The Moon changes its appearance
Moon phases come from the changing view of the Moon’s sunlit half as it orbits Earth.
In 30 seconds
- Sunlight illuminates approximately half of the Moon at a time.
- Our viewing geometry changes across its orbit.
- Earth’s shadow causes a lunar eclipse, not the ordinary monthly phases.
Make it concrete
Use a lamp and a ball in a classroom. Walk around the ball to see different fractions of its lit half without changing the lamp.
Change a value. Watch the relationship.
29.53059-day mean synodic cycle. 1 screen second = 1 lunar day. Shown from north of the orbital plane; illuminated fraction is (1 − cos θ)/2. Sizes are schematic; no eclipses modelled.
Check your understanding
Are normal Moon phases caused by Earth’s shadow?
- Sunlight illuminates approximately half of the Moon at a time.
- Our viewing geometry changes across its orbit.
- Earth’s shadow causes a lunar eclipse, not the ordinary monthly phases.
Go to the source
Sources reviewed: 13.09.2026