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DevBench

How to read the Moon phase display

  1. The tool opens on the current instant. The disc shows exactly how much of the Moon's face is lit — a slim crescent near new moon, a full circle at full moon — with the phase name, illuminated percentage, waxing or waning direction, moon age and distance beside it.
  2. The Next phases list gives the next six principal instants (new, first quarter, full, last quarter) with local date, time and a live countdown from the selected moment. The highlighted full-moon row answers the question everyone actually has.
  3. Change the date and time to look at any moment — past or future — and the disc, statistics and event list all recompute. Press Now to jump back to the present.

Where the phase times come from

The instants are solved from the Moon's ecliptic longitude crossing the Sun's — conjunction for new moon, opposition for full, quadrature for the quarters — using the truncated lunar theory of Meeus chapters 47–48, the same mathematics behind official almanac phase times. A bracketed binary search pins each crossing to about a millisecond, so the times you see typically sit within a minute or two of the published ones. Illumination comes from the Sun–Moon–Earth angle, which is why it rarely lands on exactly 0% or 100%: the Moon's orbit is tilted, so perfect alignment only happens during eclipses.

Also useful: Date Pattern Checker, On This Day, and Time Zone Converter.