3 ms·

> Moreover, only half of the Moon sees the light of the Sun. I'm no astronomer but the moon is tidally locked to the earth and not the sun, which means that th

by jgh 9y ago

> Moreover, only half of the Moon sees the light of the Sun. I'm no astronomer but the moon is tidally locked to the earth and not the sun, which means that the moon should have sunrises/sunsets...

drostie 9y ago
Yeah, it does. The real concern is that the moon-day is a full month long and there is no atmosphere transferring heat; any permanent structures on the surface will get very very cold during those two weeks in the dark. If we don't solve this with suitable solar-charged batteries we would need multiple outposts -- say 4 of them dotted around the Moon -- with colonists rotating through the cities to stay in the Sun. But that's also kind of a non-starter because the distance between those cities is something akin to the distance between Denver and New York City -- it's not impossible but it's also a nontrivial drive to be doing every week just to remain in the sunshine.
- WalterBright 9y ago
  Put a couple large mirrors in orbit around the moon.
  - WalterBright 9y ago
    So, why wouldn't a parabolic mirror in orbit that bounces the light down to solar arrays on the moonbase work?
- aaronblohowiak 9y ago
  Why not poles?
  - drostie 9y ago
    I'm glad you asked! So there's an important physics reason why the equator is warm and the North and South poles are cold on this planet, and it has to do with how you point a surface. If you ever find yourself in a dark office with a bright desk lamp (or maybe your home office or kid's bedroom might be suitable) you might even try this experiment: take a flat surface, a notepad or steno pad, and hold it up facing the light source, then slowly tilt it away and look at the color of the surface; you'll notice that it doesn't sharply transition from illuminated at 0 degrees to dark at 90 degrees -- instead it smoothly varies like the cosine of that angle. This mathematical effect is incredibly important, it means that sunlight, during the day, averages out to being half as strong as its maximum over the whole of the Earth's surface. The calculation isn't even particularly difficult: the surface area of a sphere is well-known to be 4πr², half of that or 2πr² is illuminated at any one time, but the actual irradiation that we receive is proportional to the cross-section area, which is just the area of the circle: πr². So if the Sun-directly-overhead light were to be illuminating that entire half of the world, we would get k·2πr² light for some k, but instead we only get k·πr² light for that same k, so it works out to be 1/2 when averaged over the whole surface of the Earth. While I was at university a fellow student asked me to guess the coldest place in the Solar System. I guessed "the middle of the dark side of Mercury." I guessed this for a couple reasons: (1) I knew I needed a rock without an atmosphere since atmospheres sustain convection currents that transmit energy, and (2) I figured since Mercury is so close to the Sun it's probably tidally locked to the Sun and therefore this part probably has not seen a speck of sunlight in millions of years. It turns out that general relativity makes point #2 wrong and my friend gleefully informed me that he was looking at an article (there were many, so let's take [1] as representative) suggesting that it might be in a crater on the South pole of the Moon. This has basically the same reasoning of (1) and (2) above, except substituting the shadow of a crater for the shadow of tidal locking. But Mercury is still in the running -- the only issue is that we might not be looking at the dark side of it, but rather, again, at its poles. [1] https://www.space.com/7311-moon-craters-coldest-place-solar-system.html https://www.space.com/7311-moon-craters-coldest-place-solar-...
    - alansammarone 9y ago
      That's a very interesting point, thank you for the explanation!
    - codesnik 9y ago
      why general relativity makes mercury tidal locking wrong? I've heard it's not 1:1, but I don't see any connection
      - drostie 9y ago
        Actually looking into it a bit more to answer your question, I'm entirely confusing two different things. Mercury is tidally locked 3:2, it rotates 3 times for each 2 times it orbits -- but that is a result of history and not general-relativistic corrections, which are necessary to explain a different value (the precession of its orbit). In fact our best understanding of why Mercury has the 3:2 relationship appears to be (if I'm reading these papers right) "because of historical reasons." That is, now that it's in that state, that state is stable and unlikely to decay to the 2:1 or 1:1 state, but getting into that state is much less clear and potentially requires that in the past, the other planets of the Solar system had nudged Mercury into a more eccentric orbit than it has now. The claim is that the eccentricity of that orbit caused the 3:2 resonance, then in more recent history these perturbations from other planets averaged out more to make Mercury's orbit less eccentric, but it still holds on to the resonance from its past.
    - keenerd 9y ago
      Mercury won't be under any serious consideration for habitation for a long time. It is down at the bottom of the biggest gravity well within a few light years. Too much delta-V to come or go. It is the least explored inner planet for a reason. I think you are drastically overstating the difficulty of heating a lunar polar base. You are surrounded by vacuum, one of the best insulators around. It can be used. Of course a lunar base would probably be underground (for more radiation protection) and the lunar soil would quickly wick heat away. Simple to deal with. Excavate a small cavern and suspend the base inside the cavern. Aluminize the cavern walls if possible. Now the colony has its own thermos bottle and staying warm is easy. Sunlight is just as intense at the lunar poles as the equator. The ground receives less solar energy, but that doesn't mean that less energy is present. In fact there is more solar energy available at some sites since the panels could receive light continuously instead of in 28 day cycles. (Might require putting the panels up on a tower to help reduce libration shadows?)

