01
New → full Moon
Jump to New Moon, then Full Moon. The Moon itself stays half sunlit; only the viewing geometry changes.
Moon · Question
Moon phase simulator: why the Moon changes shape
The Moon does not change shape. Half is always lit by the Sun; what changes is how much of that lit half we can see from Earth. Drag the orbit slider or jump to a major phase and compare both views.
System view — top-down
What you see from Earth
Full Moon
Day 14.8 of 29.5 · 100% illuminated as seen from Earth
Drag to move the Moon around Earth. The system view is top-down; sunlight always comes from the left.
29.5 daysOne synodic month — new moon to new moon.
8Named phases in one cycle.
~5°The Moon's orbit is tilted, so eclipses don't happen every month (not shown here).
Same sideThe Moon is tidally locked, so we always see the same face.
| Major phase | Moon position in this model | Visible disk illuminated | What the geometry means |
|---|---|---|---|
| New Moon | 0° | ~0% | The Moon is roughly between Earth and the Sun. |
| First Quarter | 90° | ~50% | The Moon is about one quarter of the way through the cycle. |
| Full Moon | 180° | ~100% | Earth is roughly between the Sun and Moon. |
| Last Quarter | 270° | ~50% | The Moon is about three quarters of the way through the cycle. |
Try the simulator
Three experiments that explain Moon phases.
Change one thing at a time and use the Earth view to test your prediction.
02
Quarter does not mean 25% lit
At first and last quarter, roughly half of the visible disk is illuminated. "Quarter" describes the Moon's position in the cycle.
03
Separate phases from eclipses
Ordinary phases happen every month. Earth's shadow only causes a lunar eclipse when the Sun, Earth, and Moon line up closely enough.
What's simplified
This model compresses a few things to stay readable.
Every visualization on Space Atlas states what has been simplified, so a model is never mistaken for the evidence itself.
01
Distance
The real Earth–Moon distance is about 30 Earth diameters. Here it's compressed so the Moon stays visible next to Earth.
02
Tilt
The Moon's orbit is tilted about 5° from Earth's orbit around the Sun. This model flattens that tilt, which is also why real eclipses are rarer than this diagram would suggest.
03
Size
Earth and the Moon are enlarged relative to the orbit so both stay clearly visible and clickable-scale.
Misconception check
What most people get wrong about Moon phases.
Quick check
Go deeper
Keep exploring.
Next in this series →
Solar & lunar eclipses ↗
Use the same Sun-Earth-Moon geometry to see why eclipses do not happen every month.
Scale
Earth–Moon scale
See the real Earth–Moon size and distance ratio that this simulator compresses.
Rotation
Tidal locking
Why the Moon rotates once per orbit and keeps nearly the same face toward Earth.