Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Can you use a magnifying glass and moonlight to light a fire? (2016)

what-if.xkcd.com

Post reply on HN
Print view View original

81–90 of 277 posts

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#81

Post by crazygringo »

Wow... separately I had no idea the surface of the moon reaches (and goes above) 100°C... that's hot! Literally boiling hot. Turns out at night it goes down to almost –200°C. That's insane.

Quick searching of how the astronauts survived this, turns out it seems they timed landings to the lunar dawn for an in-between temperature, that the lunar surface doesn't conduct heat well (all dust?), of course there's no atomsphere to conduct heat, and that their boots were extremely well-insulated.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#82

Post by jxcl »

crazygringo wrote: post #81

Wow... separately I had no idea the surface of the moon reaches (and goes above) 100°C... that's hot ! Literally boiling hot. Turns out at night it goes down to almost –200°C. That's insane . Quick searching of how the astronauts survived this, turns out it seems they timed landings to the lunar dawn for an in-between temperature, that the lunar surface doesn't conduct heat well (all dust?), of course there's no atom…

Also, since the moon is tidally locked to the earth, it rotates at the same rate it revolves, which means that sunrise and sunset only happen once a month, which makes dawn a pretty long time.

There's a pretty good song about how the moon's day/night cycle would affect a lunar mining colony: https://www.youtube.com/watch?v=GDPUdUGJpjc

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#83

Post by stcredzero »

Analemma_ wrote: post #79

Oh god, can we please get a real physicist in here? This entire thread is a mess of computer programmers “well actually”ing other computer programmers and everyone being wrong.

It's just like the old USENET days. It's also how Slashdot used to work. Enough of us programmers would spout enough nonsense to make a real expert angry enough to inform the heck out of us.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#84

Post by taneq »

btilly wrote: post #58
nickparker wrote: post #34

Earlier quoted context omitted.

You've found the right question to ask. Your mirror in sunlight works because the reflectivity or albedo of the mirror is very high relative to whatever target you're lighting on fire. In a magical closed system where radiative heat transfer was the only factor, objects of differing reflectivity would eventually reach temperature equilibrium through black body radiation. We aren't interested in closed systems though.…

Really? According to the Stefan-Boltzmann Law, the energy put out in blackbody radiation is proportional to temperature to the 4th power. Therefore something that is 5000 degrees reflected off of an object with albedo 0.12 is putting out 0.12 times the energy it originally did, while something that is 2500 degrees only puts out (1/2)^4 = .0625 times as much. So the "temperature of the Moon's light" should be more tha…

> something that is 5000 degrees reflected off of an object with albedo 0.12 is putting out 0.12 times the energy it originally did, while something that is 2500 degrees only puts out (1/2)^4 = .0625 times as much

I don't understand this bit. I don't think there's any power law involved in reflection, if something has an albedo of 0.12 then it just reflects 0.12 times the incident radiation, doesn't it?

I think I agree with your overall point, though, which is that the moon isn't a black body radiator (well it is but only at a couple of hundred degrees C at most) but is just reflecting the sun's light (and those photons are hot enough to start fires).

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#85

Post by nomel »

Dylan16807 wrote: post #76
jarfil wrote: post #70

Earlier quoted context omitted.

A penny at a temperature high enough to radiate the same amount of energy as the entire surface of the sun, would be at a far higher temperature than the sun.

If you surround the penny with sun with no gaps, all the light hits the penny. If you add more sun, you add gaps, and the amount of light hitting the penny stays constant. It never has more than one penny-surface-area of light hitting it, so it never has to get hotter than the sun.

Thanks, this is what allowed it to click for me.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#86

Post by CydeWeys »

Analemma_ wrote: post #79

Oh god, can we please get a real physicist in here? This entire thread is a mess of computer programmers “well actually”ing other computer programmers and everyone being wrong.

The linked article is correct and the arguments are well known and accepted in the physics community and have been for a long time (much longer than Randall Munroe has been alive). A lot of the counter-arguments/speculation here in the comments is wrong.

Reading this is is equivalent to reading a thread on a physics forum where with people arguing about an article saying that O(n*lgn) is really the best possible runtime complexity for a comparison-based sorting algorithm, and trying to disprove it.

It's also worth pointing out that Randall Munroe is a physicist (to the extent an undergrad degree counts anyway).

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#87

Post by mikeash »

Analemma_ wrote: post #79

Oh god, can we please get a real physicist in here? This entire thread is a mess of computer programmers “well actually”ing other computer programmers and everyone being wrong.

My thoughts exactly. Know the limits of your knowledge, folks. You’re doing the physics equivalent of arguing that you can write a lossless compression algorithm that always reduces the size of the input no matter what it is, or that you can write a comparison-based sort algorithm that’s better than nlogn if you’re sufficiently clever.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#88

Post by crazygringo »

jxcl wrote: post #82
crazygringo wrote: post #81

Wow... separately I had no idea the surface of the moon reaches (and goes above) 100°C... that's hot ! Literally boiling hot. Turns out at night it goes down to almost –200°C. That's insane . Quick searching of how the astronauts survived this, turns out it seems they timed landings to the lunar dawn for an in-between temperature, that the lunar surface doesn't conduct heat well (all dust?), of course there's no atom…

Also, since the moon is tidally locked to the earth, it rotates at the same rate it revolves, which means that sunrise and sunset only happen once a month, which makes dawn a pretty long time. There's a pretty good song about how the moon's day/night cycle would affect a lunar mining colony: https://www.youtube.com/watch?v=GDPUdUGJpjc

Ah, that makes so much more sense then... two weeks of straight unfiltered sunlight is gonna heat things up... and two weeks of total darkness gives it time to freeze, freeze, and freeze some more.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#89

Post by 08-15 »

08-15 wrote: post #43
DenisM wrote: post #14

Thermodynamics argument seems iffy to me. I can cover the entire surface of the earth with solar panels to harvest the moon light and use the combined electricity to melt iron. If this is ok with thermodynamics then so is using lenses. The rest of the argument seems to be that light cannot be optically condensed to a single point as there will always be some dispersion due to diffraction, and the size and shape of th…

> I can cover the entire surface of the earth with solar panels to harvest the moon light Are you sure about that? First off, take an off-the-shelf solar panel, point it at the moon in the middle of the night. You get a grand total of nothing. Okay, it was a cheap panel, that might not generalize to anything. But more importantly, by the argument laid out in the article, your solar panels cannot work in moon light. (…

I'll repeat for the reflexive downvoters:

Solar panels will work in moonlight if and only if you can make fire from moonlight with a magnifying glass.

The answer depends on how good a mirror the moon is. It calls for a real experiment, not a thought experiment. I don't really know which way it will go.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#90

Post by stcredzero »

baddox wrote: post #35
nealabq wrote: post #27

Earlier quoted context omitted.

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…

> 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.

While 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.)

Post reply on HN