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Our Moon has fascinated cultures worldwide, but beyond myth it is a well‑studied natural satellite that shapes Earth’s environment. Its unique characteristics and the way it is classified have intrigued astronomers and laypeople alike.

Basic Facts About the Moon

The Moon is Earth's only natural satellite. Its diameter is about 2,160 mi (3,475 km)—roughly one‑fourth of Earth's diameter—making it a comparatively large satellite. Although it occupies only about 1 % of Earth's mass, the volume ratio means Earth could fit roughly 50 Moon‑sized bodies inside it.

It orbits Earth at an average distance of 239,000 mi (384,000 km) in 27.3 days, the sidereal month. Light takes approximately 1.3 seconds to travel between Earth and the Moon. The Moon’s apparent size is about 0.5° of the sky, the same as the Sun’s apparent diameter.

Because of the Moon’s slightly elliptical orbit, a full Moon can occur when the Moon is at perigee, producing a visibly larger “super‑Moon”. The full lunar cycle, or synodic month, lasts 29.5 days.

Earth–Moon Dynamics

The Moon’s rotation period matches its orbital period (synchronous rotation), so the same hemisphere faces Earth continuously. This explains why we never see the far side from the surface.

The Earth–Moon pair has an unusual diameter ratio of 4 : 1, placing the Moon among the solar system’s five largest satellites and nearly the size of Mercury.

Phases of the Moon

The Moon’s phases result from the changing geometry between Earth, Moon, and Sun. Starting with the new Moon (fully dark to an Earth observer), the sequence proceeds: waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, third quarter, and waning crescent before returning to new Moon.

• During the first quarter, the right side is illuminated and the Moon rises near noon.
• At the full Moon, the Moon rises at sunset.
• During the third quarter, the left side is illuminated and the Moon rises near midnight.

Knowing the phase allows a rough estimate of the time of day and direction of the Moon’s appearance.

Notable Full‑Moon Phenomena

• Blood Moon – the reddish hue seen during a total lunar eclipse, caused by Earth’s atmosphere filtering sunlight.
• Blue Moon – the second full Moon in a single calendar month, occurring roughly every 2 ½ years.
• Harvest Moon – the full Moon nearest the autumnal equinox, historically aiding farmers by extending evening light.
• Super‑Moon – a full Moon occurring at perigee, appearing up to 14 % larger and brighter.

Other cultural names, such as the “pink Moon” for the April full Moon, stem from folklore rather than color.

Surface Characteristics

The lunar surface is covered in craters, a testament to billions of years of impacts without atmospheric erosion. Temperatures range from –233 °C to 123 °C (–387 °F to 253 °F) due to the absence of a substantial atmosphere.

Seismic activity, or “moonquakes,” indicates a molten core. The Moon has been the destination for 12 astronauts (all U.S. men) during the Apollo missions; future private ventures and NASA’s Artemis program aim to return humans—including the first woman—to the Moon by 2024.