# Difference in time between sunset and moonrise on a full moon

So as a Sailor I have experienced some times where sunset and moonrise are very close together (this happens when it's full moon) which is particularly beautiful. I assumed that this was always the case but looking more closely at time difference between sunset and moonrise they can change quite dramatically.

2019 (Porto-Vecchio France)
Date        sunset   moonrise  diff
04/20       20:08    21:24     1:16
05/19       20:39    21:20     0:41
06/18       20:59    22:05     1:06
07/17       20:54    21:31     0:37
08/16       20:21    21:12     0:51


As you can see there is some degree of variation, can someone explain why this happens and how I can find out where and when they are closest together?

• All of your dates are off by 1 day for the Full Moon (according to the Astronomical Almanac for 2019). Also, you need to look at the sunset/moonrise OR sunrise/moonset that is closest to the time of Full Moon to determine how close they occur. If the Full Moon is at 10 o'clock in the morning, then you should not be comparing the time of moonrise that occurs at 9 o'clock at night. – JohnHoltz Sep 28 '18 at 2:47

Technically, the Moon is Full at a specific time. Therefore you need to check the time of sunset/moonrise OR sunrise/moonset that is closest to the time of Full Moon. Then you can more accurately compare the variation. Here is the list for 2019 created using the U.S. Naval Observatory website.

Longitude E9° 8', Latitude N41° 35'
All times in Universal Time (UT)
Date of Full Moon   -------- Sun ----------     -------- Moon --------  Moon-Sun
Date    Time              Date    Time              Date    Time     Diff
1/21/2019    5:16   Rise  1/21/2019    6:45     Set 1/21/2019    6:54   +0:09
2/19/2019   15:53    Set  2/19/2019   17:01    Rise 2/19/2019   16:55   -0:06
3/21/2019    1:43   Rise  3/21/2019    5:26     Set 3/21/2019    5:58   +0:32
4/19/2019   11:12    Set  4/19/2019   18:08    Rise 4/19/2019   18:14   +0:06
5/18/2019   21:11    Set  5/18/2019   18:39    Rise 5/18/2019   18:14   -0:25
6/17/2019    8:31   Rise  6/17/2019    3:49     Set 6/17/2019    3:54   +0:05
7/16/2019   21:38    Set  7/16/2019   18:56    Rise 7/16/2019   18:49   -0:07
8/15/2019   12:29   Rise  8/15/2019    4:32     Set 8/15/2019    4:09   -0:23
9/14/2019    4:33   Rise  9/14/2019    5:02     Set 9/14/2019    4:59   -0:03
10/13/2019   21:08    Set 10/13/2019   16:46    Rise10/13/2019   17:02   +0:16
11/12/2019   13:34    Set 11/12/2019   16:07    Rise11/12/2019   16:25   +0:18
12/12/2019    5:12   Rise 12/12/2019    6:40     Set12/12/2019    6:42   +0:02


The difference is much smaller than your original list.

Why this happens MCG and Carl Witthoft explained why it happens, namely because:

1. If the moonrise/set is not the same time as the exact Full Moon, then moonrise/set occurs earlier or later than the sunset/rise.
2. Because the Moon passes above and below the ecliptic (the path of the Sun in the sky), the Full Moon is rarely directly opposite from the Sun. Therefore, it can rise earlier or later than the Sun even when exactly "Full".
3. Even if the Full Moon were exactly opposite from the Sun (180 degrees apart, and in a total lunar eclipse), there would still be a difference between the time of sunset/moonrise. Those times are when the limb of the sun/moon is visible, and the limbs are closer together than 180 degrees apart. Also, atmospheric refraction shrinks the separation a small amount.

Where and when are they closest?

This is more involved but can be reasoned using the following figure. The Sun is in the direction of 1, so the ring of sunset-sunrise is the circle A-A. The Full Moon is in the direction of 2, so the ring of moonrise-moonset is the circle B-B. Where these two circles cross at C is where the sunset and moonrise (or sunrise and moonset) occur at the same time. The highlighted area is where the Moon and Sun are both above the horizon at the same time. Note that this figure is highly exaggerated for illustrative purposes.

