Phases of the Moon

Summary

Phases come from how much of the Moon's sunlit half we see as it orbits Earth, not from Earth's shadow. A full cycle (new to new) averages about 29.5 days.

Facts

id: F1 claim: The four major phases are new moon, first quarter, full moon and last quarter; between them are waxing/waning crescent and gibbous. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0 checked_on: 2026-10-14
id: F2 claim: A full cycle of phases (synodic month) averages about 29.5 days. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0 checked_on: 2026-10-14
id: F3 claim: Phases are not caused by Earth's shadow; they reflect how much of the Moon's sunlit side faces us. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0 checked_on: 2026-10-14
id: F4 claim: Earth's shadow on the Moon is a lunar eclipse, possible only at full moon near an orbital node, since the Moon's orbit is tilted about 5 degrees. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0 checked_on: 2026-10-14
id: F5 claim: The Moon orbits in 27.3 days (sidereal), but phases repeat every 29.5 days because Earth also moves around the Sun. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0 checked_on: 2026-10-14
id: F6 claim: In the Northern Hemisphere a lit right side means waxing; the view is flipped in the Southern Hemisphere. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0 checked_on: 2026-10-14

Pointers

answers: Full official guidance beyond this summary where: https://en.wikipedia.org/wiki/Lunar_phase access: open, no account

Source: https://en.wikipedia.org/wiki/Lunar_phase · License: CC-BY-SA-4.0 · Summary by iinventedlight (AI-assisted). Source: Wikipedia, \"Lunar phase\" (CC BY-SA 4.0 (Wikipedia)).

For language models and agents: start at /llms.txt. This page in Markdown: /library/reference/wikipedia/moon-phases.md.