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Can I not reflect some sunlight off a mirror, then do the magnifying-glass-to-start-a-fire trick in daytime? Doesn't the mirror stay cool? The mirror is a part

Can I not reflect some sunlight off a mirror, then do the magnifying-glass-to-start-a-fire trick in daytime? Doesn't the mirror stay cool? The mirror is a part of you hybrid reflective/refractive light concentration system. Of course it stays cool, otherwise your system is inefficient. Doesn't the mirror stay cool? Isn't the moon just a (poor) mirror for the sun's light? Precisely. So the maximum temperature that system can achieve is much less that your mirror/magnifying glass system. A perfect black body radiator is just about the poorest mirror there is. The moon's light isn't what you'd get from a perfect black body. However, it's a long, long ways away from being a perfect mirror and probably far closer to being a black body.

saagarjha 8y ago
The moon is about as reflective as asphalt.
- stcredzero 8y ago
  So it's approximately a black body. Even asphalt has tiny bits of specular reflection coming off of it.
baddox 8y ago
> So the maximum temperature that system can achieve is much less that your mirror/magnifying glass system. Why? If the moon were a perfect black body, but with a few small perfect mirrors on the surface to bounce a bit of sunlight towards Earth, couldn't we (in theory) focus that tiny bit of sunlight to heat something up to the temperature of the Sun? There may not be enough solar radiation to make it feasible in practice, but it seems like you need to actually do the math instead of just saying "the moon's surface temperature isn't high enough to start a fire, Q.E.D."
- stcredzero 8y ago
  Why? If the moon were a perfect black body, but with a few small perfect mirrors on the surface to bounce a bit of sunlight towards Earth, couldn't we (in theory) focus that tiny bit of sunlight to heat something up to the temperature of the Sun? To get past the diffraction limits, those "few small perfect mirrors" on the Moon would have to be gargantuan. (If I'm remembering correctly, they'd be the size of the alien space ships in Star Trek that make the Enterprise-D look teeny.) Otherwise, viewed from the surface of the Earth, those mirrors would just constitute a minuscule brightening of the sky in the direction of the Moon.
  - neltnerb 8y ago
    But you could instead cover the whole moon with some kind of dust that reflects 12% of incoming irradiation in average. You're arguing technical problems with a toy example illustrating an unrelated point.
    - stcredzero 8y ago
      Each of those specks of dust modeled as a tiny mirror would be very much subject to diffraction limits. This would basically make what you propose equivalent to an approximate black body that's like asphalt with 12% albedo. Dandy.

saagarjha 8y ago

The moon is about as reflective as asphalt.

stcredzero 8y ago
So it's approximately a black body. Even asphalt has tiny bits of specular reflection coming off of it.

baddox 8y ago

> So the maximum temperature that system can achieve is much less that your mirror/magnifying glass system. Why? If the moon were a perfect black body, but with a few small perfect mirrors on the surface to bounce a bit of sunlight towards Earth, couldn't we (in theory) focus that tiny bit of sunlight to heat something up to the temperature of the Sun? There may not be enough solar radiation to make it feasible in practice, but it seems like you need to actually do the math instead of just saying "the moon's surface temperature isn't high enough to start a fire, Q.E.D."

stcredzero 8y ago
Why? If the moon were a perfect black body, but with a few small perfect mirrors on the surface to bounce a bit of sunlight towards Earth, couldn't we (in theory) focus that tiny bit of sunlight to heat something up to the temperature of the Sun? To get past the diffraction limits, those "few small perfect mirrors" on the Moon would have to be gargantuan. (If I'm remembering correctly, they'd be the size of the alien space ships in Star Trek that make the Enterprise-D look teeny.) Otherwise, viewed from the surface of the Earth, those mirrors would just constitute a minuscule brightening of the sky in the direction of the Moon.
- neltnerb 8y ago
  But you could instead cover the whole moon with some kind of dust that reflects 12% of incoming irradiation in average. You're arguing technical problems with a toy example illustrating an unrelated point.
  - stcredzero 8y ago
    Each of those specks of dust modeled as a tiny mirror would be very much subject to diffraction limits. This would basically make what you propose equivalent to an approximate black body that's like asphalt with 12% albedo. Dandy.