Moon phases explained
A Moon phase describes the illuminated fraction of the lunar disc visible from Earth. The cycle is continuous; the eight familiar names divide it into useful calendar sections.
The eight common Moon phases
- New Moon
- The Moon and Sun have the same geocentric ecliptic longitude. The near side is minimally illuminated from Earth.
- Waxing Crescent
- The visible illuminated fraction grows after new Moon and remains below half.
- First Quarter
- The Moon is about 90 degrees east of the Sun and appears half illuminated.
- Waxing Gibbous
- More than half of the disc is illuminated and the lit fraction continues to grow.
- Full Moon
- The Moon is about 180 degrees from the Sun in ecliptic longitude and its near side is maximally illuminated.
- Waning Gibbous
- More than half remains illuminated, but the visible lit fraction is shrinking.
- Last Quarter
- The Moon is about 270 degrees from the Sun and appears half illuminated while waning.
- Waning Crescent
- Less than half is illuminated before the cycle returns to new Moon.
Waxing, waning and illumination
Waxing means the illuminated fraction is increasing; waning means it is decreasing. Illumination is a percentage, while phase angle records the Moon's position in the cycle. Two dates can have similar illumination but opposite directions.
Using the phase names in a calendar
The monthly Moon calendar calculates a phase at local noon for each date. Exact new, quarter and full Moon instants appear separately in the phase schedule.