Work out moon phase on a date instantly with clear inputs, formula shown and shareable results.
Moon age is the time since the last new moon within the 29.53 day synodic month, and illumination follows a cosine of that fraction. The phase names divide the cycle into eight segments. Because the orbit is elliptical the true illumination can differ from this mean-motion estimate by a few percent.
Moon age
age = days since a known new moon, modulo 29.530589
Illuminated fraction
illumination = (1 - cos(2 pi x phase fraction)) / 2
Because the terminator sweeps across a sphere. Illumination changes fastest around the quarters and slowest near new and full.
Two full moons in one calendar month, which happens roughly every 2.7 years because the synodic month is shorter than most months.