Moon

Earth's only natural satellite — the fifth largest moon in the solar system, the most studied extraterrestrial body, and the only world beyond Earth where humans have stood. The Moon shapes Earth's tides, stabilises its axial tilt, and has structured human calendars, navigation, mythology, and religion across every culture.

Key Properties

Origin — The Giant Impact

The leading hypothesis: ~4.5 billion years ago, a Mars-sized body called Theia collided with the proto-Earth at a glancing angle. The impact vaporised Theia and much of Earth's mantle; debris was flung into orbit and coalesced into the Moon within thousands of years.

Evidence: the Moon's bulk composition closely matches Earth's mantle; its core is small (the iron-rich cores of both bodies merged into Earth); lunar rocks returned by Apollo are nearly identical isotopically to Earth rocks — distinct from meteorites from elsewhere.

The impact also tilted Earth's axis to ~23.5°, setting up the seasons.

Tidal Locking

The Moon rotates exactly once per orbit — its rotation period equals its orbital period. This synchronous rotation means the near side always faces Earth; the far side (not "dark side" — it receives the same sunlight) was unknown until Soviet Luna 3 photographed it in 1959.

Tidal locking results from tidal friction: Earth's gravity raises a bulge on the Moon; as the Moon rotated in the past, this bulge was dragged slightly ahead of the Earth-Moon line, creating a torque that slowed rotation until it matched the orbital period.

The same process is slowly slowing Earth's own rotation. Days were shorter billions of years ago; the Moon is slowly receding (~3.8 cm/year) as the system loses angular momentum to tidal friction.

The Moon's Role in Earth's Stability

The Moon stabilises Earth's axial tilt at ~23.5°, preventing the chaotic wobble that models predict for moonless Earth (potentially swinging between ~0° and 85°). This stability moderates climate variation over geological time — a moonless Earth might have far more dramatic climate swings, with implications for the evolution of complex life. See fermi-paradox for the Rare Earth argument this connects to.

Navigation

The Moon was fundamental to navigation before GPS:

Latitude — the Moon's altitude above the horizon, like any celestial body, encodes latitude for an observer who knows the Moon's declination.

Longitude — the Lunar Distance method — before accurate chronometers, sailors measured the angle between the Moon and a reference star (lunar distance). Comparing this to tabulated predicted angles gave Greenwich time, and thus longitude. This was the primary longitude method from ~1767 until Harrison's chronometer became affordable. See longitude-problem and celestial-navigation.

Calendar — virtually every ancient culture used the lunar cycle for timekeeping. The Islamic calendar remains strictly lunar (12 months of 29–30 days = 354-day year, cycling through seasons over 33 years). The Hebrew calendar is lunisolar. The Christian Easter and Islamic Ramadan are determined by lunar phases.

Apollo — Six Landings

Twelve humans walked on the Moon between July 1969 and December 1972:

Apollo 17 remains the last crewed lunar landing. The programme returned 382 kg of samples — rocks now ranging up to 4.5 billion years old, providing a chronological anchor for the inner solar system's impact history.

Surface Geology

Maria (Latin: seas) — dark basaltic plains formed by ancient volcanic flooding 3–3.5 billion years ago. Named by early astronomers who mistook them for seas: Mare Tranquillitatis, Mare Imbrium, Oceanus Procellarum.

Highlands — lighter, older, heavily cratered terrain, 4+ billion years old, composed of anorthosite (calcium-rich feldspar).

Water ice — confirmed in permanently shadowed craters near the poles by LCROSS (2009), which intentionally impacted a polar crater and detected water in the debris plume. These ice deposits may support future lunar bases.

Mythology

The Moon appears in every mythological tradition:

• Greek: Selene (Moon goddess); Artemis (goddess of the hunt, associated with the Moon); Hecate (dark Moon)
• Roman: Luna; Diana
• Egyptian: Thoth (god of wisdom, scribe of the gods, associated with the Moon)
• Norse: Máni (the Moon personified, driving a chariot across the sky, chased by the wolf Hati)
• Islamic: The crescent moon is the symbol of Islam; the lunar calendar governs Ramadan, Eid, and Hajj

The word lunatic derives from *luna* — the belief that the full Moon caused madness. Month, Monday (*Moon's day*), and menstruation all share the ~29.5-day cycle.