๐ Moon Phases
Watch the Moon orbit Earth and see how sunlight creates the 8 lunar phases. Use the slider or press Play!
Watch the Moon orbit Earth and see how sunlight creates the 8 lunar phases. Use the slider or press Play!
This interactive model shows why the Moon appears to change shape over a synodic month of roughly 29.5 days. As the Moon orbits Earth, the simulation tracks the angle between the Sun, the Moon and the viewer, and shades the lunar disc so you see only the sunlit portion that faces us. The illuminated fraction follows a smooth cosine curve, peaking at the Full Moon and vanishing at the New Moon, giving an accurate sense of the eight named lunar phases.
A top-down view of the Earth, Sun and orbiting Moon. The lit fraction is computed as 50 โ 50ยทcos(2ฯยทday / 29.5), so illumination runs from 0% at the New Moon to 100% at the Full Moon. A separate disc redraws the visible crescent or gibbous shape using clipped arcs, matching the percentage shown in the readout.
Drag the Day slider (0 to 29.5) to scrub through the month, or press Play cycle to animate it, adjusting the Speed slider from 1ร to 10ร. The phase chips let you jump straight to any of the eight phases, while the readout panel reports the current phase name, illumination percentage and day of the cycle.
The Moon always shows Earth the same face. Its rotation period exactly matches its orbital period, an effect called tidal locking, which is why we never see the far side from the ground.
The Moon produces no light of its own; it only reflects sunlight. As it orbits Earth, the angle between the Sun, Moon and Earth changes, so we see varying amounts of the Moon's sunlit half. When the lit side faces away from us we see a New Moon, and when it faces us fully we see a Full Moon.
One complete cycle from New Moon to New Moon takes about 29.5 days, known as the synodic month. The simulation uses this exact figure, which is why the Day slider runs from 0 to 29.5. It is slightly longer than the 27.3-day orbital period because Earth itself is moving around the Sun.
The Day slider sets the position in the cycle, the Play button animates the orbit, and the Speed slider changes how fast it runs. The phase chips snap to specific phases, and the statistics panel updates the phase name, the illuminated percentage and the day of the cycle in real time.
The model uses a cosine relationship to map day of cycle to illuminated fraction, which closely approximates real lunar illumination as seen from Earth. It is an idealised, smoothly varying curve rather than an exact ephemeris, so it captures the correct shape and timing of the phases without modelling orbital eccentricity or libration.
Waxing means the illuminated portion is growing, from New Moon toward Full Moon, while waning means it is shrinking, from Full Moon back to New Moon. In this top-down view the difference comes from which side of the Moon faces the Sun as it moves around its orbit, which the simulation reflects by lighting the correct edge of the disc.