Why doesn't the moon rise in the same place every night?
February 14, 2021 4:54 PM   Subscribe

I was never good at astronomy, obviously. But since the moon circles the earth shouldn't it rise and set in the same place in the sky night after night?

I can picture being in the center of a merry-go-round, and watching an individual horse going around and around, and it would appear in the same place every time it orbited me. So this is a bad model for understanding the moon's movement, but it's the only model I have.
Here's the thing - a paragraph of text probably isn't going to help me. I think I need to see a video. The videos I can find are about why there are phases of the moon, not why the moon rises and sets where it does.
What videos help explain this?
posted by Vatnesine to Science & Nature (6 answers total) 6 users marked this as a favorite
 
It's because the orbit of the moon is inclined with respect to the ecliptic and the equator. The first diagram here at Wikipedia's page on the orbit of the moon shows pretty clearly how your merry-go-round model of off. This video may also help.

The main point is the planes all have to line up to get the kind of regular, unchanging pattern that you're picturing.
posted by SaltySalticid at 5:09 PM on February 14, 2021


Best answer: It might also help you to know that the moon doesn’t rise each night because it’s going around the earth. It rises each night because the earth is spinning on its own axis. The moon, meanwhile, is going around the earth on a much slower cycle, taking nearly a month to go around, and the path it follows is not perpendicular to the earth’s axis.
posted by jon1270 at 5:22 PM on February 14, 2021 [6 favorites]


Best answer: ...i.e. you’re not in the center of the merry-go-round, you’re on the horse, going around fast, and the moon is slowly walking around the carousel. The carousel is built on a hillside, so the moon’s height relative to you is a little different each time your horse passes it, and this analogy is getting strained...
posted by jon1270 at 5:29 PM on February 14, 2021 [49 favorites]


Best answer: Let's go back to the merry-go-round, but use it a little differently. The merry-go-round is the Earth, spinning around once every day. For the purposes of this argument and a short enough time period, the Sun is that building wayyyy over there.
You're on the merry-go-round. For about half the go-round, you can see the building (sun), somewhere in your field of view. That's daytime.
There's also a guy walking around the merry-go-round. Just walking, so you spin round a bunch of times while he walks around once. The path the guy walks on makes him just about average eye-level to you on your horsey on one side, but on the other side of the ride, you can see his bald spot no matter if your horse is up or down.
Here's the trick: the edge of the ride is held up by stripey poles. That's your frame of reference - the horizon. If you were to note every time the guy lines up with the pole, sometimes his head lines up with one stripe, sometimes another stripe.
So every time you see the dude past the stripey pole you're using for reference, his height seems to jump - more than you expect. it's the combination of you seeing him at a different part of his path as he comes into view, and your horsey going up and down.

And his background is different, too. That's the background stars...
posted by notsnot at 6:45 PM on February 14, 2021 [3 favorites]


Best answer: The word you are looking for is "ecliptic". That is the path the sun traces through the stars and constellations, as seen from the earth.

Because the solar system is like a big, very flat pancake, with (nearly) all the planets and moons rotating around the sun in the same flat plane, as seen from the earth, all planets and moons are lined up along the ecliptic. As you watch a particular planet--or, for example, our moon--throughout a month or a year, or even a decade or two, what you will see is that the path of that planet/moon will move around on the ecliptic, sometimes back and forth, but never getting more than a few degrees away from the ecliptic.

This video shows the ecliptic & flat/pancake shape of the entire solar system.

The moon, in particular, takes a trip around the entire circle of the ecliptic once every month.

Now the trick is this: If the ecliptic were exactly aligned with the earth's equator, then the sun would, indeed, rise from the same direction/same spot every day of the year and set in the same direction/same spot every day.

Since the moon stays on the line of ecliptic, too (very closely) it would do the same.

BUT: The ecliptic is NOT exactly aligned with the earth's equator. In fact it is 23.5 degrees offset from the equator. In practical terms, that means the place the sun sets depends on exactly where the sun is on its annual journey around the ecliptic.

When the sun is exactly lined up with earth's equator (spring & fall equinoxes) it will rise due east and set due west. But anytime but those two days of the year, the sun is either north or south of the equator, meaning that it will rise and set somewhat north or somewhat south of due east.

Here is as pretty decent video explaining all this. You've noticed this--this is why we have seasons.

Now here is the takeaway: The same path the sun takes along the ecliptic over the course of an entire year, the moon takes in just ONE MONTH.

So the same thing I just explained that happens to the sun over the course of every year (east to southeast back to east then northeast and back to east) happens to the moon over the course of every single month.

This is actually really, really cool to watch once you are onto it. Over the course of a single month you can watch the moon follow the course the sun takes in the winter, spring, summer, and fall. Sometimes it rises much more in the NE and is very high in the sky all night for many hours, setting in the NW--just like the sun in summertime. Other times it rises in the SE and scuttles very low across the horizon after just a few hours, setting in the SW--just like the sun does in wintertime. Other times it is between the two.

(Note the above description is for the northern hemisphere, mid-northern latitudes. If you're in the southern hemisphere, near the equator, near the pole, etc etc etc you'll have to adjust accordingly. Regardless, whatever your sun does in one entire year of seasons, you'll observe the moon doing over the course of any given month.)
posted by flug at 7:50 PM on February 14, 2021 [2 favorites]


Here is a pretty good way to get a visualization of how things move over time, as seen from earth:

Stellarium online planetarium

What you do is: Set it to show the ecliptic, set the time to say 12 noon. Zoom out until you can see the entire sky, with the horizon a circle. Then click the time settings to move along one day or one month at a time throughout the year. Click through an hour at time, sunrise to sunset, for a few days and you'll get an idea of how the sun moves across the sky during a day and how that relates to what the ecliptic and other constellations are doing.

Remember that the moon also follows the ecliptic path (to within a few degrees). But it moves around the entire 360 degrees of the ecliptic every 28 days rather than the 365 days the Sun takes. So what the sun is doing every year the moon is doing every month.

If you want to follow the moon through an entire month to see exactly what it does, it is a bit more complicated that following the sun for a year, but you can do it. For example, move the hours until the moon is just a moonrise. Then move through the days of the month. Every few days you'll need to adjust the hour of the day to stay at moonrise (because the hour of moonrise changes--almost one hour different per day).

Similarly follow the moon's "noon" (moon's highest point in the sky) for a month, and then follow the moonset point.

Doing all these things is easy with a planetarium program and gives you a good feel for how things changes for a day, a month, and a year.
posted by flug at 8:27 PM on February 14, 2021 [2 favorites]


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