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Q & A: starting a fire with moonlight

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Most recent answer: 05/28/2017
theoretically could you build a large enough lens to concentrate moonlight to start a peice of paper on fire. My 16 year old son and I are locked in this debate and we need an expert opinion.
- Glen Hawkins (age 44)
Clinton,Ut USA
That's a very intriguing original question. I think the answer is no.

There are several ways to make the argument. They all rely on a key fact- that the moon scatters the sunlight that hits it into more or less random directions.  That greatly limits the ability of any subsequent optical device (lens, mirror,...) to refocus the light.

Here's one argument. Let's imagine that the moon's surface was hot enough to glow with the same energy intensity as we actually see in its reflected sunlight. (The spectrum would be different, but that's not very relevant for the heating power. The important point is that the thermal radiation also heads off in all directions.) No passive device (such as a lens) could cause this thermal system to spontaneously heat up another system to a temperature higher than the surface temperature. How hot would that surface temperature be? Since the moon is at about the same distance from the sun as us, and  we are at the right temperature to emit about as much radiation as hits us from the sun, that means that the focused moonlight would only heat things up a little above ordinary room temperature. (I've used the law that the absolute temperature only goes as the 1/4 power of the thermal radiation flux to get that even adding the focused moonlight to our ordinary radiant energy wouldn't get things very hot.)

That was fun- took some thought.

Mike W.

(published on 10/21/2011)

Follow-Up #1: flashlight burner

I have a 2 000 000 candelas portable halogen flashlight. I'm able to burn things with it when I focus the light on a dark surface with my magnifying glass. Surely with a large enough magnifying glass I could also do the same with moon light.
- Anonymous
No, this actually makes sense.The filament of your halogen bulb is very hot. With good  clear optics, you should be able to get something else nearly as hot as the filament. The surface of the sun is also very hot, so that's why with good optics you can use sunlight to also burn paper. The surface of the moon is cool, and radiates only as much as a lukewarm object. You can't use its light to burn paper.

If you were to put some frosted glass a few inches in front of your halogen bulb, diffusing its light like the sunlight bouncing off the moon, you'd find you couldn't burn paper with it either.

Mike W.

(published on 10/23/2011)

Follow-Up #2: unfocused light

Say the energy intensity of reflected light is x watts per off the moon's surface S. If you have a magnifier glass with surface also S (as big as moon) located close to the moon then you have x watts going through each of the glass. This means the glass is emitting x watts per each of its Now if you focus this energy to an area s of paper, then according to conservation law of energy, you will have to have S/s * x watts of energy falling on each of paper. For example, if S/s is 1,000,000 then each of paper will receive 1M Watts. So the question simplifies to will 1M watts per burn the paper? If not then s can be reduced (focus sharpened) to reach S/s * x watts that is enough to burn the paper. I say, the answer is yes.
- Mehran (age 61)
Miami, FL

Hi Mehran- It won't work because the light is heading all different directions so it won't focus. The product of the angular range and the spatial range stays constant as you manipulate the beam with lenses or mirrors. So you can take a spread-out beam traveling in nearly complete collimation and focus it down to a small spot, with the beam at the spot coming in from a wide range of directions.  Here you start out with a wide range of directions and positions, so you're out of luck.

Mike W.

(published on 02/25/2012)

Follow-Up #3: focusing moonlight

The light from the sun is also heading in all different directions.. It will focus down to an image of the sun, and likewise the moonlight will focus down to an image of the moon. I think that the answer to this question is in fact yes.
- Kevin (age 50)
Newark, DE, USA
Both the sunlight and moonlight head out in all directions.  With optimum focusing of either one, you can get a radiant energy flux at the focus that's about the same as the radiant energy flux at the surface where it came from. In the case of the sun, that energy flux is what's radiated by a roughly 6000K surface. In the case of the moon the net energy flux is very roughly equivalent to what would be radiated by a 300K surface. That's a big difference. So you can  easily start fires with focused sunlight but not moonlight. Try it!

Mike W.

(published on 03/01/2013)

Follow-Up #4: burning paper with moonlight?

Since it is generally accepted that bright moonlight flux is approx 1/5000000 that of the accepted solar standard used to calculate photovoltaic device efficiencies (1-sun at 1.5 air-mass = 1000 watts/sq. meter), moonlight insolation striking the earth's surface equates to (1000W/sq.meter x 1/500000) = 0.002W/sq.meter. So, if you used a 5.0 sq/meter lens to focus that moonlight down to a 1.0 spot, the resultant flux intensity would be 100 watts/, which is probably enough impinging concentrated radiant energy to ignite a black piece of paper. -KPS
- Kevin S. (age 50)
Newark, DE, USA

The general outline of your calculation makes sense, and the relative size of the image and the lens is also consistent with optical laws. The problem is right at the end, where you accidentally switched meters and cm.

0.002W/sq.meter * 5 sq.meter= 0.01 W.

So in your 1 cm2 image, you've only got 0.01 W/cm2 or 100 W/m2. That's only around 10% of what you get from unfocussed sunlight. It won't burn the paper.

Mike W.

