiters are all live.

The Moon itself

Terminator: The moving line between lunar day and night. Because a lunar day lasts 29.5 Earth days, it creeps about 15 km per hour at the equator. Craters look most dramatic near it, where the Sun grazes the surface sideways.
Libration: The slow apparent wobble of the Moon as seen from Earth. The Moon always shows us the same face, but its tilted, elliptical orbit lets us peek slightly around the edges; over time 59 percent of the surface is visible from Earth.
Far side ("the dark side"): The hemisphere that never faces Earth, popularly called the dark side of the Moon. The "dark" means unseen rather than unlit: it gets exactly as much sunlight as the near side. It is dark to radio, though, which is why far-side missions need a relay satellite.
Mare (plural: maria): The dark plains visible from Earth: ancient basaltic lava floods, not seas. Early astronomers named them as if they held water: Mare Tranquillitatis, the Sea of Tranquility, is where Apollo 11 landed.
Permanently shadowed regions: Crater floors near the poles that sunlight never reaches, some colder than Pluto. Radar and neutron data suggest they trap water ice, which is why nearly every announced future landing site aims at the south pole.
Regolith: The layer of crushed rock and glassy dust covering the Moon, made by eons of micrometeorite impacts. Sharp, abrasive, and clingy; Apollo crews found it smelled like spent gunpowder.
Selenographic coordinates: Latitude and longitude on the Moon. Longitude 0 runs through the centre of the near side; the far side is around longitude 180. Every site on this globe is placed by its selenographic coordinates.

Orbits and tracking

Perilune / apolune: The low and high points of a lunar orbit, the Moon equivalents of perigee and apogee. Queqiao-2 dives to about 254 km at perilune and swings out past 16,900 km at apolune.
Frozen orbit: An orbit designed so the Moon’s lumpy gravity does not slowly twist it. A frozen elliptical orbit can keep its high point parked over one region for years, which is how relay satellites hover usefully over the south pole.
Ephemeris: A table of where a spacecraft is over time. Lunar orbiters have no public catalog like Earth satellites do; this globe uses NASA/JPL Horizons ephemerides, refreshed twice daily, and shows "no data" when one lapses.
Retroreflector: A mirror array that bounces light straight back where it came from. Five sites on the Moon still return laser pulses fired from Earth (Apollo 11, 14, 15 and both Lunokhod rovers), measuring the Earth-Moon distance to millimetres.
Tidal locking: Why the Moon always shows Earth the same face: its rotation and orbit take the same 27.3 days, a lock created by billions of years of tidal friction. The wobble left over is libration.

Open the Moon globe →

azmth Luna is the Moon half of azmth, the free real-time satellite tracker: an interactive 3D Moon with every verified landing site, the live orbiters, and a catalog of 76 landmark missions. No signup, no ads.

Satellite and orbit glossary →