[dperfect]

Does this mean there's a location in space where – on the side of the earth opposite to the sun – the sun's light is focused into a point of extremely high intensity due to the earth's atmosphere acting like a magnifying glass?

If so, that could make for either a very unfortunate surprise (i.e. a spacecraft passing through that point suddenly melting to a crisp) or an interesting source of energy if it could be harnessed.

[mehwoot]

It's not focused on a point, it's focused on an area, and while it will be hot it won't be unbelievably hot. See this explanation which is far better than I can explain- https://what-if.xkcd.com/145/

[cbolton]

I think this what-if has serious issues in its argumentation (though I agree with the conclusion):

The argument about reversibility is kind of a straw man: it answers "why can't you concentrate all light on a single point" while the real question would be "why can't you concentrate light on a smaller surface" (which you can do actually).

Similarly for the conservation of étendue: maybe you can't "swoosh the light rays closer together" but that also doesn't say you can't concentrate beams on a small surface, which might be sufficient to start a fire.

So really it all comes down to the thermodynamic argument, which has its own problem: it only works if you assume that moonlight has the same temperature as the moon. There's nothing in the article that mentions or justifies this assumption. And obviously a mirror can reflect light that is much warmer than itself, so you definitely have to explain why that's not the case with the Moon (e.g. its albedo and heat dissipation are too low).

(However I love the drawing of the encircling sun, it's great to make the point that no matter how much light you concentrate, it won't heat the body warmer than the light's temperature).

[sandworm101]

It's a great explanation, but a "spherical chickens in a vacuum" textbook explanation. Add in realworld atmospheric effects (diffusion) and non-lens things like the internalized reflections of fiber optics and the infinite reversability of optical systems breaks down. Then look to how easy it is to start a "fire" in some substances and the moonlight-to-fire concept becomes less difficult. Greenhouse effects also throw a wrench at reversability.

[jahnu]

That's brilliant, thanks! I never would have guessed. It goes completely against my intuition.

[bartislartfast]

Does that mean you could use light from Mercury to start a fire?

[Out_of_Characte]

Mercury can get up to 430c, so Maybe yes. But you might as well use the sun

[bartislartfast]

coming soon: rooftop mercurial panels

[dvh]

If a YouTube video I saw whether you can use moonlight to start a fire is to be believed, you cannot achieve higher temperature using lens than the source temperature, so your earth lens will achieve same temperature as 5cm lens from dollar store.

[hutzlibu]

But isn't the actual source the sun (6000K) and not the moon as it is acting only as a mirror?

With a mirror you can also burn things if you focus sunlight, but the surface of the mirror stays cool.

[ben_w]

At a guess, the moon is a diffuse rather than a reflective surface, so it's going to be closer to a maximum of the lunar surface temperature than to solar surface temperature.

An atmospheric lens, however, will reach a maximum of somewhat closer to solar surface temperature, though still lower because of scattering and absorption which definitely isn't trivial on this kind of scale.

[PaulHoule]

The moon is pretty dark, reflecting about 12% of the sunlight that falls on it

https://www.universetoday.com/19981/moon-albedo/

If it had an albedo of 0% it would be a black body and be radiating only thermal radiation at its surface temperature; it's not too far away from that but it is reflecting some "black body" radiation from the sun representative of a higher temperature.

[bartislartfast]

a 5cm lens from a dollar store can burn things in the focal point