How it works
The Moon does not make its own visible light: sunlight illuminates half of it at all times. The phase you see is the part of that lit half that faces Earth. Dragging horizontally moves the Moon through its orbit so you can see the geometry change continuously rather than memorizing a diagram.
The eight phases
From new Moon to full Moon and back again, the familiar sequence is new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, and waning crescent. The simulation labels these positions as you move around the orbit.
Why eclipses are rare
A solar eclipse needs a new Moon, while a lunar eclipse needs a full Moon. But the Moon's orbit is tilted relative to Earth's orbit around the Sun, so most new and full Moons pass above or below the exact line-up. When the geometry does align, the simulator can jump to the next eclipse and show the changing shadow geometry.
Questions visitors ask
Why does the Moon change shape?
Half of the Moon is always lit by the Sun. As the Moon orbits Earth, we see different amounts of that sunlit half from our viewpoint, producing the phases.
How long is a lunar cycle?
The cycle from one new Moon to the next is about 29.5 days, called the synodic month.
Why is there not an eclipse every month?
The Moon's orbit is tilted by about 5 degrees relative to Earth's orbital plane. Most new and full Moons therefore pass above or below the alignment needed for an eclipse.
Does the simulator use real astronomical dates?
Yes. The scene uses astronomical ephemerides to place the Sun and Moon, and the eclipse controls use the same underlying calculations to move to real eclipse events.
Sources and further reading
- Moon Phases — NASA Science
- Eclipses — NASA Science
- Astronomy Engine — Cosine Kitty
Cite this exhibit
Eurekarium. (2026). Moon Phases Simulator. https://eurekarium.com/astronomy/moon-phases/
Last updated September 27, 2026.