Can you always see the Moon from the Earth?

May 24, 2009 8:25 PM

Is there anywhere on Earth, where you can't see the Moon?

Of course, I mean the surface of the Earth!, and when the Moon is visible from other places on Earth

I suppose what I'm wondering is can you always see the Moon from the Poles. If not, is there a latitude above/below which a full Moon might be below the horizon.

BTW: I've no idea if this is a sensible question... I just woke-up with this thought in my head, and couldn't quickly find an answer.

posted by Dub to science & nature (16 answers total)

The full moon is opposite the Sun in the sky. So from the poles, you'd be able to see the full moon during the dark half of the year. But you wouldn't see the full moon during the light half.

(Of course, the full moon isn't exactly opposite the Sun, because otherwise there would be a lunar eclipse every month. But it's a pretty good approximation.)
posted by madcaptenor at 8:36 PM on May 24, 2009

The moon over Antarctica.
posted by bigmusic at 8:37 PM on May 24, 2009 [3 favorites]

A scientist answers the question:

Can you see the moon all day or not at all?

You can see the moon only during half the month, and it is the half when the moon is a crescent. The closer to a quarter moon it is, the closer to the horizon it is. During a full moon, the moon is on the opposite side of the Earth from the Sun, and now the south pole is tilted towards the Sun and away from the Moon. But to prove it is visible, I took this picture from out on the ice shelf.
posted by bigmusic at 8:40 PM on May 24, 2009 [1 favorite]

Other than those days of the month when the moon is obscured by the sun due to their proximity in the sky (ie, a new moon), I'd say you can always see the moon in a 24 hour period from any place on earth given a) a clear sky and b) an unobstructed 360 degree horizon.
posted by TDIpod at 8:40 PM on May 24, 2009

No. At some point in time, you can see the Moon from anywhere on the surface of the Earth. This diagram should make it relatively clear why. (Keep in mind that the Earth-Moon distance is not to scale!)

Even if the Moon's orbit was exactly on the Earth's equator, as seen from the poles of the Earth, the Moon would not be entirely below the horizon -- just intersecting it.
posted by xil at 8:41 PM on May 24, 2009

Bigmusic has a good point. I stand corrected.
posted by TDIpod at 9:36 PM on May 24, 2009

To build on the original question, since it has been more or less answered -- does the moon fully rise up over the horizon when standing on either pole, or does it remain partially sunken?

I suppose one could try to work out the necessary heigh using the diagram xil linked to and a bit of trigonometry (the radius of the moon would also be needed), but perhaps there is a page somewhere that explains this.
posted by hiteleven at 9:38 PM on May 24, 2009

I don't have a mathematical proof or anything but I would think that the above statements would also be true of any 3-dimensionally convex shape in general, as well as the Earth.
posted by XMLicious at 10:31 PM on May 24, 2009

To build on the original question, since it has been more or less answered -- does the moon fully rise up over the horizon when standing on either pole, or does it remain partially sunken?

It depends entirely where the moon is in its orbit. I would imagine some days it does not rise above the horizon at all at a given pole, but then a few days later would rise several degrees. It wouldn't ever be directly overhead at the poles, probably the best you'd get would be 31 degrees (wild ass guess)...
posted by barc0001 at 10:45 PM on May 24, 2009

Would it be possible, say, in a valley between mountains, especially by the poles, that you could stick a camera down there and it would never ever see the moon? That would count, no?
posted by Mach5 at 8:17 AM on May 25, 2009

All the above answers assume a cue-ball Earth. There are lots of places on the Earth where you can never see the moon. The simplest example would be at the base of a pole facing box canyon with sufficiently high walls at any non tropical latitude. The higher the latitude the lower the walls or wider the canyon can be.
posted by Mitheral at 9:09 AM on May 25, 2009

All the above answers assume a cue-ball Earth. There are lots of places on the Earth where you can never see the moon. The simplest example would be at the base of a pole facing box canyon with sufficiently high walls at any non tropical latitude. The higher the latitude the lower the walls or wider the canyon can be.

I think it's safe to say that this question assumes that one is standing as sea level, with no natural or man-made obstructions in the way. By your logic, one could climb into a cardboard box anywhere on the Earth and not see the moon.
posted by hiteleven at 9:45 AM on May 25, 2009 [1 favorite]

Other than those days of the month when the moon is obscured by the sun due to their proximity in the sky (ie, a new moon), I'd say you can always see the moon in a 24 hour period from any place on earth given a) a clear sky and b) an unobstructed 360 degree horizon.

This is incorrect. Keep in mind that the moon's orbit around the earth takes about 27 days. It appears to move around the earth in about 25 hours only because of the earth's rotation. Secondly, keep in mind that the moon does not orbit in the earth's equitorial plane: the plane of the moon's orbit is angled about 5° from the ecliptic, which itself is 23° from the earth's equitorial plane. Since the plane of the moon's orbit precesses with an 18.6-year cycle, that means the moon's orbit is inclined at an angle between 18° and 28° from the earth's equitorial plane, varying on an 18.6-year cycle. Depending on where we are in that cycle, points within 18-28 degrees of the poles will not be able to see the moon for at least a 24-hour period within each lunar month. However, it can be said that every point on earth (assuming flat horizon, no obstructions, clear sky, etc.) can see the moon some point within each lunar siderial month, approx. 27.3 days. (Just like every point on earth can see the sun at some time every year, although there are points near the poles which do not see it every day.)

No need to take my word for it alone, though: for an example, try out the US Naval Observatory's "Complete Sun and Moon Data for One Day" page. Try out Barrow, Alaska (71°N) on January 20, 2009, and you'll get "No Moon phenomena occur" with the explanation "Missing Moon phenomena indicate Moon below horizon for extended period of time." In fact, if you vary the dates around that at Barrow a bit, you'll find the moon was below the horizon from 8:17 a.m. (local time) on January 18, 2009, until 12:11 p.m. on January 26—out of sight for more than eight days.
posted by DevilsAdvocate at 7:31 AM on May 26, 2009

Thanks, all of you!
posted by Dub at 8:59 AM on May 26, 2009

Related: Apparently, Phobos cannot be seen from Mars's polar regions.
posted by Chrysostom at 12:47 PM on May 29, 2009 [1 favorite]

This should be your next local moon-set.
posted by hAndrew at 10:42 PM on May 30, 2009 [1 favorite]