Maria and terrae

Even with the naked eye the Moon splits into two kinds of terrain:

• Maria (singular mare, "seas") — dark, smooth, low-lying plains of solidified basalt lava that flooded giant impact basins. Because they formed later, they have had less time to be hit and so carry few craters. Examples: Mare Tranquillitatis (Sea of Tranquillity), Mare Imbrium, Oceanus Procellarum.
• Terrae (highlands) — bright, rugged, higher ground made of older, lighter rock. They are saturated with craters, showing they are the oldest surface on the Moon.

Crater counting: more craters = older surface. That single idea lets you rank any two lunar regions by age — and it is a favourite Edexcel question.

Which surface is older?

Craters, rays and rilles

Almost every circular feature on the Moon is an impact crater. A high-speed impact blasts out a bowl with:

• a raised wall / rampart around the rim,
• often a central peak (rock rebounding after the impact),
• an ejecta blanket of debris thrown out around it, and sometimes bright rays — long streaks of fresh ejecta that mark a young crater (Tycho and Copernicus are the classic ray craters).

Other named features you must recognise:

• Rilles — long, narrow channels; many are collapsed lava tubes, others are rift valleys (e.g. Hadley Rille, visited by Apollo 15).
• Mountain ranges — such as the Apennines, which are the uplifted rim of the Imbrium impact basin.
• Regolith — the layer of powdery, broken rock and dust covering everything, ground up by billions of years of impacts.

Why craters survive: the Moon has essentially no atmosphere and no liquid water, so there is no wind or rain to erode them and no plate tectonics to recycle the crust.

Why we only ever see one face

The Moon takes 27.3 days to orbit the Earth (its sidereal month) — and it also takes 27.3 days to turn once on its own axis. The two periods are locked together. This is called synchronous (or captured) rotation, and it means the same hemisphere always faces us.

Because the Moon's orbit is slightly elliptical and tilted, it appears to rock and nod slightly — this is libration. Over a full cycle libration lets us peep round the edges, so about 59% of the lunar surface can be seen from Earth in total.

Nail the wording: the Moon does rotate — a "far side" is not a "dark side". Every part of the Moon gets roughly two weeks of sunlight and two weeks of night. The far side was first photographed by the Soviet Luna 3 probe in 1959, and it turned out to have almost no maria and a much thicker crust.

Your turn — the hidden Moon

One face only

The size of the disc in the sky

The Moon is 3475 km across and lies (on average) 384 400 km away. Its angular size — how big the disc looks — follows the small-angle relationship:

The great coincidence: the Sun is about 400× wider than the Moon but also about 400× further away, so it too appears about 0.5° across. That is exactly why total solar eclipses are possible from the Earth.

Your turn — measuring a crater

Your turn — angular size

Sort the lunar features

Tap a description, then tap the type of terrain or feature it belongs to.

Match the description to the feature

Tap a description on the left, then its matching term on the right.

The far side

Why craters last

Observing and recording the Moon

The best place to look is the terminator — the line dividing the lit and unlit parts of the disc. There the Sun is low in the lunar sky, so craters and mountains cast long shadows and the relief leaps out. A full Moon, lit from straight on, is the worst time: it is dazzling and looks disappointingly flat.

• Landmarks to find: Tycho (the bright ray crater in the south), Copernicus, Plato (a dark-floored crater on the edge of Mare Imbrium), the Apennine mountains and Mare Tranquillitatis.
• Record by sketching (note the date, time, phase and instrument) or by imaging with a camera or webcam.
• Remember that many telescopes invert the image — north and south, or east and west, may be swapped compared with a map.

Earthshine: the faint glow filling in the dark part of a thin crescent Moon is sunlight reflected from the Earth onto the lunar night side — "the old Moon in the new Moon's arms". Leonardo da Vinci worked out the explanation.

When to look

The big ideas to know

That is The Lunar Disc covered for Edexcel GCSE Astronomy. Press Finish to see your score.