Understanding moon phase explorer
Move through a lunar month and see how sunlight creates the Moon’s familiar phases.
The idea behind it
The Sun lights half of the Moon. As the Moon orbits Earth, our viewing angle changes, revealing different fractions of that sunlit half. The repeating phase cycle is a synodic month.
The relationship to remember
(1 − cos(2π × 7.38 / 29.53059)) / 2 = 50% illuminated.
Make your own discovery
Slide from new Moon to full Moon. Watch the bright portion grow, then shrink during the second half of the cycle.
Try the interactive lab ↗What this model leaves out
A mean 29.53059-day cycle, not a precision lunar ephemeris. Date estimates can differ from the actual phase by about a day. The diagram uses a conventional northern-hemisphere orientation; the apparent tilt depends on location and time.
Check your understanding
Are phases caused by Earth’s shadow?
No. Ordinary phases are changing views of the sunlit half of the Moon. Earth’s shadow causes a lunar eclipse.
Why is the phase month longer than the orbit?
While the Moon orbits, Earth moves around the Sun. The Moon needs extra time to return to the same alignment with the Sun.
Continue with the formula reference or test yourself in space practice.