Part of the Moon Always Lit?

[Ithaka]

A friend claimed last night that there was a small part of the Moon that never saw darkness. His explanation involved it being a high point near the pole. Whilst this sounds vaguely plausible, I think it's highly unlikely and have never heard of such a thing. I've googled but to no avail. Anyone know about this (cites would be good). Thanks people.

[Squink]

Lunar mountain has eternal light (http://news.bbc.co.uk/2/hi/science/nature/3549667.stm)

[Ithaka]

Thanks, Squink, who'd have thought it. I will now go eat humble pie for being wrong.

[LSLGuy]

Don't be too quick to apologize. The article says that MAYBE there's such a place.

They are sure there are places on the Moon with continuous sunlight over the lunar summer (just like on Earth's polar areas) but they are NOT certain the same effect lasts through the lunar winter.

[Don't Call Me Shirley]

One has to be careful with the words "never" or "always." Even if there were such a place, it would not get any sunlight during a lunar eclipse, so it wouldn't "always" be in the sun.

[Acsenray]

While I don't know whether it's true, I don't see why it should be so unlikely. All it takes is a sufficient tilt of the axis. Look at Uranus.

[Kalhoun]

While I don't know whether it's true, I don't see why it should be so unlikely. All it takes is a sufficient tilt of the axis. Look at Uranus.
YOU look at Uranus! I'm certainly not gonna look at Uranus!

[Ethilrist]

However, there is always a part of the moon that's getting light from some star (a lot of them, actually), just not necessarily the closest one.

[The Great Sun Jester]

Well I think we can all agree that the moon (http://www.rathergood.com/moon_song/) is very useful.

[Bytegeist]

While I don't know whether it's true, I don't see why it should be so unlikely. All it takes is a sufficient tilt of the axis. Look at Uranus.

Actually that has the opposite consequence. (Not to mention that the concept of "place" is a little slippery on a gas giant like Uranus, which has no solid surface, but I'm going to ignore that complication.)

Because of Uranus's axial tilt, it has no place that's always lit by the Sun. That will be true of any spherical body that (1) has a non-zero axial tilt and (2) isn't tidally locked to the Sun. There might be an asteroid or two with those properties, but I don't think they hold for any of the planets or moons.

[Freddy the Pig]

They are sure there are places on the Moon with continuous sunlight over the lunar summer (just like on Earth's polar areas) but they are NOT certain the same effect lasts through the lunar winter.I found that aspect of the story to be kind of puzzling. Of course there are places near the pole of the sunward hemisphere that experience continuous light. The only issue, one would think, would be whether the effect persists during winter--which they don't answer.

[L. G. Butts, Ph.D.]

I found that aspect of the story to be kind of puzzling. Of course there are places near the pole of the sunward hemisphere that experience continuous light. The only issue, one would think, would be whether the effect persists during winter--which they don't answer.
Which hemisphere is the sunward one? Since the moon is tidally locked with the earth I would imagine that the "sunward hemisphere" changes over time...

[Freddy the Pig]

By "sunward hemisphere" I mean the northern lunar hemisphere during northern-lunar-hemisphere spring and summer, and the southern lunar hemisphere during southern-lunar-hemisphere spring and summer. The "seasons" change over the course of a year just like on Earth, although this has almost no effect on lunar "weather".

[L. G. Butts, Ph.D.]

By "sunward hemisphere" I mean the northern lunar hemisphere during northern-lunar-hemisphere spring and summer, and the southern lunar hemisphere during southern-lunar-hemisphere spring and summer. The "seasons" change over the course of a year just like on Earth, although this has almost no effect on lunar "weather".
OK, but then the sunward hemisphere does change as a function of time. The lunar northern hemisphere is sunward during the lunar summer and the southern is sunward during the winter.

So the only way a part of a perfect sphere would recieve continual light while orbiting the sun and rotating is if it had 0 axial tilt. Any amount of axial tilt will destroy this case.

For an irregular object like the moon orbiting the sun, a mountain near one of the poles could concievably get continual light but only if the plane perpendicular to the axis of rotation and tangent to the surface at the pole intersects the peak. With the moon it may be more complicated yet as we do not know if the plane of the moons orbit is coplaner with the plane of the earth's orbit. Wouldn't this cause the axis of the moon's rotation to precess relative to the sun?

ps sorry for any grammer and spelling errors, I am pressed for time...

[L. G. Butts, Ph.D.]

For an irregular object like the moon orbiting the sun, a mountain near one of the poles could concievably get continual light but only if the plane perpendicular to the axis of rotation and tangent to the surface at the pole intersects the peak.

Oops, another requirement is that the plane also intersects the sun when the pole is tilted away from the sun.

[Squink]

More Here:
Eternal Sunshine of the Lunar Kind (http://www.nytimes.com/2005/04/19/science/19obse.html?ex=1117339200&en=bc2bc107bad3af47&ei=5070) unlike Earth, the Moon is tilted only slightly - about 1.5 degrees - from its rotational axis. So an area near the poles that is at high enough elevation may even catch sunlight during the winter, when it is tilted away from the Sun.

