Complete cycle
A synodic month runs from one same phase to the next and lasts about 29.5 days.
Lunar cycle · geometry first
Moon Phases
Understand why Moon phases happen, how the eight familiar phase names relate to geometry, and why phase is not the same as local moonrise or visibility.
Direct answer
Moon phases are changing views of the sunlit half of the Moon as the Moon orbits Earth. The sequence is geometric, not caused by Earth’s shadow; Earth’s shadow is involved only during a lunar eclipse.
Waxing / waning
Waxing means the illuminated portion is increasing; waning means it is decreasing.
Phase vs eclipse
Ordinary phases come from viewing geometry. A lunar eclipse requires special Sun–Earth–Moon alignment near a node.
Verified current positions · refreshes hourly
The Active Transits Sky Map
Current geocentric longitudes. No local rise, set, altitude, direction, weather, or future visibility is inferred.
The phase family is derived from current Sun and Moon longitude. It is not an exact phase-event time, illuminated fraction, moonrise calculation, or local visibility forecast.
The eight familiar phase names
The common sequence is new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last or third quarter, and waning crescent. These labels divide a continuous cycle into useful neighborhoods. Only new, first quarter, full, and last quarter describe exact geometric event moments in standard almanacs; the crescent and gibbous phases fill the intervals.
Why the Moon changes shape
Sunlight always illuminates roughly half of the Moon. From Earth, the fraction of that lit hemisphere we can see changes as the angle among Sun, Earth, and Moon changes. Near new Moon the lit side mostly faces away from us. Near full Moon the lit side mostly faces Earth. The terminator—the boundary between lunar day and night—reveals relief especially well through binoculars or a telescope.
Phase, age, and illumination are different
Lunar age counts time since new Moon, phase names describe parts of the cycle, and illuminated fraction estimates how much of the visible disk is sunlit. They correlate but are not interchangeable. A broad phase family derived from longitude is useful for orientation; precise illumination and exact event timing come from a lunar ephemeris that accounts for the Moon’s real orbit.
Phase does not supply a local schedule
The full Moon generally rises near sunset and a new Moon travels near the Sun, but exact rise and set times change with location and date. Twilight definitions, refraction, the observer’s horizon, and the Moon’s changing declination all matter. Use a location-aware table rather than adding a fixed number of minutes to yesterday’s time.
How to use a phase calendar responsibly
A phase calendar is excellent for planning moonlight-sensitive observing, photography, or a simple nature journal. It is not a substitute for a local visibility forecast. If you also use lunar phases symbolically, label that practice as reflection or tradition. Claims about medical outcomes, emergencies, markets, or human behavior require evidence beyond the calendar.
Sources and live-data notes
- NASA: Moon phases ↗Phase geometry and the repeating sequence of illuminated lunar appearances.
- NASA: Moon Phase and Libration, 2026 ↗A year-specific visualization from NASA; use it for the 2026 lunar sequence, not local rise or set.
- U.S. Naval Observatory: Astronomical Data ↗Authoritative astronomical data services and event tables.
- U.S. Naval Observatory: Rise, Set, and Twilight Definitions ↗Definitions for location-dependent rise, set, and twilight calculations.
- The Great Almanac live APISame-origin gateway for the validated hourly current-position snapshot; the upstream source is identified in the response metadata.
Continue through the Sky & Time Almanac
Cluster hubSky & Time AlmanacAll observation guides, live instruments, and source boundaries.
Lunar almanac · observed nowMoon TodayRead the Moon’s current geocentric longitude, zodiac degree, approximate phase family, and the limits of non-location-aware lunar data.
Sun, Earth, Moon · alignmentEclipsesA grounded guide to solar and lunar eclipse geometry, paths of visibility, local circumstances, Saros cycles, and safe observation.
Debris, atmosphere, dark skyMeteor ShowersUnderstand meteor-shower radiants, parent bodies, peak rates, Moon interference, and the local conditions that determine what you can see.
Observing calendar · verify locallyCelestial EventsLearn the major kinds of celestial events, how to verify an observing opportunity, and what a current longitude feed can and cannot establish.
Common questions
Are Moon phases caused by Earth’s shadow?
No. Normal phases come from our changing view of the Moon’s sunlit half. Earth’s shadow crosses the Moon only during a lunar eclipse.
What is the difference between first quarter and half Moon?
At first quarter the Moon is about one quarter of the way through its phase cycle, while its visible disk appears approximately half illuminated.
Can a phase calendar tell me when the Moon rises?
Not by itself. Rise and set are location-dependent calculations and should come from a service using your coordinates and date.
