WHAT just happened to the moon?!

June 11, 2013 9:49 PM

What did we just see? Please tell me we didn't just witness the beginning of the end of the world...

My boyfriend and I, currently living in the mountains in southwestern-ish Colorado, just witnessed the moon (a waxing crescent, I think, but I'm not quite sure) going from being completely and fully visible, to completely sinking below the horizon in the span of about two minutes. As in, we saw the moon move, sink below the horizon, bottom to top, with our naked eyes. This was at about 10 or 10:15 MT. Please, someone enlighten us, WHAT just happened? I never thought it was possible to see the moon move so quickly like that! Right now we're imagining apocalypse scenarios...

So, please...someone reassure us that this isn't the day after tomorrow and explain this phenomenon?

(Unfortunately, I recently read "The House on the Borderlands"...eek)

posted by Emms to Science & Nature (28 answers total)

Is it cloudy there at all?
Were you moving, like driving in a car?
posted by LobsterMitten at 9:52 PM on June 11, 2013

My guess is that it has something to do with the moon being really close to earth; that makes it appear to move faster. I noticed this on the last full moon.
posted by desjardins at 9:54 PM on June 11, 2013 [2 favorites]

Response by poster: We were standing outside of our house, looking west-ish, and watching the moon sink below the mountains on the horizon. It definitely wasn't a cloud; it seems to be a fairly clear night.
posted by Emms at 9:54 PM on June 11, 2013

Sounds like you saw the moon set. Ever watched the sun set? It happens fairly quickly, and the moon, being the same size of the sun in the sky, sets just as fast.
posted by zsazsa at 9:59 PM on June 11, 2013 [16 favorites]

As in, we saw the moon move, sink below the horizon, bottom to top, with our naked eyes.

This sounds right, I've never seen it happen any other way. When the moon starts setting top to bottom, get up off the floor and maybe have some water.
posted by carsonb at 10:01 PM on June 11, 2013 [40 favorites]

Looks like moonset time for Colorado was 10:33PM tonight.
posted by zsazsa at 10:02 PM on June 11, 2013 [1 favorite]

That's how the moon sets. The sun too, but you can't really stare at it.
posted by Justinian at 10:04 PM on June 11, 2013 [4 favorites]

I hate to do this but the nice folks at Yahoo answers have observed this same phenomenon. In short, the moon appears to set quickly sometimes based on a variety of stuff. This is normal. In fact if you check Google you can even watch other people's videos of the moon setting in about 2-3 minutes. Warning: they are dull. You are fine, this is the moon doing what the moon does.
posted by jessamyn at 10:06 PM on June 11, 2013 [14 favorites]

The sun too, but you can't really stare at it.

The other factor in the perception of speed variance between the sun and moon is that you can see the moon moving against the background of the stars in the night sky. When the sun is in the sky it blots out the star background, so it's more difficult to accurately gauge its speed across the sky.
posted by carsonb at 10:07 PM on June 11, 2013

Yeah, you don't notice the motion of the moon when it's in nothing but sky, but place it near the horizon and things go fast.
posted by mazola at 10:07 PM on June 11, 2013

As in, we saw the moon move, sink below the horizon, bottom to top, with our naked eyes.

Please, someone enlighten us, WHAT just happened?

The earth rotated so that the moon's disk disappeared below the western horizon. I am sure you have seen the sun do this, too. It is not surprising that it was so quick. Moonset (and sunset) only take a few minutes.
posted by Tanizaki at 10:08 PM on June 11, 2013 [5 favorites]

The angular diameter of the moon is about 30 arc-seconds, or 1/2 a degree, or 1/720 of a full circle.

It takes (approximately) 24 hours for the moon to go a full circle around the Earth.

24 hours * 1/720 = about 2 minutes.

Which means you are really good at estimating time.
posted by benito.strauss at 10:08 PM on June 11, 2013 [49 favorites]

Also: the moon is about half a degree of arc in the sky. So that's about 2 minutes.

Edit: Damn you, benito.strauss!
posted by mazola at 10:10 PM on June 11, 2013 [2 favorites]

It takes (approximately) 24 hours for the moon to go a full circle around the Earth.

*Cough* The moon is actually moving the opposite direction, taking about 28 days to go around the earth.
This is why it rises about an hour later every night.
The motion you are referring to, and the motion the OP is talking about, is caused by the earth's rotation. One rotation takes 24 hours. No big surprise.
Next time you are watching the moon set, imagine you are standing on a huge ball, moving 1000 miles per hour in the opposite direction, because that's what's happening.
posted by weapons-grade pandemonium at 10:14 PM on June 11, 2013 [11 favorites]

Actually, the length of moonset is a bit more complicate than simply dividing a day's length by 720. Latitude matters, as does the date.

