The Lunar Disc: Features, Rotation and Libration
Get to know the Moon you observe. Read its surface features, name the seas and craters, and understand why we always see the same face.
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Our nearest neighbour
The Moon is the brightest and closest object in the night sky, and the first target for most observers. This module reads its surface: the dark seas, the bright highlands and the craters, and explains why we only ever see one face of it.
How big is the Moon?
The Moon has a mean diameter of about 3500 km. Enter that value in km.
The surface up close
With the naked eye and binoculars you can pick out four kinds of feature:
Find the maria
This is a schematic of the lunar disc, not a map of the real nearside. Tap BOTH of the MARIA: the large, smooth, dark plains of solidified lava.
Match each feature to its description
- Craters
- Maria
- Terrae
- Mountains
- Bowl-shaped hollows made by impacts
- Dark, flat plains of solidified lava
- Light, heavily cratered highlands
- Raised ranges, such as the lunar Apennines
Name the features
Learn a few named features you can point to: - Maria: the Sea of Tranquility (where Apollo 11 landed), the Ocean of Storms, the Sea of Crises. - Craters: Tycho (with bright rays), Copernicus and Kepler. - Mountains: the Apennine range.
Match each named feature to what it is
- Sea of Tranquility
- Tycho
- Copernicus
- The Apennine range
- A mare (dark lava sea)
- A bright crater with rays
- A large crater
- A mountain range
The same face
The Moon takes about 27.3 days to spin once on its axis, and about the same time to orbit the Earth once. Because its rotation period equals its orbital period, the Moon always keeps the same face turned toward Earth. This is called synchronous rotation.
How long is a lunar month?
The Moon takes the same time to rotate once on its axis as it does to orbit the Earth once. How many days is that? Give it to one decimal place.
Why one face?
Why do we always see the same side of the Moon from Earth?
- Its rotation period equals its orbital period
- The Moon does not rotate at all
- The far side is always in darkness
- Earth's shadow hides the other side
A slight wobble
The Moon does not move perfectly steadily, so from Earth it seems to rock slightly from side to side and up and down. This apparent wobble is called libration, and it changes how much of the surface we ever get to see.
True of the Moon
Select the TWO statements that are true.
- Libration lets us see slightly more than half the surface over time
- The maria are dark plains of ancient lava
- The Moon does not rotate at all
- The Moon is larger than the Earth
How were craters made?
How were most of the Moon's craters formed?
- By impacts from meteorites and asteroids
- By erupting volcanoes
- By water eroding the surface
- By wind wearing the rock away
Lunar disc summary
The Moon has a mean diameter of about _____ km. Its dark plains of solidified lava are the _____, and its bright cratered highlands are the _____. Because its rotation period equals its orbital period, it always shows the same _____ to Earth.