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.

  • 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.

    illustration of moonrise and sunset planes

    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 !!

  • 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.

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

    enter image description here

    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.

    I said that with respect to the drawing after stating I am imagining a clockwise rotation. I can change it to counter clockwise and first quarter if you'd like? I was just using a quick example to explain about rise and set times using the picture

    This doesn't appear to address the question at hand.

    @Carl how does it not? OP asked how they can find out when they are closest. I answered by using visual observations and saying how to do just that

    I just made sure it was simplified for OP

    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

    @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

    Ahh, yes, the famous "moving target" question. I've been burned by that too. Sorry.

    @CarlWitthoft no need to apologise! These things happen. Had it a few times on the EE stack exchange too. I'm sure it won't be the last time either! I assume that was the reason for the downvote? Unless there was something else I missed too?

  • 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.

    @PM2Ring can you quantify that claim, i.e. how much time difference +/- 5 degrees actually causes?

    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.

    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.

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Content dated before 7/24/2021 11:53 AM

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