Colorado's southern latitude is 37 N, so I will use that. On the equinox, moonset would take about 160 seconds (2 minutes, 40 seconds). On the solstice, moonset would take about 192 seconds (3 minutes, 12 seconds). These are actually figures for the duration of sunset because I have a handy formula for calculating sunset duration, but they are a close approximation given that the sun and moon have very close angular diameter.

The equinox is right around the corner, so tonight's moonset is one of the shortest of the year.
posted by Tanizaki at 10:24 PM on June 11, 2013 [6 favorites]

Strike that - the solstice is right around the corner, making for one of the longest moonsets. (although still pretty quick)
posted by Tanizaki at 10:34 PM on June 11, 2013 [3 favorites]

Why would the equinox or solstice influence the pace of moonset?

The earth's tilt, which causes equinoxes and solstices, is in relation to the plane of its orbit about the sun. The moon doesn't figure into that.
posted by notyou at 11:29 PM on June 11, 2013

Wow. You got an excellent does of awe, wonder, mystery, fear and ignorance/speculation (I don't mean that negatively) that the ancients must have had/felt when they saw unexplained activities up there. Way cool. Mystery and magic in the dark of night. You should take some time with those fresh feelings of fear and wonder and write some poetry. No kidding.

Last week, a huge ass asteroid passed within 65000 miles of the earth with less than a day's notice. (The moon is 240,000 miles away.) Lots of stuff up there. This one was only 1/3 Tungaska sized, but it's a good illustration of the kind of stuff we could encounter spinning around in our not-so-empty plot of real estate in the cosmos.

Feel free to worry, but it's no more effective than prayer. There's really no place to go. But those feelings of fear, awe and immediate danger from the usually benign universe... yum. The stuff of magic.

Consider this... you are going to die anyway. Would you want to miss a show like a once in a billion year fireworks display? Worse things could happen, like a quiet death in bed. Meh!
posted by FauxScot at 12:40 AM on June 12, 2013 [17 favorites]

Actually, the truth is that tonight's moonset is just one step towards the beginning of the END OF THE WORLD.

as the semimajor axis of the Moon's orbit continues to increase in the future, this stabilizing effect will diminish. At some point perturbation effects will probably cause chaotic variations in the obliquity of the Earth, and the axial tilt may change by angles as high as 90° from the plane of the orbit. This is expected to occur within about 1.5–4.5 billion years, although the exact time is unknown

THE END TIMES APPROACH. IN AS LITTLE AS 1.5 BILLION YEARS!

Now you can prepare yourselves appropriately.
posted by yohko at 1:03 AM on June 12, 2013 [7 favorites]

It was normal.

A friend of mine, who used to be a Phantom pilot (stationed in Hawaii), told me that, on his training flights, he liked to fly in large circles, gradually increasing altitude, until he reached his workable ceiling, then quickly descend until he was only a few hundred feet above the ocean. He would do this several times during the mission. When he took off around sunset, he said, he could watch the sun rise and set a few times before returning to base.

One of my pastimes in the high country (of the Sierras) was to sit facing the sunset, when the sun was low on the horizon--a red ball, dim enough to look at without being blinded. The sun seemed to hang for a little while just above what seemed to be the rim of the Earth, then sink, pausing just before the last of it sank below the horizon.

Some of this is caused by refraction--the red rays actually tend to bend around the horizon a bit more than the blue ones. Just after the sun has disappeared, the light dims, the air seems to turn blue, then, for a brief moment, the red rays seem to--well, flash isn't the word, but the air takes on a quick red glow for a few seconds, then quickly fades back to blue. It's subtle, and you can miss it if you are distracted. We called it Red Ray Time. It is especially dramatic when you are amidst granite canyons.
posted by mule98J at 1:59 AM on June 12, 2013 [3 favorites]

Here's a pretty clip with the moon crossing the horizon in ~3 min:

Full Moon Silhouettes is a real time video of the moon rising over the Mount Victoria Lookout in Wellington, New Zealand
posted by tinker at 4:01 AM on June 12, 2013 [1 favorite]

Why would the equinox or solstice influence the pace of moonset?

The earth's tilt, which causes equinoxes and solstices, is in relation to the plane of its orbit about the sun. The moon doesn't figure into that.