[Squink]

You can calculate how high a polar peak would have to be to still get sun when it's tilted 1.5 degrees away from the sun.

The formula for distance to the horizon (http://www.rossmack.com/ab/RPG/traveller/AstroHorizonDistance.asp) is d = ( h2 + 2hr )1/2, where h = height, and r = the moon's radius (1,738 km).

A degree on the moon is 30.3 km, 1.5 degrees (see previous post) is 45.5 km.
solve for h:
45.5 = ( h2 + 3476h )1/2
h2 + 3476h - 2070 = 0

Apply the quadratic formula
h = ( -3467 +/- 3465.8 )/2
h = .2/2 = 0.1 km

If I calculated correctly, a polar peak need only be 100 meters tall to get sun year round. It'll still go dark during lunar eclipses.

[L. G. Butts, Ph.D.]

Would the plane of the moons revolution not being co-planar with the plane of the earths revolution effect this?

I would think so. Taking the most extreme case, the axis of the moons revolution lying in the plane of the earth's revolution would make it impossible (Think conservation of angular momentum here) for any part of the moon to have eternal sunlight.

Anyway, is the axis of the moon rotation or revoltution (1.5 degrees apart, I know) around the earth coaxial with the axis of the earth's revolution around the sun? And if not, wouldn't this effect also have to be added in to determine if a location of eternal sunlight exists on the moon (without setting up a camera to watch for years on end as the Nasa guys in Squink's link did)?

Several relavant quotes from Squink's link (bolding mine):

Poring over images from a 10-year-old lunar mission, they have located an area near the Moon's north pole that is fully illuminated, at least in summer.
Because the images were obtained only during what is the lunar summer, it's not known whether they are illuminated in winter as well.

I'm kind've suprised, this should not be too hard to calculate. All you need is an elevation for the mountain/high spot, the size of the two spheres (sun and moon), distance between them, and the relevant rotation/revolution vectors... Take the worst case for the axis of moon rotation pointing away from the sun (which may be affected by the plane of the moons rotation around the earth) and place the mountain on the far side from the sun. Construct the line from the mountain tip to the pole and see if it intersects the sun. If you want to get more complicated you could add in other mountains around you eternal sunshine place and instead of a line use a plane as I mentioned above.

[L. G. Butts, Ph.D.]

You can calculate how high a polar peak would have to be to still get sun when it's tilted 1.5 degrees away from the sun.

The formula for distance to the horizon (http://www.rossmack.com/ab/RPG/traveller/AstroHorizonDistance.asp) is d = ( h2 + 2hr )1/2, where h = height, and r = the moon's radius (1,738 km).

A degree on the moon is 30.3 km, 1.5 degrees (see previous post) is 45.5 km.
solve for h:
45.5 = ( h2 + 3476h )1/2
h2 + 3476h - 2070 = 0

Apply the quadratic formula
h = ( -3467 +/- 3465.8 )/2
h = .2/2 = 0.1 km

If I calculated correctly, a polar peak need only be 100 meters tall to get sun year round. It'll still go dark during lunar eclipses.
Good stuff, but it does not even need to be that tall. You also need to add in the angular extent of the sun from the moons surface. After all you only need to see a little of the sun to be in the sun, not the center.

[Squink]

Good stuff, but it does not even need to be that tall.Sure. The moon isn't perfectly round either, so the calc will be a little off from that too, and any peaks over a hundred meters tall within 45.5 km of the pole could throw shade on a polar peak. Without great maps, the only way to tell for sure is to watch for several years.

[Freddy the Pig]

Would the plane of the moons revolution not being co-planar with the plane of the earths revolution effect this?No. The plane of the Moon's revolution around the Earth is inclined 5.1 degrees to the plane of the Earth's revolution around the Sun (the ecliptic). It doesn't matter. All that matters is the inclination of the Moon's axis of rotation relative to the ecliptic--1.5 degrees.

Now, I suspect that the latter figure changes over long time scales. The Earth's axial tilt varies (over tens of thousands of years) between 21.5 and 24.5 degrees, so the Moon's tilt may vary as well. If so, it may sometimes be greater than 1.5 degrees, and the fact that a mountain is experiencing year-round light today may not mean that it does so at all times. But this is speculation on my part.

[Ithaka]

Golly, this is more complicated than I first thought - thanks for all the replies.

Can I paraphrase and say that the general consensus is that this is perfectly possible but that we don't have enough infomation to give a 100% yes that there is such a spot?

[Mangetout]

Even if there is such a place, sunlight to the moon is occasionally eclipsed by the Earth... Just sayin'.

[Ithaka]

Even if there is such a place, sunlight to the moon is occasionally eclipsed by the Earth... Just sayin'.

Pedant.
;)