What does the time attached to a full moon date signify?
August 5, 2022 8:35 AM   Subscribe

Astronomers of Metafilter: I'm looking at a calendar of full moons for 2022, and each of them has a time attached to it. What does that time mean?

Is that when the full moon will rise? Or some sort of 'peak' time? If I wanted to view the full moon, how much window would I expect to have around that time to do so?

In particular, I'm looking at the full moon on October 9, at 4:55pm EST and want to confirm that it would still be visible at nightfall, around 7pm?

Thanks!
posted by geegollygosh to Science & Nature (14 answers total) 2 users marked this as a favorite
 
It’s the time when the Moon and Sun are in exact opposition in ecliptic longitude, that is, completely full. Visibility will vary by location; the exact full moon can happen at any time, including in the middle of the day in a given location, so you can only view it the night before or after.
posted by Atrahasis at 8:44 AM on August 5 [4 favorites]


Looking at the source of that calendar (the NASA SKYCAL page) I don't see any way of indicating a location. Without knowing where you are, the generated data can't "know" the moonrise time, since that's dependent on where you are. I think the time listed is the peak full moon, when the earth is exactly between the sun and the moon.
posted by mrgoldenbrown at 8:45 AM on August 5


I think that is the moment that the moon is at its fullest. In fact when I search for 'moonrise on October 9th' it seems to be much later than that (do your own search, that way I won't cock up the time zone conversion and put you wrong).

Full moon is when the moon is opposite to the sun in the sky, so it should come up a bit after sunset on those days (at least, you know they aren't going to be in the sky at the same time). Kind of obvious when someone says it out loud but it took me a long time to think that through.
posted by How much is that froggie in the window at 8:48 AM on August 5 [1 favorite]


Response by poster: Got it, thanks all-- that makes sense and seems somewhat obvious now that I think about it!

Looking at a location specific moonrise calendar like this one, is there anything I should know about what viewing the moon is like at different times between when it rises and when it sets?

For instance, on that Oct 9 date, in my area the moon is supposed to rise around 6:30pm. Does that mean it would be just starting to come across the horizon at that time, or fully visible? Again, from a viewing perspective, does it tend to look larger at the beginning/ending/middle of it's path?
posted by geegollygosh at 9:01 AM on August 5 [1 favorite]


Yes. There is a moment of "full moon" when the moon is the "fullest" it ever gets; when the Sun, Earth, and Moon are lined up. Well, as lined up as they get, since if they were exactly lined up in the same plane, we'd get a lunar eclipse (side note: this is why lunar eclipses always happen during the full moon).

This is also true for lots of other events in our solar system: there are specific times for all phases of the moon, but also things like the summer and winter solstices and the vernal and autumnal equinoxes.
posted by Betelgeuse at 9:01 AM on August 5


Response by poster: Does that mean it would be just starting to come across the horizon at that time, or fully visible?

I think I answered this question myself by looking at the tool in more detail! Still looking for answers on the other part, though: Again, from a viewing perspective, does it tend to look larger at the beginning/ending/middle of it's path?
posted by geegollygosh at 9:04 AM on August 5


For instance, on that Oct 9 date, in my area the moon is supposed to rise around 6:30pm. Does that mean it would be just starting to come across the horizon at that time, or fully visible? Again, from a viewing perspective, does it tend to look larger at the beginning/ending/middle of it's path?

The times noted for moon rise, etc, are when the upper limb of the moon (i.e. the top) crosses the horizon. It likely will not be visible at that time unless you have a completely clear horizon (i.e. clear of trees and mountains/hills).

Generally, we perceive the Moon as larger when it's closer to the horizon. There's some debate over the cause of it, but there's universal agreement that this is a matter of our perception rather than any actual physical size change.
posted by Betelgeuse at 9:05 AM on August 5 [1 favorite]


Moons always look bigger to humans at rising and setting, compared to highest in the sky, due to some psychology and vision/optical processing stuff, see a nat geo bit about that here.

