2 ms·

Thanks for the explanations. The current top comment here seems to agree with my original intuition, though. https://news.ycombinator.com/item?id=18739120 htt

• Dylan16807 8y ago
  As a few people pointed out, the reflection off the moon ruins the étendue, so there's no concentrating it beyond what all the rocks on the surface already experience.
  • AlexCoventry 8y ago
    Thanks again for your help. I think that led me in the right direction. This is the optical argument for maximum concentration given conservation of étendue (the same page has an optical argument for the conservation): https://en.wikipedia.org/wiki/Etendue#Maximum_concentration https://en.wikipedia.org/wiki/Etendue#Maximum_concentration The angle subtended by the moon is approximately 0.54 degrees, or about 0.01 radians, so the maximum concentration factor is about 10,000. The moon provides about 0.1 lux of illumination, so the maximum illumination you can achieve by optics is about 1000 lux. The sun subtends about the same angle, and we receive 30,000-100,000 lux from it, and the maximum illumination you can achieve from concentrating it optically is about 1B lux. I'm willing to believe that you can't light a fire from the moon, if the intensity's a million times lower.