Why does each phase of the moon stay up a different number of hours per phase, so that, for example, January’s full moon was up twice as long as the new moon? Plus every month this bulge shifts and at some point the opposite phase is doubled. Three June full moon nights takes six March 1st quarter nights. Why does this happen?

  • $\begingroup$ The moon is always above the horizon for approximately 12 hours (various offsets for latitude and whatnot). You seem to be confusing visibility with duration. $\endgroup$ Feb 14, 2019 at 16:22
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    $\begingroup$ Hi @Calydon. It is not clear what you are asking. What does "duration of moon phase" mean? Does that mean the length of time the Moon is above the horizon? Please clarify your original question by editing it. $\endgroup$
    – JohnHoltz
    Feb 14, 2019 at 17:13
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    $\begingroup$ I'm guessing that Calydon is asking why the number of days that the Moon is "full" (or at another phase) seems to change from one lunar month to the next. But this isn't answerable, because there's no definition of how "long" a phase lasts. The Moon is "full" at the moment (measured down to the precise minute or second) that it reaches exact opposition. It's close enough that it "looks" full for roughly 12 hours either side of that. It's "nearly full" (but visibly not quite full) for maybe another day or two either side of exact opposition. But what's "nearly full" to me may not be to you :) $\endgroup$ Feb 14, 2019 at 22:45
  • $\begingroup$ Thanks I have edited the question for clarity and to narrow the scope of explanations. $\endgroup$
    – Calydon
    Feb 15, 2019 at 19:37
  • $\begingroup$ Still unclear. What do you mean by "stay up", and how did you measure this? $\endgroup$
    – Hobbes
    Feb 15, 2019 at 20:02

1 Answer 1


The interval between moonrise and moonset varies for the same reason that summer days are longer than winter days. How long a celestial object appears above the horizon depends on the observer's latitude and the object's declination (angle north or south of the celestial equator). At full moon on 2019-01-21, the Moon's declination was +20$^\circ$, and there were 15 hours between moonrise and moonset at 40$^\circ$N latitude. At new moon on 2019-02-04, the Moon's declination was -18$^\circ$, and moonrise and moonset at 40$^\circ$N were only 10h10m apart.

Another way to think of this is that when the Moon is in the first quarter, it's within about 5 degrees of where the Sun would be 3 months later; last quarter, the Moon is where the Sun would be 3 months earlier, etc. A full Moon in June is up about as long as the Sun in December; a first quarter Moon in March is up about as long as the Sun in June.


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