<p>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 <a href="https://en.wikipedia.org/wiki/Declination" rel="nofollow noreferrer">declination</a> (angle north or south of the celestial equator).
At full moon on 2019-01-21, the Moon's declination was +20<span class="math-container">$^\circ$</span>, and there were 15 hours between moonrise and moonset at 40<span class="math-container">$^\circ$</span>N latitude.
At new moon on 2019-02-04, the Moon's declination was -18<span class="math-container">$^\circ$</span>, and moonrise and moonset at 40<span class="math-container">$^\circ$</span>N were only 10h10m apart.</p>

<p>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.</p>

CC BY-SA 4.0
2019-02-16T04:27:23.157Z
2019-02-16T04:27:23.157Z
Mike G
<p>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?</p>

CC BY-SA 4.0
2019-02-14T16:22:36.483Z
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.
Carl Witthoft
CC BY-SA 4.0
2019-02-14T17:13:06.987Z
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.
JohnHoltz
CC BY-SA 4.0
2019-02-14T22:45:18.420Z
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 :)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2019-02-15T19:37:14.747Z
Thanks I have edited the question for clarity and to narrow the scope of explanations.
Calydon
CC BY-SA 4.0
2019-02-15T20:02:36.363Z
Still unclear. What do you mean by "stay up", and how did you measure this?
Hobbes
CC BY-SA 4.0
2019-02-15T21:19:04.787Z
Not sure exactly what you're asking, but any phase of the moon will seem to take a but longer if it occurs during apogee, when the moon's relative, linear velocity is less than when it is at perigee. How observable that is, is another question.
BillDOe
CC BY-SA 4.0
2019-02-16T00:42:14.390Z
You mean the time interval between moonrise and moonset?
FSimardGIS
CC BY-SA 4.0
2019-02-16T18:17:28.393Z
If you're getting the Moon phases from a typical calendar, they are rounded to the nearest day, which can exaggerate the differences in the durations of the phases. Also, Moon phase data in printed calendars isn't necessarily calculated for the locale where the calendar is distributed, it may just use data for a different longitude, eg Greenwich, although that should not be the case if the calendar gives Moon rising & setting times.
PM 2Ring
<the-moon><moon-phases>
What causes the duration of moon phases (the time the moon is visible above the horizon) to vary for an observer in the northern hemisphere?
Calydon
# What causes the duration of moon phases (the time the moon is visible above the horizon) to vary for an observer in the northern hemisphere?

(Asked by: username_0 on 2019-02-14)

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?

**Tags:** the-moon, moon-phases

> **Comments:**
> **username_1**: 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.  
> **username_2**: Hi @username_0. 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.  
> **username_3**: I'm guessing that username_0 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 :)  
> **username_0**: Thanks I have edited the question for clarity and to narrow the scope of explanations.  
> **username_4**: Still unclear. What do you mean by "stay up", and how did you measure this?  
> **username_5**: Not sure exactly what you're asking, but any phase of the moon will seem to take a but longer if it occurs during apogee, when the moon's relative, linear velocity is less than when it is at perigee. How observable that is, is another question.  
> **username_6**: You mean the time interval between moonrise and moonset?  
> **username_7**: If you're getting the Moon phases from a typical calendar, they are rounded to the nearest day, which can exaggerate the differences in the durations of the phases. Also, Moon phase data in printed calendars isn't necessarily calculated for the locale where the calendar is distributed, it may just use data for a different longitude, eg Greenwich, although that should not be the case if the calendar gives Moon rising & setting times.  

---

## 1 answers

### Answer by username_8 on 2019-02-16. Score: 2

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](https://en.wikipedia.org/wiki/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.


---

