The Lunar Disc: Features, Rotation and Libration
Our Nearest Neighbour
The Moon is Earth’s only natural satellite and the most studied body in the night sky. Its mean diameter is approximately 3 500 km — roughly a quarter of Earth’s mean diameter. Even with the naked eye you can see broad bright and dark patches across its face; through binoculars or a small telescope a stunning world of craters, mountains and ancient lava plains comes into view.
Types of Lunar Feature
The lunar surface is divided into two broad terrain types. The bright, rugged highland regions are called terrae (singular: terra). These ancient rocks record billions of years of meteorite bombardment. The darker, lower-lying regions are the maria (singular: mare). Despite their watery name, the maria are vast plains of solidified basaltic lava that flooded low-lying impact basins between roughly 3 and 4 billion years ago. They appear darker because basalt reflects less sunlight than highland rock.
Three named maria you are expected to know are the Sea of Tranquility (Mare Tranquillitatis — landing site of Apollo 11 in 1969), the Ocean of Storms (Oceanus Procellarum — the largest dark region, visible to the naked eye) and the Sea of Crises (Mare Crisium — a nearly circular mare in the north-east quadrant of the near side).
The lunar surface also bears mountains and valleys. The most prominent range is the Apennine mountain range, which arcs around the south-eastern rim of Mare Imbrium and contains peaks exceeding 5 km in height — comparable to the European Alps.
Named Craters
Craters cover most of the lunar surface. Three prominent craters form a useful observational triangle near the southern highlands:
- Tycho — a young, bright crater in the southern highlands, famous for its spectacular ray system that stretches thousands of kilometres across the surface. The rays are ejected material blasted out at impact and are most visible at full moon.
- Copernicus — a large, well-preserved impact crater with a terraced inner wall and central peaks, located in the Oceanus Procellarum region. Roughly 93 km in diameter.
- Kepler — a smaller but well-defined crater west of Copernicus, also showing a bright ray system.
Key named features: Maria (Sea of Tranquility, Ocean of Storms, Sea of Crises); Craters (Tycho — youngest rays; Copernicus — terraced walls; Kepler — bright rays); Mountains (Apennine range, peaks to 5 km).
Synchronous Rotation and Tidal Locking
If you observe the Moon over many nights, you will notice that it always shows the same face. This is not a coincidence — it is the result of tidal locking (also called synchronous rotation). The Moon’s rotation period (the time it takes to spin once on its axis) is 27.3 days, which is exactly equal to its revolution period (the time it takes to complete one orbit of Earth). Billions of years of tidal friction — gravitational tugging between Earth and the Moon’s slightly non-uniform mass — gradually slowed the Moon’s spin until the two periods matched and stabilised.
The result is that the near side of the Moon always faces Earth, while the far side — sometimes called the ‘dark side’ (a misleading label, since it receives just as much sunlight as the near side) — was never seen by humans until the Soviet Luna 3 spacecraft photographed it in 1959.
The far side receives sunlight for approximately half its month, just as the near side does. The term ‘dark side’ is sometimes used loosely to mean ‘the side we cannot see from Earth’. In exam answers, call it the far side to avoid confusion — precise terminology matters. The far side is actually more heavily cratered than the near side because a thicker crust prevented the volcanic flooding that created the near side’s maria.
Libration: Peeking Around the Edge
Although we always see the same face of the Moon on average, we can actually observe slightly more than 50% of its total surface over time. This effect is called libration. There are three main causes:
- Libration in longitude: The Moon travels in an elliptical orbit so its speed varies — faster at perigee (closest point) and slower at apogee (furthest point). Its spin rate, however, is constant. This mismatch means the Moon appears to rock from side to side slightly, letting us peek around its eastern and western limbs alternately.
- Libration in latitude: The Moon’s equator is tilted slightly with respect to its orbital plane, so it nods gently north and south during each orbit, exposing a little of each polar region in turn.
- Diurnal libration: Because Earth rotates, an observer shifts position between moonrise and moonset, viewing the Moon from slightly different angles and seeing marginally around one limb or the other.
Combined, librations allow us to observe approximately 59% of the lunar surface over time, rather than exactly 50%.
Libration allows about 59% of the Moon’s surface to be seen from Earth over time. There are three types: libration in longitude (elliptical orbit speed variation), libration in latitude (axial tilt of the Moon) and diurnal libration (observer’s shift as Earth rotates).
The best time to observe craters is not at full moon, despite the Moon being brightest then. At full moon, sunlight falls almost straight down onto the surface, leaving craters without shadows and making them hard to distinguish. The best views come in the days around first quarter (waxing crescent / quarter phase) when the terminator — the line between lit and unlit hemispheres — crosses the surface. Here, low-angle sunlight casts long, dramatic shadows across crater rims and mountain peaks. The exception is ray craters like Tycho and Copernicus, which are actually most prominent at full moon because their bright ejecta rays reflect a great deal of sunlight.