Moon (Earth's natural satellite)

Physical characteristics

The Moon lacks a global atmosphere and liquid water on the surface today. Its terrain includes dark, basaltic plains called maria, brighter highlands, craters of many sizes, and regions of regolith made by constant micrometeorite impacts. The lunar day (a full rotation relative to the Sun) equals about one lunar month, producing long periods of daylight and darkness at any given location.

Origin and orbital behavior

Most scientists explain the Moon’s origin by a large collision early in Earth’s history that ejected material which later coalesced into the Moon. The Moon orbits Earth and is slowly receding at a measurable rate due to tidal interactions, a process that also transfers angular momentum and influences Earth’s rotation.

Exploration, uses, and importance

Human interest in the Moon spans navigation, calendars, and culture. In the 20th century, robotic probes and crewed missions visited and sampled the surface, advancing knowledge of planetary formation and geology. The Moon also serves as a laboratory for testing technologies for deeper space exploration and is discussed as a potential site for scientific bases.

Notable facts and distinctions

• It is the largest natural satellite relative to its planet in the inner Solar System and uniquely influential on Earth’s tides and axial stability.
• Its apparent size in the sky is about 0.5°, a coincidence that allows solar eclipses to occur when the Moon aligns with the Sun.
• Its surface gravity is about one-sixth of Earth’s; this affects weight and motion on the surface.

For basic definitions and further reading see Earth and Moon relations, materials on natural satellites at natural satellite references, observations of apparent size at apparent angular size, and resources on lunar gravity and tides at tidal and gravity studies.