Tukuk-OS🌙 Moon Phase
ℹ️ What this tool does & how it works
The Moon is a three-body problem, but the mean synodic month — 29.530588853 days between new moons — answers "what phase is it roughly" to within a day. Anchor the cycle to a known new moon (6 January 2000, 18:14 UTC), take the day difference modulo the cycle length, and age, illumination (a cosine) and phase name all fall out. For garden planting, tides of mood and curiosity this is the right tool; for a launch window, take the ephemeris.
Why "full moon tonight" is a fuzzy fact
The disk stays above 99% illuminated for roughly half a day around opposition, so two consecutive evenings both look full — astronomers speak of an instant of fullness, cultures of a day. The mean synodic month handles this cleanly because it averages the orbit\u2019s eccentricity: the Moon swings faster near perigee, so true new moons drift up to ±14 hours from the mean, and that drift is why ancestor calendars that trusted only the mean eventually accumulated errors (the reform-era Easter tables needed real corrections for exactly this reason). For tide timing, fishing lore and garden calendars the mean cycle is more than sharp enough — and it is the only version arithmetic can reproduce.