Beyond Earth · Lesson 5 of 7
The Moon and Natural Satellites
“Investigate why the Moon orbits Earth and what its cratered surface tells us.”
• Define a natural satellite and distinguish a moon from a planet. • Describe the Moon’s revolution, distance, and size relative to Earth. • Explain the formation and long survival of lunar impact craters. • Describe how spacecraft and images help investigate the Moon.
Objects travelling around planets
Planets are not the only bodies that travel around another object. A moon is a natural satellite of a planet. Earth has one Moon; Mars has two, and each of the four outer giant planets has many. The term satellite can be used more broadly for a body moving around a larger body, but in this lesson a natural satellite means an object such as the Moon, rather than a human-made spacecraft.
A naturally occurring body that moves around a planet; a moon is a natural satellite.
A planet travels around the Sun, while a planet’s moon travels around that planet. The Moon also travels with Earth around the Sun, so its motion is more than a single flat circle on a page. Earth’s Moon is our nearest neighbour in space and appears bright because sunlight falls on it and some of that light reaches our eyes.
The Moon’s size, distance, and motion
The Moon is roughly 384,000 kilometres from Earth. Its diameter is about one quarter of Earth’s diameter, so it is physically smaller than Earth. It takes about 27 days to make one revolution around Earth. Do not confuse this period with Earth’s roughly 24-hour rotation or nearly one-year revolution around the Sun. Each time describes a different movement.
Why the Moon is covered with craters
The Moon’s surface contains circular bowl-like hollows called craters. Many formed when asteroids or other space rocks struck it. On Earth, air, water, living processes, and geological changes alter many surface features over time. The Moon has hardly any atmosphere, no flowing water like Earth’s, and no known life, so many impact features remain visible for a very long time.
A bowl-shaped hollow left when a rock from space strikes a surface.
Look at a whole-Moon image and then a close view. The close image makes the circular crater shapes easier to see; it does not mean the whole Moon is a perfectly smooth circle with only one hole. Ask what evidence in the image supports the interpretation and what you could not learn from the picture alone. The chapter includes an image from Cartosat and a close image from cameras aboard Chandrayaan-3.
Learning through spacecraft
The Moon is too far to investigate only by standing on Earth and looking up. Orbiters, landers, rovers, and their cameras can collect different kinds of information. India’s Chandrayaan-1 launched in 2008, Chandrayaan-2 in 2019, and Chandrayaan-3 in 2023. In August 2023 the Chandrayaan-3 Vikram lander brought the Pragyan rover safely to the lunar surface near the south-polar region. Images and measurements from such missions let us test ideas about a place we cannot easily visit.
Problem
A lunar photograph shows many circular depressions. A student says wind carved them, just as wind affects land on Earth. What explanation fits better?
- 1.The Moon has almost no atmosphere, so strong ordinary winds are not a suitable main explanation.
- 2.Impacts by rocks from space can make bowl-shaped depressions.
- 3.With little atmosphere and water to erase them, many old craters remain visible.
The Moon appears bright but does not produce starlight. It reflects sunlight. A satellite is described by its motion around a planet, not simply by how bright it looks.
Quiz
What is Earth’s Moon?
Approximately how long does one lunar revolution around Earth take?
How large is the Moon’s diameter compared with Earth’s?
What made many lunar craters?
Why can many Moon craters remain for a long time?
Practice Problems
- Distinguish the Sun, Earth, and Moon by what each revolves around and what produces its own light.
- Draw a labelled Earth–Moon sketch. Include the approximate revolution time and note that your sizes and distances are not to scale.
- Compare 384,000 km, the approximate Earth–Moon distance, with the Earth–Sun distance of roughly 150 million km.
- Explain the two steps of reasoning from a space-rock impact to a long-lasting Moon crater.
- Describe what a spacecraft image can show about the lunar surface and one question an image alone cannot settle.
- Research one observation made by a Chandrayaan mission and state its date and source separately from the chapter’s basic Moon facts.
Key Takeaways
• The Moon is Earth’s natural satellite, orbiting our planet in about 27 days. • It lies about 384,000 km away and is roughly one quarter Earth’s diameter. • Space-rock impacts formed many craters that can last because the Moon has little atmosphere and no flowing water. • Spacecraft and close images provide evidence about the Moon’s surface.