Moon Phase
Computes the current moon phase (new, waxing, full, waning) and the illumination percentage for any date.
How is the phase computed?
The method rests on a simplified astronomical computation driven by the moon age, that is, the days elapsed since a reference new moon (Jan 6 2000). Each lunar cycle lasts 29.530589 days.
It's the position within that cycle that sets the phase. Illumination, in turn, comes from (1 − cos(2π × age / cycle)) / 2, which gives the visible fraction.
Being a simplified version, accuracy lands within ±a few hours, plenty for general use.
What's the moon phase today?
From tides to sky-watching and on into planting traditions, the moon phase stirs up plenty. This tool works out the lunar phase for any date (new, waxing, full or waning) and shows the disc's illumination percentage as well.
That percentage gives a far finer reading than the phase name on its own: you can tell whether the moon is nearly full, at an exact quarter or just a sliver of light. It pays off for night photographers, sky-watchers, fishers and anyone tracking the lunar rhythm for some activity, or out of plain curiosity.
Type in the date and you'll see the phase and the illumination right away. The calculation happens in the browser, so it doesn't rely on a connection.
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