How does the Moon compare with Earth?

Quick facts and useful context

The headline number or idea is: The Moon's diameter is about one-quarter of Earth's, while its mass is about 1.2% of Earth's.

One important detail is easy to miss: That large relative size makes the Earth–Moon system unusual among planetary satellite systems.

A second useful point is that lunar facts often connect to several fields at once. The same Moon can be studied through astronomy, geology, physics, planetary science and spacecraft engineering. Looking at the topic from more than one angle is often the fastest way to understand why it matters.

Why this matters for Moon exploration

The Moon is close enough to Earth to study in extraordinary detail, yet distant enough to preserve a record of processes that shaped rocky worlds. That combination makes lunar science useful for understanding impacts, geology, orbital mechanics, planetary history and the practical realities of spaceflight.

Future missions have to turn scientific knowledge into practical decisions: where to land, how to navigate, how to communicate, how to manage power and how to protect people and equipment. A seemingly small lunar fact can therefore become part of a much larger mission-design problem.

What to remember

• Core fact: The Moon's diameter is about one-quarter of Earth's, while its mass is about 1.2% of Earth's.
• Context: That large relative size makes the Earth–Moon system unusual among planetary satellite systems.
• Big picture: Lunar exploration combines science, navigation, engineering and human operations.