I did not say the equinox or solstice influence the pace. I said what matters is one's latitude when observing and the date of observation. The length of the sunset/moonset depends on the date and latitude of observation for the same reason that daylight hours for a day vary depending on date and latitude. I used the solstice and equinox because they provide the extremes of the range of possible duration times.

On the solstice, for example, the moon will be setting the maximum distance north or south of west. Thus, the moon sets at the shallowest possible angle giving the slowest moon set. This angle will vary with date and one's latitude, as I previous explained. Watch the sun set sometime and you will notice that it does not approach the horizon at the perpendicular but slants down to it.
posted by Tanizaki at 5:09 AM on June 12, 2013

The angular diameter of the moon is about 30 arc-seconds, or 1/2 a degree, or 1/720 of a full circle.
It takes (approximately) 24 hours for the moon to go a full circle around the Earth.
24 hours * 1/720 = about 2 minutes.
Which means you are really good at estimating time.

That is all good in theory, but as you can see in tinker's video above, the moon actually took 3 minutes to rise.

That formula gives an estimate of the fastest you will see the moon rise or set. The fuller answer is provided by Tanizaki above. Depending on the time of year (because this determines when daytime/nighttime are) and the phase of the moon (this determines the moons place in its orbit relative to the sun), the moon rises higher or lower in the sky. And this also varies the angle at which it intersects the horizon.

I couldn't find a good table of this online but someone has done it for the sun. As you can see the sun can rise or set in as little as a couple minutes and as long as 6 minutes if you happen to be at 55N latitude in the summer. And much longer if you're farther north.
posted by vacapinta at 6:04 AM on June 12, 2013 [2 favorites]

Another factor influencing the length of moonrise and moonset is the distance from the earth to the moon, which is not constant. The angular diameter of the moon varies from about 29.3 to 34.1 minutes of arc. Moonset will take longer when the moon is closer to us, other things being equal.
posted by brianogilvie at 7:00 AM on June 12, 2013

On the solstice, for example, the moon will be setting the maximum distance north or south of west. Thus, the moon sets at the shallowest possible angle giving the slowest moon set. This angle will vary with date and one's latitude, as I previous explained. Watch the sun set sometime and you will notice that it does not approach the horizon at the perpendicular but slants down to it.

I should have asked a better question. I understand how latitude would influence the perceived course of the moon -- it would determine the observer's position WRT to the moon's position on its orbit. I get how the same is true of the sun, and I get how the tilted Earth moving along it's orbit affects the sun's position in the sky.

What I'm not seeing is how that -- the Earth's tilt -- would influence the moon's relative position in the sky, and do so according to the the earth's position on its orbit.

I found two explanations.

1. The moon's phases are determined by the relative positions of earth, moon, and sun. The moon is only full when it is opposite the sun, and so will follow the sun's track across the sky.

2. The moon has its own priorities:

Changing Direction of Moonrise and Moonset:
The pattern in the Moon's movements is more complex than the Sun's for several reasons:

1. The Moon completes one orbit around us in a much shorter time than 1 year (in 29.5 days or about a month, in fact, in contrast to our orbit around the Sun with a period of 365.25 days).
2. The plane of the Moon's orbit is close to the plane of Earth's orbit around the Sun, but not identical -- the Moon orbits us in a plane which is tilted by 5.1° relative to the ecliptic.

• Because of the rapid orbit of the Moon around us in a plane which is close to the plane in which we always see the Sun, THE MOON DOES IN A MONTH WHAT THE SUN DOES IN A YEAR, in terms of the changing rising and setting direction along the horizon. That is, the moonrise and moonset directions change like a pendulum, swinging back and forth along the horizon and completing one full swing each month. The Sun's pendulum-like swing continues much slower, with the Sun completing one full swing each year.
• Because of the tilt of the Moon's orbital plane in relation to our orbit around the Sun, the outer extremes of the Moon's monthly range of rising and setting are NOT THE SAME as the outer extremes of the Sun's yearly range of rising and setting.
• Furthermore, the outer extremes of the Moon's monthly range of rising and setting are not fixed, but change slightly from year to year with an 18.6-year cycle.

posted by notyou at 7:29 AM on June 12, 2013 [1 favorite]

The equinox is right around the corner, so tonight's moonset is one of the shortest of the year. Strike that - the solstice is right around the corner, making for one of the longest moonsets. (although still pretty quick)

This is incorrect. The maximum altitude of the moon, which is when is sets the fastest each month is when the moon is new (in the summer, northern hemisphere). The minimum altitude, and longest moonset occurs when the moon is full (in the summer). Right now the moon is near a new moon in the summer so it is near its highest altitude of the month and the fastest moonset. The longest moonset will occur when the moon is full on the 23th and at lowest altitude.

