3 ms·

In that case.... there goes their moonlight claim. https://what-if.xkcd.com/145/ https://what-if.xkcd.com/145/

by splawn 10y ago

teraflop 10y ago
You could definitely use moonlight to heat up an engine, if you have a big enough concentrator. You just can't reach a temperature higher than the surface of the moon itself. Stirling engines are capable of operating with relatively small temperature differences.
- LeifCarrotson 10y ago
  > You just can't reach a temperature higher than the surface of the moon itself. False. You can't reach a temperature hotter than the black body which emitted the light. The moon is a reflector which is as cold as -153 C, not a black body emitter. Try reflecting sunlight with a large mirror cooled by air to 23 C through a magnifying glass onto an object in the shade. You can still light things on fire.
  - caspianm 10y ago
    A better analogy is a piece of paper. Like a mirror, it reflects rather than thermally emits the light, but unlike a mirror, it scatters the reflected light, limiting how much it can be focused.
FiatLuxDave 10y ago
I'm a huge Randall Munroe fan, and so I was hugely disappointed when he posted that "What if", because it is 100% wrong. Moonlight is pretty much partially reflected sunlight. The moon is acting more like a dirty mirror than like a blackbody emitter. This is easily apparent through two simple facts: 1) If the moon was emitting light due to its temperature rather than reflecting light from a higher temperature object, we couldn't see it all. Objects at 120 C don't really emit light in the visible range. 2) The color temperature of moonlight is around ~4000K, about 1000 K lower than sunlight. It's not like we haven't measured this stuff. So, yes, you could start a fire with moonlight. I leave this as an exercise for the Mythbusters-inspired reader. [1] https://en.wikipedia.org/wiki/Color_temperature https://en.wikipedia.org/wiki/Color_temperature [2] http://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature http://physics.stackexchange.com/questions/244922/why-does-m...
- splawn 10y ago
  Im not following your reasoning. Which I admit has more to do with my lack of knowledge than anything else, so don't read that as "I think you are wrong". I did find other places with the question asked... some have similar answers to the what-if... others came to a different conclusion as you did. Either way even if it is on some boundary of what is possible, it seems weird for this company to mention it as a feature considering it would be many many times less effective than using it with sunlight and even then only on a few nights out of the month if that.. EDIT: After giving TFA another look, I see that they do mention using PV and not just steam engines... so I guess you could use moonlight sometimes to charge a PV, then switch it to the steam engine during the day or something.
  - FiatLuxDave 10y ago
    Another way that you could look at it, is as LeifCarrotson said below, that the moon is like a mirror. If RM's idea that the sun's light reflected from the moon could not start a fire was true, what makes the situation different if you are reflecting the sun's light from a cold mirror? Can the light from the mirror not cause a higher temperature than the temperature of the mirror? We know that mirrors (and even lenses made of ice) can be used to create temperatures at their focus high enough to melt them. So, what is it about the moon that is different from a mirror, besides a lower reflectivity? I suspect that RM just got a little lost in the weeds on this one. I remember making a similar mistake in undergrad when I was thinking about a problem with two light sources with an optical filter in between. The key insight I needed to make was that the filter was a physical object with its own temperature. Thinking about things like this is how you develop what physicists call "physical intuition".

In that case.... there goes their moonlight claim. https://what-if.xkcd.com/145/ https://what-if.xkcd.com/145/

by splawn 10y ago

In that case.... there goes their moonlight claim. https://what-if.xkcd.com/145/ https://what-if.xkcd.com/145/

You could definitely use moonlight to heat up an engine, if you have a big enough concentrator. You just can't reach a temperature higher than the surface of the moon itself. Stirling engines are capable of operating with relatively small temperature differences.

LeifCarrotson 10y ago
> You just can't reach a temperature higher than the surface of the moon itself. False. You can't reach a temperature hotter than the black body which emitted the light. The moon is a reflector which is as cold as -153 C, not a black body emitter. Try reflecting sunlight with a large mirror cooled by air to 23 C through a magnifying glass onto an object in the shade. You can still light things on fire.
- caspianm 10y ago
  A better analogy is a piece of paper. Like a mirror, it reflects rather than thermally emits the light, but unlike a mirror, it scatters the reflected light, limiting how much it can be focused.

FiatLuxDave 10y ago

I'm a huge Randall Munroe fan, and so I was hugely disappointed when he posted that "What if", because it is 100% wrong. Moonlight is pretty much partially reflected sunlight. The moon is acting more like a dirty mirror than like a blackbody emitter. This is easily apparent through two simple facts: 1) If the moon was emitting light due to its temperature rather than reflecting light from a higher temperature object, we couldn't see it all. Objects at 120 C don't really emit light in the visible range. 2) The color temperature of moonlight is around ~4000K, about 1000 K lower than sunlight. It's not like we haven't measured this stuff. So, yes, you could start a fire with moonlight. I leave this as an exercise for the Mythbusters-inspired reader. [1] https://en.wikipedia.org/wiki/Color_temperature https://en.wikipedia.org/wiki/Color_temperature [2] http://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature http://physics.stackexchange.com/questions/244922/why-does-m...

splawn 10y ago
Im not following your reasoning. Which I admit has more to do with my lack of knowledge than anything else, so don't read that as "I think you are wrong". I did find other places with the question asked... some have similar answers to the what-if... others came to a different conclusion as you did. Either way even if it is on some boundary of what is possible, it seems weird for this company to mention it as a feature considering it would be many many times less effective than using it with sunlight and even then only on a few nights out of the month if that.. EDIT: After giving TFA another look, I see that they do mention using PV and not just steam engines... so I guess you could use moonlight sometimes to charge a PV, then switch it to the steam engine during the day or something.
- FiatLuxDave 10y ago
  Another way that you could look at it, is as LeifCarrotson said below, that the moon is like a mirror. If RM's idea that the sun's light reflected from the moon could not start a fire was true, what makes the situation different if you are reflecting the sun's light from a cold mirror? Can the light from the mirror not cause a higher temperature than the temperature of the mirror? We know that mirrors (and even lenses made of ice) can be used to create temperatures at their focus high enough to melt them. So, what is it about the moon that is different from a mirror, besides a lower reflectivity? I suspect that RM just got a little lost in the weeds on this one. I remember making a similar mistake in undergrad when I was thinking about a problem with two light sources with an optical filter in between. The key insight I needed to make was that the filter was a physical object with its own temperature. Thinking about things like this is how you develop what physicists call "physical intuition".