Quick Facts
What Is the Moon?
The Moon is a rocky satellite that orbits Earth. It has no significant atmosphere, no liquid water on its surface, and extreme temperature variations between day and night. Its surface is covered with impact craters, vast dark plains called maria, and highlands.
The Moon is tidally locked to Earth, meaning the same side always faces our planet. It is the largest moon relative to its parent planet in the solar system and plays a major role in Earth’s tidal systems and long-term climate stability.
Origin of the Moon
The leading scientific explanation for the Moon’s origin is the giant-impact hypothesis. Early in the solar system’s history, a Mars-sized body (often called Theia) is thought to have collided with the proto-Earth. Debris from the impact formed a disk around Earth that eventually coalesced into the Moon.
This model helps explain the Moon’s composition (similar to Earth’s mantle), its relatively large size compared with Earth, and the angular momentum of the Earth–Moon system.
Surface Features
Maria
Dark, relatively smooth plains formed by ancient basaltic lava flows. They appear as the “seas” visible to the naked eye.
Highlands
Brighter, heavily cratered regions that make up most of the lunar surface and are older than the maria.
Craters
Impact features of all sizes, formed by collisions with asteroids and comets over billions of years. The Moon preserves this record because it lacks erosion and plate tectonics.
Regolith
A layer of fine dust and broken rock produced by continuous micrometeorite bombardment. It covers nearly the entire surface.
Orbit and Phases
Orbital Period
The Moon takes about 27.3 days to complete one orbit relative to the stars (sidereal month) and about 29.5 days to return to the same phase (synodic month).
Phases
The changing appearance of the Moon—from new to full and back—is caused by the varying angles of sunlight as seen from Earth.
Tidal Locking
The Moon rotates on its axis in the same time it takes to orbit Earth, so the same hemisphere always faces us.
Far Side
The side of the Moon never visible from Earth is often called the far side. It is more heavily cratered and has fewer maria.
Influence on Earth
Tides
The Moon’s gravity is the primary driver of Earth’s ocean tides, with the Sun playing a secondary role.
Axial Stability
The Moon helps stabilize Earth’s axial tilt, contributing to relatively steady seasons over long timescales.
Length of Day
Tidal friction between Earth and the Moon has gradually slowed Earth’s rotation and lengthened the day over geologic time.
Exploration
The Moon was the first celestial body beyond Earth visited by spacecraft and by humans. Robotic missions began in the late 1950s and 1960s. The Apollo program (1969–1972) landed twelve astronauts on the lunar surface and returned rock and soil samples that transformed scientific understanding of the Moon’s history and composition.
Ongoing and planned missions continue to study the Moon’s resources, geology, and potential as a platform for further exploration of the solar system.
Earth’s Companion
The Moon is far more than a light in the night sky. It is a key influence on Earth’s tides and axial stability, a preserved record of early solar-system impacts, and the first world beyond our own that humans have walked upon.
Studying the Moon deepens understanding of planetary formation, the Earth–Moon relationship, and the possibilities for future exploration beyond Earth.