Moon Phases: Geometry, Illumination and Common Misconceptions
Why phases happen, why everyone sees the same phase and how phase differs from moonrise time.
Key takeaways
- Moon phases are caused by changing Sun-Earth-Moon geometry, not by Earth's shadow.
- Except during an eclipse, roughly half the Moon is illuminated by the Sun at any moment.
- Observers around Earth see essentially the same phase at the same time, although orientation differs.
- Moonrise and moonset depend on location even though phase does not.
The basic geometry
Sunlight always illuminates roughly half of the spherical Moon. As the Moon orbits Earth, our viewing angle onto that illuminated hemisphere changes.
Near new Moon, the illuminated half mostly faces away from Earth. Near full Moon, it mostly faces us. Quarter phases occur when the Sun-Earth-Moon angle lets us see approximately half of the illuminated hemisphere.
Why phases are not Earth's shadow
Earth's shadow is involved only during a lunar eclipse, when the full Moon passes through or near that shadow.
Ordinary waxing and waning happen every month because of orbital geometry. A crescent Moon is not partly hidden by Earth.
Waxing and waning
After new Moon, the illuminated fraction visible from Earth increases: the Moon is waxing. After full Moon it decreases: the Moon is waning.
The same illumination percentage can occur twice in one cycle, once waxing and once waning. Those two dates illuminate lunar terrain from opposite sides and therefore produce different shadows.
Phase and observing time
A waxing crescent is generally prominent after sunset, first quarter is visible through afternoon and evening, full Moon rises around sunset, and waning phases increasingly favour late-night or morning observation.
Exact rise and set times vary with date, observer latitude, longitude and orbital geometry, so these are useful patterns rather than universal clock times.
Why everyone sees the same phase
The Earth-Moon distance is small enough compared with the scale of the illumination geometry that people across Earth see essentially the same lunar phase at a given moment.
What changes geographically is orientation in the sky and the local time at which the Moon is above the horizon.
Phase versus illumination percentage
Illumination percentage measures how much of the visible lunar disk is lit, while phase name divides the continuous cycle into familiar categories.
A precise illumination value can be useful, but it does not by itself tell an observer which named features are close to the terminator. For that, the waxing/waning state and lunar longitude of sunrise or sunset matter.
Put the guide into practice
Use the Moon Observation Planner, Field of View Simulator and Observation Log.
Sources and further reading
- U.S. Naval Observatory - Phases of the Moon: Moon phase geometry and terminology
- NASA Science - Moon Facts: Moon size, distance, synchronous rotation, surface, exosphere, exploration and returned-sample context