I would go for rising personally, Bc it's nice to see it go from eg half above to all the way above horizon.
posted by SaltySalticid at 9:21 AM on August 5


Note that if the moon is full at 1 a.m. tomorrow; it will appear most full to me when I walk the dog tonight at 10 or 11 and will be slightly less so tomorrow night, though the date is tomorrow.

FYI, the moon rises approximately 1 hour later every day. Full moons rise @ 6pm, set @ 6am with variations for time zone, so the next night, 7pm., 5am.

The leaves are out in the N. hemisphere, which absorb a lot of light, but I can still read by the full moon. in winter, with no leaves on trees and snow on the ground, it's easy because it's so bright. City lights have taken away some of the beauty of the moon, but it can still be found.
posted by theora55 at 9:42 AM on August 5 [1 favorite]


IF the moon is about half a degree in diameter and the earth earth rotates once every 1440 minutes THEN it takes 2 minutes for the lower limb to follow its upper oppo to make the moon fully visible on rising. Don't be late and get your sight-lines sorted prior.
posted by BobTheScientist at 9:54 AM on August 5


After writing out a comment about full moons' rising related to sunset or absolute clock times, I realized something important to internalize is that, (statistically for most locations on earth) that the moon hits peak fullness about half the time during your day. Yes, the moon could be hitting full "full" at your noon! And that evening it will rise a little bit (less than 1hr) after sunset. The day before it would have risen just before sunset. And most people would look at the moon on both evenings and call them full. If it's hitting full in your nighttime then it will rise a little before the sunset before that nighttime. I think I got that right.
posted by achrise at 11:04 AM on August 5 [2 favorites]


I'm Wiccan and my coven meets at the full moon; for October's moon we will arrange in advance to meet on the evening of either October 8, 9, or 10 depending on which one is best for members' schedules, since the moon will be visibly "close enough to full" for that entire period, even though "peak full" is at 4:55pm EDT on the 9th. I hope this conveys some sense of what the "full moon" means to a casual viewer -- basically it's going to look pretty much full for about a 3-night period around "peak full".

If your needs are specific enough that you want to do a trial run, the next full moon is next Thursday evening (August 11). It will rise in the east around sunset, be maximally overhead around 1am (this would happen at midnight, but daylight saving time pushes it back by an hour), and set in the west around sunrise. The time at which the rising of the moon will be viewable *by you* depends on what's on your local horizon: it's a very different experience depending on whether you live in a flat region with a clear view to the eastern horizon, or live among a bunch of hills, trees, and/or buildings that obscure your view of the lower parts of the eastern sky.

The biggest risk to actually being able to see the full moon is that it will be overcast on the night in question.
posted by heatherlogan at 1:26 PM on August 5 [2 favorites]


Nthing the comments about horizons. I live in a valley surrounded by hills that are officially tall enough to be mountains, and visible moon rise here is at least 30min and sometimes 45min after the "official" moon rise time.

And yes, the moon will look very full on the 8th, 9th, and 10th.
posted by lapis at 5:19 PM on August 5


IF the moon is about half a degree in diameter and the earth earth rotates once every 1440 minutes THEN it takes 2 minutes for the lower limb to follow its upper oppo
An object in the sky moves at four minutes per degree or slower depending on its distance from the equator.

As an extreme example, imagine a moon in a polar orbit, so that it's just five degrees away from the north pole. Meanwhile, you are on the equator so the north star is right on the horizon. The moon will rise five degrees east of due north, spend 12 hours slowly moving in a tiny arc around the north star, and then set five degrees west of the north star.

In this case, the moon spent twelve hours traveling 180 degrees around the circle, but it's so close to the pole, that it only traveled (10 * pi = 31.4) degrees in that time; it would take about twelve minutes to move an entire lunar diameter.

Celestial motion is complicated.
posted by Hatashran at 9:10 PM on August 5


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