This one is really the reason why I registered - although on seeing some of the other threads now I’m hooked. We need your help to resolve one of those pointless office debates - does the moon rise and set? I say, “yes, of course it does - just like the sun” but cannot seem to deliver them a convincing proof.
We’ve been distracted by discussing phenomena including phases of the moon, that the moon and the sun are visible simultaneously sometimes, that the same side of the moon always faces the earth etc but still the central issues lingers on unresolved. The fact that it is a satellite, and of the earth to boot and not a star or a planet (which I think all are agreed certainly rise and set), probably has something to do with the answer.
Leaving with an understanding of the movement generally would be the ideal but we’ll settle for sorting out the rising and setting thing. BTW we are talking about movement relative to an observation from the earth here - before some smart Alec really tries to complicate matters!!
Of course it rises and sets–I’m not sure how the question even arises.
In fact, beginning with the new moon (which is in fact when the moon and sun occupy the same point in the sky as seen from the Earth), the moon appears to rise a little bit later each evening. A little less than an hour later, actually, which is why 14 days after new moon, it will be exactly opposite the sun in the sky, and will appear full.
The U.S. Naval Observatory has a page at http://aa.usno.navy.mil/AA/data/docs/RS_OneYear.html which allows you to view a table listing the moonrise and moonset times for your location. In fact, the USNO has a number of more detailed answers for you than I can give, such as:
Moonrise and moonset times are computed for exactly the same circumstances as for sunrise and sunset [when the upper edge of the disk of the moon is on the horizon]. However, moonrise and moonset may occur at any time during a 24 hour period and, consequently, it is often possible for the Moon to be seen during daylight, and to have moonless nights. It is also possible that a moonrise or moonset does not occur relative to a specific place on a given date.
Moonrise and moonset. Moonrise and moonset are defined similarly, but the situation is computationally more complex because of the nearness of the Moon and the eccentricity of its orbit. If the computations are carried out using coordinates of the Moon with respect to the Earth’s center (the usual method), then moonrise or moonset is defined to occur when the geometric zenith distance of the center of the Moon is
90.5666 degrees + Moon’s apparent angular radius - Moon’s horizontal parallax
Under normal atmospheric conditions at sea level, the upper limb of the Moon will then appear to be tangent with a level, unobstructed horizon. No account is taken of the Moon’s phase; that is, the Moon is always regarded as a disk in the sky and the upper limb might be dark. Here again, a constant of 34 arcminutes (0.5666 degree) is used to account for atmospheric refraction. The Moon’s apparent radius varies from 15 to 17 arcminutes and its horizontal parallax varies from 54 to 61 arcminutes. Adding all the terms above together, the center of the Moon at rise or set is geometrically 5 to 10 arcminutes above the observer’s “geocentric horizon” - the horizontal plane that passes through the Earth’s center, orthogonal to the observer’s local vertical.
Although we orbit the sun and the moon orbits the earth I think we can still say the both rise and set. The ancients believed everything revolved around the earth. Why? Because it looks that way from earth (at least until you start watching closely).
Since neither is really doing any such thing (rising or setting) I believe everyone goes from our perspective here on earth. I have frequently heard the term moon-rise and moon-fall. When astronauts were on the surface of the moon I believe they described seeing the earth coming over the moon’s horizon as earth-rise.
Even though the moon orbits the earth the earth rotates quick enough that we catch-up (as it were) to the moon once per revolution. The moon takes roughly a month to make one complete orbit of the earth but you pass ‘under’ the moon once every 24 hours. You also catch up to the sun once every 24 hours (not counting the earth’s tilt) to see it rise and then set. Hence I think the same can be said of the moon (rising and setting).
The moon always shows the same face because it so happens that its rotation is proportional to its orbital speed such that the same side is always pointed towards us. I do not know if this is just weird happenstance or if orbital mechanics make it a requirement of a moon. Kinda weird if it’s just dumb luck it worked that way.
I think the other posts covered this pretty well. I’d just like to add that I didn’t it was possible to not know the answer to this.
Jeff_42 wrote:
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I think this only happens to astronauts orbiting the moon. The same side of the moon always faces the Earth, so if you are standing on the moon the Earth is either up or it isn’t, and it will stay that way.
You are correct in your feeling - it’s an orbital dynamics thing. The moon is slightly oblong (egg-shaped), and the mass is not centered. This imbalance in the mass contributes to the need for the moon to assume the orientation that it has, keeping one face to the earth at all times. The other factor that played a role is the tidal forces. Back when the moon was formed, it rotated faster. The same way that the moon creates tides on the Earth, the Earth made tides on the moon. These tidal forces were what stabilized the moon.
It is the same all across the solar system. Any bodies small enough have had enough time to stabilize, and have the same behavior to their planet.
The same way that the moon is tidally locked to the Earth, so the Earth will one day be with the moon (well, depending on how long that takes vs. the sun running down). The tides are slowing the earth’s rotation.
Lance Turbo said:
Not quite entirely. The moon has a slight libration, which means it rocks back and forth. Very close to the equator, you would see a change in position of the Earth.
However, I think the term “earthrise” may have been a poetic reference to seeing it low in the sky. I’m not sure.
Wait a minute. Wouldn’t this be a solar eclipse? They certainly don’t happen once a month, and they don’t last very long when they do come around. I think a new moon is just when the earth-side face of the moon is completely un-illuminated.
In fact, if anything, it’s more correct, since the Moon actually does go around the Earth (to first approximation). Of course, the effect is still due primarily to the Earth’s rotation, but that’s the case with the Sun, too.
Yeah, Boris caught me, but I didn’t think about it until several hours later, by which time dtilque already got the rebound.
Wouldn’t it be way cool if there WAS a total eclipse every month, though? A total solar eclipse is about one of the most interesting things you can experience.
pldennison: Wouldn’t it be way cool if there WAS a total eclipse every month, though? A total solar eclipse is about one of the most interesting things you can experience.
This would only happen if the Moon’s orbit were equitorial. Then, only the tropics would ever see total eclipses. In N. America, we’d probably see partials in the summer months.
This is what has me stumped, and it’s bugging me. notquitekarpov, how do they think it doesn’t rise and set? Do they think the moon is always visible? If not, how do they think it gets from visible to not visible?
I found this thread using the search feature, because I was looking for information about why the moon always shows the same face. I think Irishman did a great job answering the question here, however I would like any additional information anyone could provide concerning tides, both here on earth and on the moon. Specifically, how long has the moon been stabilized by tides and when is the earth expected to stabilize and always show the same face to the moon?
