Moon Phases (Month Chart)

You want to know which phase the Moon was in on a past date, to explain why the fishing went the way it did, or which phase it will be in on the weekend of a hike, where the gap between a dark sky and full-moon glare changes every photo. A wall calendar is no help when the date is old or months away.

The calculation splits the mean synodic cycle, 29.530556 days, into eight slices and measures your date against a reference new moon on 7 January 1970. The result is the slice number, 0 to 7, evaluated at midnight on the chosen day, with the legend 0 new, 2 waxing, 4 full and 6 waning. The four decimals are formatting only; the value is a whole number.

Because the maths uses the mean cycle rather than the Moon's actual position, the point where one slice becomes the next can slip by roughly a day against the exact new or full moon. Treat the number as a rough guide and check an ephemeris when the date really matters. The page handles one day at a time, with no chart or monthly table, and runs in your browser.