Moon Phase Simulator
Move through one lunar month and watch the same geometry from space and from Earth. The orbit, phase name, and illuminated fraction stay synchronized.
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Lunar phase simulator guide
Follow a complete Moon phase cycle
This simulator connects two views that are often taught separately. The space view shows where the Moon sits relative to the Sun and Earth. The sky view shows how much of the illuminated half faces an observer on Earth. Move the Moon-age slider and both diagrams update together.
How to use the simulator
- Drag Moon age from day 0 through the 29.53-day cycle.
- Choose any of the eight named phases to jump to its midpoint.
- Play the orbit, pause it, or step one day at a time.
- Compare the orbit position with the illuminated disk percentage.
Why the Moon changes shape
Sunlight illuminates half of the Moon at every ordinary phase. As the Moon orbits Earth, our viewing angle changes, so we see a crescent, a half-lit disk, a gibbous Moon, or the full illuminated face. Earth's shadow only darkens the Moon during a lunar eclipse.
The illumination formula
Let p be the fraction of one synodic month since new Moon. The simulator estimates the illuminated share k of the visible disk with a simple geometric model:
The formula gives zero at new Moon, one half at either quarter, and one at full Moon. It describes the illuminated area, not the Moon's apparent brightness, which also depends on surface scattering and observing conditions.
The eight Moon phases
| Phase | Approximate age | Disk illuminated |
|---|---|---|
| New Moon | 0.00 days | 0% |
| Waxing Crescent | 3.69 days | 15% |
| First Quarter | 7.38 days | 50% |
| Waxing Gibbous | 11.07 days | 85% |
| Full Moon | 14.77 days | 100% |
| Waning Gibbous | 18.46 days | 85% |
| Last Quarter | 22.15 days | 50% |
| Waning Crescent | 25.84 days | 15% |
Model assumptions and limits
- One cycle lasts a fixed average synodic month of 29.530588853 days.
- The orbit is circular and advances at a constant rate. Real lunar motion varies because the orbit is elliptical and tilted.
- The space diagram enlarges the Moon, Earth, orbital radius, and Sun distance independently. Do not use it to compare physical sizes or distances.
- The sky diagram uses the common north-up teaching orientation. The Moon can look rotated at a real location and time, and the Southern Hemisphere usually sees the familiar pattern rotated.
- The model does not calculate calendar dates, rise and set times, libration, atmospheric effects, or eclipses.
Astronomy references
The cycle length and geometry follow NASA's explanation of the mean synodic month. NASA's Moon guide also distinguishes ordinary phases from the brief passage through Earth's shadow during a lunar eclipse.
Continue exploring the Moon
Moon phase simulator FAQ
What causes the phases of the Moon?
The Sun always lights half of the Moon. As the Moon orbits Earth, we see a changing share of that illuminated half. Earth's shadow is not responsible for the ordinary phases.
How long is one Moon phase cycle?
The average time from one new Moon to the next is about 29.53059 days. Astronomers call this a synodic month. Individual cycles vary slightly because the Moon's orbit is not perfectly circular.
What is the difference between waxing and waning?
The Moon is waxing while the visible illuminated share grows from new Moon toward full Moon. It is waning while that share shrinks from full Moon toward the next new Moon.
Why does a quarter Moon look half lit?
First quarter and last quarter name the Moon's position one quarter or three quarters through its orbit. At either point, half of the visible disk is illuminated from Earth's viewpoint.
Is this lunar phase simulator to scale?
No. The space view enlarges the bodies and shortens the Sun and Moon distances so the geometry fits on one screen. The phase angle and illuminated fraction follow the model, but the drawing is not a scale diagram.
Educational geometry model. Distances and body sizes in the space view are not drawn to scale.