Forgive me if his has been answered before; I did a search on “moon” and came up with over 2400 threads- far to many to review.
Anyway, my question is this:
So, the moon revolves around the earth, right? It takes approximately 28 days per revolution. To my thinking this means that half the time, the moon should be on the sunlit side of the earth; that is, you should only see it in the night sky a maximum of 14 days per month.
So how is it that almost every night, there’s a big ol’ moon in the sky?
Well, yes, half the time the moon is in the sky during the day. Frequently you can see it when it is. The thing is, excepting for right at the new moon, it’s either trailing or leading the sun by enough of a distance that it will be clearly visible for at least a little while at dusk or dawn. Those little skinny crescents are only in the night sky for a short time, however. They spend most of their time in the daytime sky.
No matter where the moon in its orbit, we spin beneath it every 24 hours, right? Therefore we really only don’t see it when its dark side is directly toward it.
The moon is not out there almost every night; you only notice it when it’s there. At any given time of day or night, the moon has a 50% chance of being in the sky, assuming the day is random. But why don’t you wait a week before testing this assertion, huh?
When the Moon is full, it’s on the opposite side of the Earth from the Sun, and it’s visible all night, and not at all during the day. The full moon rises at sunset, and then sets at sunrise.
When it’s at third quarter, or waning (the lit portion is shrinking), the Sun-Earth-Moon angle is 90 degrees, so when the Moon is setting, the Sun is at its highest point in the sky. The third quarter Moon rises at midnight, is highest at sunrise, sets at noon, and is “below” the Earth at sunset.
When it’s new, and we don’t see it, it’s on the same side of the Earth as the Sun, and at approximately the same position in the sky, so it rises and sets at about the same time as the Sun.
And when it’s at first quarter, or waxing (lit portion growing), it rises at noon, is at its highest at sunset, sets at midnight, and is below the Earth at sunrise.
It might help if you draw a diagram, looking from the top down at the Earth, Moon, and Sun. And remember that you can see half the sky at a time, not just straight up.
While the major player is that the earth is spinning upon its axis, it is also important to note that the moon is very slowing moving eastward in its orbit…making moonrise about 45 minutes later each night, on average.
Nothing is as simple as we were taught. Even our 24 hour day (i.e. solar day) is a simplification - as we easily accept that ONLY the earth’s axial rotation determines this…never accounting for the (lesser) orbital (revolutionary) effects which plays a role, too.
In short, don’t trust your sundial, either, pilgrim!
Not quite… The moon and the earth rotate around an axis somewhere between the two of them. If you imagine that the earth and moon are attached by a majorette’s baton, then they move as if the ‘baton’ was being twirled at a point along its length that is somewhat closer to the earth than to the moon, due to the difference in size and gravity between the two bodies.
I wouldn’t say it quite like that. The so-called barycenter of the Earth-Moon system, the point that they orbit around, is inside the Earth, about 1700 km below the surface.
Call me crazy, but isn’t the moon visible somewhere in the sky sometime during the night most of the lunar cycle? It may only be visible at any designated time (say midnight) half the cycle, but if you allow +/- 6 hours of earth rotation, you increase the number of nights it is visible sometime between sunrise and sunset.
Yeah, this was kind of my point. We spin beneath the moon every 24 (or so) hours (I didn’t think the moon’s orbital velocity to be significant to the example). The moon is in our half of the sky every day. It may have it’s dark side to us or it may be in our daytime sky but it should be (technically) a daily moon.
Yes, by the way, I knew the earth’s center wasn’t the rotational center, center of mass, GM[sub]1[/sub]M[sub]2[/sub]/r[sup]2[/sup], and all that but that, too, wasn’t relevant to the OP.
On the other hand, if you can’t pick a nit on the SDMB, then where can ya?
The moon’s velocity is very significant. The Moon does a meridian crossing in an average of about 24.85 hours. For an observer located at the equator, the moon would be, on average, 12.42 hrs above the horizon and 12.42 hrs below the horizon. For an observer located at a latitude of the same name as the declination of the moon (N or S) the moon would be above the horizon longer and below the horizon shorter (just like the sun in the summer). For an observer located in a latitude of contrary name as the declination of the moon the moon would be below the horizon longer than above (like the sun in winter). But, as the moon moves from N to S declination regularly, it all averages out, just like the sun. BUT the average time of the moon above and below the horizon is 12.42 hrs, not 12 hrs.