• Ok, I think i was a day off because the night where the moon rises as completely full is they day after the full moon. Checking your data source for Greenwich 17/06/19 it will rise incomplete and be full at 08:31. So the night after the rise will be a fair bit after sunset everywhere !! – Iain Watt Sep 29 '18 at 8:59

I'm wondering where you got your list. It appears that other days might be better choices. As an example, for Porto-Vecchio, April 20,2019 is a day late:

Apr 19, 2018 sunset: 20:07, moonrise: 20:13, diff: 0:06

The moon rises about 50 minutes later on consecutive days. In addition, the sunset time might be changing by a couple of minutes a day as well. Since sunset only happens once a day, then we'd expect the rise after a full moon to be anywhere in that ~50 minute period late. On average then, I'd expect the closest sunset/moonrise of a month to usually be a quarter of that range (cut in half because closest could be before or after, and another half for variation) or about 15 minutes.

There are a lot of sites with accurate sunset/moonrise times, but I don't know of any that directly give the delta between them. Could probably create one with some python scripts, but not something I've done.

Going out on a limb a bit here -- depending on the month, the compass location where the sun sets varies as well as the length of time the sun is above the horizon (note also there are different definitions of "sunrise" - visible, half up, etc). The moon's compass location varies with a different cycle because the moon's orbit is not perfectly over the equator.
Combine those two variables, keeping in mind that a "perfect" full moon happens when the sun and the moon are exactly $$\pi$$ radians apart from our point of view, and it would appear quite reasonable to have significant variation in their relative rise/set times.

• The equatorial plane isn't very relevant here. The plane of the Moon's orbit is around 5 degrees from the ecliptic plane. – PM 2Ring Sep 28 '18 at 8:48
• @PM2Ring can you quantify that claim, i.e. how much time difference +/- 5 degrees actually causes? – Carl Witthoft Sep 28 '18 at 12:19
• The point of my previous comment is that the ecliptic plane is what we need to consider for this question, since that's the plane of the Sun's apparent motion, and it's also fairly close to the plane of the Moon's motion (which is pretty unusual since satellites are normally equtorial). The equatorial plane is about 22 degrees away from the ecliptic. The details of the Moon's motion are rather complex, eg its orbit is relatively eccentric, and it precesses fairly quickly. Its apparent speed varies quite a bit even on a short timescale. – PM 2Ring Sep 28 '18 at 12:38
• The inclination of the lunar orbit to the ecliptic is only one of several factors affecting the variation in the period of one Full Moon to the next. To compute that period with precision of around one minute takes about a dozen trigonometric terms. To answer your question re the 5 degree inclination I'd have to do some spherical trig, but at a rough guess, it means that Full Moon can be more than an hour off from the time when the Moon has an ecliptic longitude 180 degrees from that of the Sun. – PM 2Ring Sep 28 '18 at 12:44

The reason for this is quite simple. While we orbit the sun, the moon is orbiting us.

Look at the direction of the sunlight (this will show where the Sun is), then look at the position of the Moon.

Let us take the Full Moon as an example. Imagine, in this 2D image, the Earth is rotating anti-clockwise. As the Sun is on one side, the moon is on the other. So Sunset and Moonrise will occur at close to the same time.

Now let us look at the first quarter. At sunset, the moon is going to be almost directly overhead, which means moonrise occurred hours previously.

Using a 2D visual representation makes it easy to visualise. Of course, it is not always so black and white, as the moons orbit is not a perfect circle same as ours is not perfect around the sun (hence different rise times throughout the year during the same phases), then there is axial tilt etc, etc. But this will give you a pretty good idea. The closer to the full moon it is, the closer Moonrise and Sunset will be.

• The last quarter Moon is not "almost directly overhead" at sunset. The last quarter Moon is not even visible at sunset. You may have confused yourself from the diagram because the earth is rotating counter-clockwise in the diagram, not clockwise. The first quarter Moon is near the meridian at sunset. – JohnHoltz Sep 27 '18 at 16:36
• This doesn't appear to address the question at hand. – Carl Witthoft Sep 27 '18 at 19:07
• The OP asked why there is variation at full moon, and then how to find when sunset and full moon rise are closest. Not why it's different for quarter vs. full – Carl Witthoft Sep 27 '18 at 19:19
• @CarlWitthoft the question has been edited since I answered. When I did, the only mention of a full moon was OP stating that its most beautiful during a full moon, but no mention of realising that's when it is happening. Then asked why is there a difference throughout different days. Hence my answer. It's sue to the orbit of the moon. So at the time, it did make sense. I didn't see the question edit – MCG Sep 28 '18 at 7:39
• Ahh, yes, the famous "moving target" question. I've been burned by that too. Sorry. – Carl Witthoft Sep 28 '18 at 12:18