Moon Phase Calculator
lunar phase, illumination, and orbital geometry by date
The celestial geometry of lunar phases 🖖
Lunar phases are not caused by Earth's shadow, but by the changing angle of sunlight reflecting off the Moon as it orbits Earth. Statically, the synodic month lasts 29.53 days, which is longer than the sidereal month of 27.32 days because Earth also moves along its orbit around the Sun, requiring the Moon to travel an extra 2.2 days to realign with the Sun. The fraction of the illuminated lunar surface visible from Earth is given by (1 + cos(θ))/2, where θ is the elongation angle between the Moon and the Sun as seen from Earth.
Reading the Moon's age off the sky 🖖
This tool's 'moon age' is simply the number of days since the last new moon, counting up to about 29.5. That single number fixes the phase: a young Moon is a thin crescent, day 7 is first quarter, day ~15 is full. In the northern hemisphere a growing (waxing) Moon is lit on its right side; in the southern hemisphere it is flipped, which is why the tool offers a hemisphere switch.
We see 59% of a tidally locked Moon 🖖
The Moon keeps one face toward us because its spin and orbit are locked to the same 27.3-day period. You would expect exactly half its surface to be forever hidden — yet over a month the Moon appears to nod and rock slightly, an effect called libration, because its orbit is elliptical and tilted. These wobbles let us peek around the edges, so about 59% of the lunar surface is visible from Earth over time, not 50%.