The regular daily and monthly rhythms of Earth’s only natural satellite, the Moon, have guided timekeepers for thousands of years. Its influence on Earth’s cycles, notably tides, has been charted by many cultures in many ages.

The Moon moderates Earth’s wobble on its axis, leading to a relatively stable climate over billions of years. From Earth, we always see the same face of the Moon because the Moon is spinning on its axis at the same speed that it is going around Earth (that is, it is in synchronous rotation with Earth).

Five Things to Know About the Moon

1. Earth’s Moon is a cornerstone of planetary science. Without plate tectonics or weather to erase evidence of its past, our closest neighbor in space preserves a record of the geologic history that has shaped our solar system – including our own planet.
2. Evidence suggests that, about 4.5 billion years ago, a Mars-sized object crashed into early Earth. The debris from this impact likely formed the Moon.
3. Moonquakes shake it. Compared to Earth, the Moon is a quietly active world. Tidal forces, meteoroid impacts, and thermal changes in rock near the surface all trigger seismic disturbances.
4. The coldest temperatures in the solar system have been recorded at the Moon’s poles. Some polar craters, darkened by permanent shadows, harbor hidden water ice.
5. Twelve humans have walked on the Moon (all astronauts in NASA’s Apollo program in the 1960s and 1970s). Plans are now under way for humans to return to the Moon through NASA’s Artemis program.

Lunar Terrain

The light areas of the Moon are known as the highlands. The dark features, called maria (Latin for seas), are impact basins that were filled with lava between 4.2 and 1.2 billion years ago. These light and dark areas represent rocks of different composition and ages, which provide evidence for how the early crust may have crystallized from a lunar magma ocean. The craters themselves, which have been preserved for billions of years, provide an impact history for the Moon and other bodies in the inner solar system.

Lunar Origins

The leading theory of the Moon’s origin is that a Mars-sized body collided with Earth approximately 4.5 billion years ago, and the resulting debris from both Earth and the impactor accumulated to form our natural satellite. The newly formed Moon was in a molten state. Within about 100 million years, most of the global “magma ocean” had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust.

The early Moon may have developed an internal dynamo, the mechanism for global magnetic fields for terrestrial planets. Since the ancient time of volcanism, the arid, lifeless Moon has remained nearly unchanged. With too sparse an atmosphere to impede impacts, a steady rain of asteroids, meteoroids, and comets strikes the surface. Over billions of years, the surface has been ground up into fragments ranging from huge boulders to powder.

Nearly the entire Moon is covered by a rubble pile of charcoal-gray, powdery dust and rocky debris called the lunar regolith. Beneath is a region of fractured bedrock referred to as the megaregolith.