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Given a date, I'd like to return the lunar phase.

Jean Meeus's Astronomical Algorithms book and the many packages like PyEphem that make use its algorithms provide ways to calculate when a primary phase (New, First Quarter, Full, and Last Quarter). The secondary phases (Waxing Crescent, First Quarter, Waxing Gibbous, Waning Gibbous, and Waning Crescent) can be easily calculated as falling in between the primary ones.

The percent illuminated the moon (or any other body between the observer and the Sun) can also be calculated for a given point in time.

We can also calculate the subsolar point.

What isn't clear is how long the primary phases should be considered to be in play. How are labels generally determined for these phases?

The next new moon occurs 2017/7/23 09:45:34 UTC. When should it no longer be considered new but waxing crescent?

  • When it is 1.0% illuminated?
  • When the subsolar point moves 5 degrees west of 180E? 10 degrees?
  • At 2017/7/24 00:00:01 local time?
  • At 2017/7/24 00:00:01 UTC?

I'm inclined to use the last. The moon is "new" for the entire UTC day 0.0% illumination falls on. The following day it is waxing crescent and remains so until the day the first quarter falls.

Anyone know of a standard here or rule of thumb?

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  • $\begingroup$ Technically, the New Moon (also First/Last Quarter and Full) are instants in time, not ranges. The instant after New Moon, it's a waxing crescent. It then becomes First Quarter for an instant and then waxing gibbous. I made up my own person rule based on difference in ecliptic longitude between the sun and moon, but that's just me. $\endgroup$
    – user21
    Jul 3, 2017 at 17:56

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A new moon occurs 2017/7/23 09:45:34 UTC. Instantly after that it is waxing.

A crescent won't be visible for a few days. It reaches first quarter about 7 days later. The moon will appear to be about half full for maybe a couple of days. Nevertheless, first quarter, full moon, third quarter and new moon are moments in time, not intervals.

It may, sometimes be convenient to refer to "the day that new moon occurs", and in casual use people will say the moon is full while it appears as a circle to the naked eye (usually lasting several nights) These are casually defined for casual use. For precise terminiology say "it is now 3 days 4 hours after New Moon" or similar.

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  • $\begingroup$ I like the idea of outputting 'X hours until/after Full Moon" for some period around the full moon. illumnation changes very slowly around the new and full moons and phase mostly about appearance. $\endgroup$
    – rtphokie
    Jul 4, 2017 at 1:22

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