In the original question, they described a crescent moon near the new moon in the summer in the northern hemisphere, so the moon was near its monthly highest altitude and therefore fastest moonset.

You are confusing the solar and the lunar cycle. The moon goes through the highest and lowest altitude (length of moonset) each month. These extremes occur at new and full moon and reverse between winter and summer. The lunar cycle dominates the solar cycle so that the phase of the moon is more important in determining length of moonset than how close it is to the solstice.

On the solstice, for example, the moon will be setting the maximum distance north or south of west.

Not exactly. The farthest north and south occur at the full moon, same as explained above. The full moon rarely occurs on the solstices. For any one year, the maxs and mins will be at the full moon nearest the solstices. This year that happens to be June 23, not the solstice. The maxs and mins can be offset from the solstices by half a month.

Another thing to keep in mind is that the sun and full moon are on opposite sides of the earth. So this means that, in the northern hemisphere, the sun is highest in the sky in the summer and the full moon is lowest in the sky. Likewise, in winter the sun is low in the sky but the full moon is highest.
posted by JackFlash at 11:59 AM on June 12, 2013 [2 favorites]

Summarizing some of the above that is accurate:

* The apparent motion of objects through the sky due to the Earth's rotation is surprisingly fast
* It is usually not that noticeable because you don't have a point of reference
* But when, for example, the sun or moon gets close to the horizon you do have a point a reference and if you observe carefully--yup, it is indeed surprisingly fast.
* So yes, for mid latitude locations like your location in Colorado, say less than 45 degrees, both the moon & the sun will take between about 3 minutes (+/- 15-20 seconds), start to finish, depending on a number of factors. Anywhere from the equator to say 45 degrees latidude, you're looking at 2.5-4 minutes for the full disk of the moon to set. Vacapinta's chart gives a good idea of the variation in times for the sun, and as notyou notes, a similar chart for the moon would have a similar amount of variation and would look very similar in terms of the difference in times via latitude. That's because the moon happens to have a similar apparent size to the sun and follows a similar though not identical track through the sky as the sun, the big difference being that the moon does this monthly whereas the sun does it annually.

However, I think all of the above might miss the main factor that made your observation even shorter than normal:

1. Yesterday the moon was a fairly small crescent. Depending on the angle of the crescent to the horizon, that means the entire visible crescent of the moon might disappear in half the time a full moon would--or even a bit less than half.

2. You're in the mountains--was there by any chance a slope on the part of the horizon where you observed the moon set? Specifically was that part of the horizon sloping downward to the left anywhere close to 45 degrees?

If so, that would line up the crescent of the moon (the crescent was at about a 45 degree angle to the horizon) and the direction the moon was setting (also about a 45 degree angle to the horizon, moving from your upper left to your lower right as you face the setting moon) with the angle of the mountain (let's say it was also about a 45 degree angle, sloped to the left, from the flat horizon) rather exactly needed in order to minimize the time it would take the visible/lighted portion of the moon to disappear behind that mountain.

If the crescent, the direction of the moon's travel in the sky, and the angle of the mountain are all lined up just right, the moon might go from completely visible to invisible in maybe as little as 1:30 or so, whereas for a completely full moon on a completely level horizon to set at this latitude would take a good 3 minutes, give or take maybe 15 seconds.

I've been playing around with this on the Space @ Home online planetarium--which is the only online planetarium I've found that both works correctly and includes correct-sized sun & moon. If you're interested, it's quite interesting.
posted by flug at 12:28 PM on June 12, 2013 [1 favorite]

Oops. What I described above was actually not the longest and shortest moonset. I was actually describing something different, the longest and shortest "days" of moonlight, the time from moonrise to moonset. The longest moon "day" occurs when the moon is highest in the sky, at full moon and the shortest when the moon is lowest in the sky, at new moon.

On the other hand, the two longest moons sets, the time it takes the moon to slide below the horizon, are each month are at new moon and full moon, when the moon has the highest and lowest altitudes and the path at moonset is more horizontal. The two shortest moonsets each month are at the half moons, when the path is more vertical.

Each year, the longest moonsets occur at the full moon and new moon nearest the two solstices. Each year, the shortest moonsets occur on the two half moons nearest the two equinoxes.
posted by JackFlash at 9:50 PM on June 12, 2013 [1 favorite]