First Quarter
The First Quarter Moon occurs when the Moon reaches 90° angular separation from the Sun. Half of the visible disk is illuminated.
- Position in cycle
- 3 of 8
- Phase Angle Range
- 90° to 135° Moon-Sun Phase Angle
- Illumination Range
- 50%
Orbital Geometry
The Earth-Moon-Sun angle forms a right angle (90° square).
Observational Notes
High in the southern sky at sunset, setting around midnight.
Frequently asked questions
- Why is it called First Quarter if half is lit?
- Because the Moon has completed one-quarter (25%) of its full orbital cycle.