by jgh 9y ago

> Moreover, only half of the Moon sees the light of the Sun. I'm no astronomer but the moon is tidally locked to the earth and not the sun, which means that the moon should have sunrises/sunsets...

drostie 9y ago

Yeah, it does. The real concern is that the moon-day is a full month long and there is no atmosphere transferring heat; any permanent structures on the surface will get very very cold during those two weeks in the dark. If we don't solve this with suitable solar-charged batteries we would need multiple outposts -- say 4 of them dotted around the Moon -- with colonists rotating through the cities to stay in the Sun. But that's also kind of a non-starter because the distance between those cities is something akin to the distance between Denver and New York City -- it's not impossible but it's also a nontrivial drive to be doing every week just to remain in the sunshine.

WalterBright 9y ago
Put a couple large mirrors in orbit around the moon.
- WalterBright 9y ago
  So, why wouldn't a parabolic mirror in orbit that bounces the light down to solar arrays on the moonbase work?
aaronblohowiak 9y ago
Why not poles?
- drostie 9y ago
  I'm glad you asked! So there's an important physics reason why the equator is warm and the North and South poles are cold on this planet, and it has to do with how you point a surface. If you ever find yourself in a dark office with a bright desk lamp (or maybe your home office or kid's bedroom might be suitable) you might even try this experiment: take a flat surface, a notepad or steno pad, and hold it up facing the light source, then slowly tilt it away and look at the color of the surface; you'll notice that it doesn't sharply transition from illuminated at 0 degrees to dark at 90 degrees -- instead it smoothly varies like the cosine of that angle. This mathematical effect is incredibly important, it means that sunlight, during the day, averages out to being half as strong as its maximum over the whole of the Earth's surface. The calculation isn't even particularly difficult: the surface area of a sphere is well-known to be 4πr², half of that or 2πr² is illuminated at any one time, but the actual irradiation that we receive is proportional to the cross-section area, which is just the area of the circle: πr². So if the Sun-directly-overhead light were to be illuminating that entire half of the world, we would get k·2πr² light for some k, but instead we only get k·πr² light for that same k, so it works out to be 1/2 when averaged over the whole surface of the Earth. While I was at university a fellow student asked me to guess the coldest place in the Solar System. I guessed "the middle of the dark side of Mercury." I guessed this for a couple reasons: (1) I knew I needed a rock without an atmosphere since atmospheres sustain convection currents that transmit energy, and (2) I figured since Mercury is so close to the Sun it's probably tidally locked to the Sun and therefore this part probably has not seen a speck of sunlight in millions of years. It turns out that general relativity makes point #2 wrong and my friend gleefully informed me that he was looking at an article (there were many, so let's take [1] as representative) suggesting that it might be in a crater on the South pole of the Moon. This has basically the same reasoning of (1) and (2) above, except substituting the shadow of a crater for the shadow of tidal locking. But Mercury is still in the running -- the only issue is that we might not be looking at the dark side of it, but rather, again, at its poles. [1] https://www.space.com/7311-moon-craters-coldest-place-solar-system.html https://www.space.com/7311-moon-craters-coldest-place-solar-...
  - alansammarone 9y ago
    That's a very interesting point, thank you for the explanation!
  - codesnik 9y ago
    why general relativity makes mercury tidal locking wrong? I've heard it's not 1:1, but I don't see any connection
    - drostie 9y ago
      Actually looking into it a bit more to answer your question, I'm entirely confusing two different things. Mercury is tidally locked 3:2, it rotates 3 times for each 2 times it orbits -- but that is a result of history and not general-relativistic corrections, which are necessary to explain a different value (the precession of its orbit). In fact our best understanding of why Mercury has the 3:2 relationship appears to be (if I'm reading these papers right) "because of historical reasons." That is, now that it's in that state, that state is stable and unlikely to decay to the 2:1 or 1:1 state, but getting into that state is much less clear and potentially requires that in the past, the other planets of the Solar system had nudged Mercury into a more eccentric orbit than it has now. The claim is that the eccentricity of that orbit caused the 3:2 resonance, then in more recent history these perturbations from other planets averaged out more to make Mercury's orbit less eccentric, but it still holds on to the resonance from its past.
  - keenerd 9y ago
    Mercury won't be under any serious consideration for habitation for a long time. It is down at the bottom of the biggest gravity well within a few light years. Too much delta-V to come or go. It is the least explored inner planet for a reason. I think you are drastically overstating the difficulty of heating a lunar polar base. You are surrounded by vacuum, one of the best insulators around. It can be used. Of course a lunar base would probably be underground (for more radiation protection) and the lunar soil would quickly wick heat away. Simple to deal with. Excavate a small cavern and suspend the base inside the cavern. Aluminize the cavern walls if possible. Now the colony has its own thermos bottle and staying warm is easy. Sunlight is just as intense at the lunar poles as the equator. The ground receives less solar energy, but that doesn't mean that less energy is present. In fact there is more solar energy available at some sites since the panels could receive light continuously instead of in 28 day cycles. (Might require putting the panels up on a tower to help reduce libration shadows?)