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.
Revise this, the fun way
Play it interactively, earn XP and build a streak, free.
Start revising freeWhat you'll cover
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?
An interactive activity.
The surface up close 🌑
With the naked eye and binoculars you can pick out several kinds of feature: - **Craters**: bowl-shaped hollows made by impacts. - **Maria** (seas): dark, flat plains of solidified lava. - **Terrae**: the light, heavily cratered highlands. - **Mountains** and **valleys**: raised ranges and long clefts.
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
What are the seas?
The maria (the lunar "seas") are actually...
- Dark, flat plains of solidified lava
- Large lakes of liquid water
- Deep impact craters
- Bright highland regions
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
What kind of feature?
Tycho and Copernicus are examples of which kind of feature?
- Craters
- Maria (seas)
- Mountains
- Valleys
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**.
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**. Over a month, libration lets us **peek around the edges**, so we can actually see about **59%** of the surface, a little more than half.
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.