To look up the phase of the moon for any (Common Era) date,
just fill out the form below, then hit “Find Phase.”

The waning moon.

At 01:01:33 UT Tuesday 11 May 2021
the moon’s phase was

WANING CRESCENT (0.52% full)

Some factors that go into this equation:

Universal Time is 1.0425 in decimal hours.
The Julian date was 2459345.50516586.
The sun’s ecliptic longitude was 50.594771621259 degrees.
Its mean anomaly was 126.2825151092 degrees.
The moon’s mean longitude on 00 January 1990 was 318.351648 degrees.

These calculations incorporate the switch to the English New Style (N.S.)—i.e., Gregorian— calendar after Wednesday, 02 September 1752; the next day became Thursday, 14 September, skipping eleven days. (Please also note that, while fairly accurate, these calculations remain a bit buggy.)

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