Moon Phase

How this is worked out

The moon's phase repeats on a synodic month — the time from one new moon to the next — averaging 29.530588 days. This tool counts the days elapsed since a known reference new moon (6 January 2000) and takes the remainder after dividing by the synodic month length, giving the moon's age (days into the current cycle) directly, and its phase name from eight traditional bands across that cycle. Illumination is approximated as (1 − cos(2π × age/synodic month))/2, the standard idealised-sphere approximation for the illuminated fraction of a disc lit from one side as it orbits — it does not account for the small extra wobble (libration) or the Moon's slightly elliptical, tilted orbit, so it can be off by up to a percentage point or two from a full lunar ephemeris.

Frequently asked questions

Why "age" rather than a calendar date for the cycle start?

Because the synodic month isn't a whole number of days, so a fixed calendar start date would drift out of sync after a few cycles; counting age in days since a reference new moon avoids that, at the cost of needing to divide out the whole number of elapsed cycles first (which this tool does for you).

Does this tool show the Moon's distance from Earth?

Not currently — the Moon's real distance varies through each month (due to its elliptical orbit) in a way that needs a fuller lunar position calculation than this simple age-based approach, so it isn't included here to avoid showing an unreliable figure as if it were precise.

Related tools: Sunrise & Sunset Calculator, Julian Date Converter, Season Calculator.

Source / Last verified: NASA Goddard Space Flight Center, "Six Millennium Catalog of Phases of the Moon" (eclipse.gsfc.nasa.gov/phase/phasecat.html): "the length of the mean synodic month ... is 29.530588 days", read 1 October 2026. The illuminated-fraction formula is the standard idealised-sphere approximation, not a further source beyond that.