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If the light is being radiated from a black body surface, then you cannot make something hotter than that surface. But you're right, the light from the moon is
by nealabq 8y ago
If the light is being radiated from a black body surface, then you cannot make something hotter than that surface. But you're right, the light from the moon is reflected sun light, plenty hot to start a fire. The moon also radiates like a black body, but virtually all that light has longer wavelengths than the reflected visible light.
You could start a fire from the light of a single star if you had a big enough lens. You could also start a fire with the light from a hand mirror at the distance of the moon if it reflected the sun's light at you. But you'd need a very big lens.
- baddox 8y ago> But you're right, the light from the moon is reflected sun light, plenty hot to start a fire. That's my impression as well. A blackbody has albedo 0. The moon has an albedo of around 0.12. While I suspect you can't start a fire from moonlight in practice, I don't think the arguments in this article are correct.
- stcredzero 8y agoWhile I suspect you can't start a fire from moonlight in practice, I don't think the arguments in this article are correct. I guess if you're reading that as some kind of absolutely logical argument. I read stuff like that as an abstraction which just kinda works in the messy real world. Pretty much like how the typical explanation for how a wing works turns out to be an over-simplification. It only partly works that way. Actual wings are complicated, but in the aggregate, they just get enough air molecules to go downward to net out the forces to keep the plane from going downward. It turns out that there are a lot of mechanisms contributing to this all at once. (Which is something else he discusses in that series.)
- baddox 8y agoIt’s presented as an absolute logical argument of the form “you can’t use a lens to make anything hotter than the average surface of the moon.”
- stcredzero 8y agoAnd it "kinda" works in the same way the simplified Bernoulli explanation of the wing works.
- starbeast 8y ago>While I suspect you can't start a fire from moonlight in practice Ivanpah may be able to, I wonder if we can borrow it at night for an experiment - https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility
- alanh 8y agonow this is an idea i like.
- starbeast 8y agoI wonder what the protocol is for 'borrowing' Ivanpah to point it at the moon at night and at what point in the exchange the phrase, 'well, you won't be using it', will finally occur. edit - And by whom. Should probably get it in early, just in case.
- heyjudy 8y agoThunderf00t should totally make a lens to start a fire with starlight.
- AlexCoventry 8y agoI think you'd need a different argument to show that's impossible, since the surfaces of stars are very hot, and their light is unmediated.
- ummonk 8y ago>But you're right, the light from the moon is reflected sun light, plenty hot to start a fire. It's mostly not reflected. It's mostly scattered. If it were specularly reflected, there would be a bright spot on the moon (where you could see the reflection of the sun), and you'd be able to concertate light from that to light a fire.
- smaddox 8y agoWith a sufficiently isolated vacuum flask containing the tender and with an optical opening only large enough to view the moon, you wouldn't even need to concentrate the scattered light from the moon. The temperature of the inside of the flask would eventually reach the approximate blackbody temperature of the reflected light from the Sun (corrected for whatever frequencies are absorbed by the Moon). The only reason the moon's surface doesn't reach such high temperatures is that the moon's surface is not thermally isolated from the rest of the night sky or from the body of the moon itself (which is also not isolated from the night sky).