Earth’s only natural satellite and the brightest object in the night sky after the Sun. It orbits Earth at an average distance of about 384,400 kilometers (238,855 miles).
Key Facts
- Formation: The most widely accepted theory suggests the Moon formed approximately 4.5 billion years ago from the debris of a giant impact between early Earth and a Mars-sized body named Theia.
- Size and Mass: With a diameter of 3,474 km (2,159 miles), the Moon is about one-quarter the width of Earth. It is the fifth-largest natural satellite in our Solar System and the largest relative to its parent planet. Its mass is about 1.2% of Earth’s.
- Tidal Locking: The Moon is tidally locked with Earth, meaning it always shows the same face to our planet. This is due to Earth’s gravity synchronizing the Moon’s orbital and rotational periods, both lasting about 29.5 days.
- Surface and Atmosphere: The Moon has a solid, rocky surface heavily cratered from impacts by asteroids, meteorites, and comets. It has a very thin and tenuous atmosphere called an exosphere, which is not breathable. Unlike Earth, the Moon lacks plate tectonics, preserving its ancient surface.
- Gravitational Influence: The Moon’s gravitational pull is the primary driver of Earth’s ocean tides. It also causes a measurable “tide” in Earth’s solid surface.
- Distance from Earth: The Moon is slowly moving away from Earth at a rate of about 3.8 cm (1.5 inches) per year.
- Exploration: More than 105 robotic spacecraft have been launched to explore the Moon. The first crewed landing was Apollo 11 in 1969, and only 12 people, all American men, have walked on the Moon.
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