(published on 03/04/2013)

Follow-Up #5: fire from moonlight

I believe you answer to: "theoretically could you build a large enough lens to concentrate moonlight to start a peice of paper on fire. My 16 year old son and I are locked in this debate and we need an expert opinion." is incorrect. The light from the day side of the moon is just reflected Sun light, so it has about the same spectra and temperature just much fainter (because the surface of the moon is not a good mirror and absorbs most of the solar radiation). Thus, if a magnifying glass can start a fire with Sun light then so could a large telescope with moon light. It would further help is the optics of the telescope has a large enough Field of view to actually see a significant portion of the moon and that the focal plane is small (so the area it focus the moon light is smaller. P.S. Please only include my first name if you use this in any way, thank you.
- Ben (age 26)

If the moon were a good mirror, your argument would be right. The Moon, however, is a fuzzy reflector, scrambling the directions that the light goes. As a result, it can't be focused the way a simple beam can. 

Mike W.

(published on 03/18/2014)

Follow-Up #6: Does moonlight cool things?

There are a multitude of videos on youtube right now claiming that moonlight actually makes things colder and magnified moonlight makes things colder still. I would tend to think these videos are hoaxes. Are there any legitimate tests on the temperature of magnified moonlight?
- Keighsee (age 40)
London, ON Canada

I found an amazingly amateurish video of a guy stumbling around in the dark trying to use an infrared thermometer on a wallet. It's been debunked:  . The biggest problem was that the thermometer was looking at a much bigger region than the wallet, so that any differences in its readings were just from the big temperature differences of things (rocks, pool, ...) in the vicinity.

OK, now with a smarter search, I found another. Somebody takes the temperature in two spots on a mat but doesn't do anything to show what the temperatures were without moonlight. We have no idea what's underneath the mat, etc. And another, with another crude thermometer sort of pointing at a bar or maybe off its edge,...

You've pretty much ruined my day.  There seems to be a large cult of people who use digital infrared thermometers, trusting fully that these complicated devices work just as modern physics say they should, who think that incredibly sloppy experiments with them disprove the entire basis of modern physics! They record this garbage with their iphones and post it via computers on the Web, with no sense of irony. Even if the people posting the videos are all hoaxers, they seem to have true-believer followers. Their belief that the moon landings were faked is by far the least crazy of their ideas.   With all their toys, these flat-earthers (not a metaphor, that's who they are) are hundreds of years behind Aristarchus (~250 BC) in understanding our world. He just had his eyes, but they were enough.

Mike W.

p.s. Just to try to get the creepy feeling out of my head, let me describe an experiment by which one might mistakenly conclude that exposure to the moon cooled things. Things cool at night via infrared radiation heading out into cool space. Take a big parabolic reflector with a thermometer at the focus. Point it at a more or less empty part of the sky. The temperature will drop via that infrared cooling. Point it at the moon. It'll still drop, but not quite as much because a little light is coming back from the moon. Now point it at a cloud or tree. It won't drop as much because there's pretty much infrared light coming back. (That's why the ground cools more on a clear cloud-free night.) If you just compared the times when a cloud was in front and when the reflector "saw" the moon, you could think that the moonlight cooled the thermometer. Actually, it's the cloud that helps keep it warm.

(published on 12/31/2015)

Follow-Up #7: starting fire with moonlight

This is of course an old question. Here is my 2nd attempt to say yes, you can burn a paper with moonlight. There is no question that the moonlight right here on earth has energy. Now, say you convert this energy into electricity and run a very tiny motor. Then you connect millions of such motors to rotate a well-lubricated shaft. This assembly creates a sizable torque which you may feed to energize a high-torque high power-output generator. The large current output from this generator can burn a paper. By the way why do you recycle these old questions and what criteria do you use? Because they are so interesting?
- Mehran (age 65)
Arlington Heights, IL

Sure, you could also put the current from a huge array of photocells into a standard voltage converter (chopper plus transformer) and make a high voltage version to drive a standard heater. The questions were not about that, though, but about using big magnifying glasses.

Every now and then we pick a question that seems of general interest and put it into a batch that are randomly recycled by the server.

Mike W.

(published on 02/05/2016)

Follow-Up #8: cool moonlight?

Allow me to help you keep your answer clear" there is absolutely no way that moonlight is cooler than darkness. Therefore there is no experiment possible to discern this, other than beaming far out into the sky.Surely if moonlight is OBVIOUSLY not cooling anything, you could design a bullet-proof SIMPLE experiment to demonstrate that, where the light is diffused (as is sunlight) it's heat is detectable.Or, are you saying that moonlight is so completely perfect as light that there is zero heat given off.Quite being so stupid in officious terms.
- john (age 65)
omaha, ne

I'm not sure I follow your point. Certainly moonlight has energy and it thus warms things up when it's absorbed. Perhaps I got too complicated in trying to imagine experiments where an honest person could fool themselves into thinking that moonlight cooled things. It doesn't.

Mike W.

(published on 03/29/2016)

Follow-Up #9: focussing scattered light?

This is a follow-up to answers to "fire from moonlight" question.Mike W., you keep bringing up the light diffraction. While Moon surface does scatter light, there must evidently be some (reasonably well collimated) light beams hitting the Earth (otherwise we wouldn't be able to see the Moon because the light beams would be missing the Earth and viewer's iris completely).Why would it be impossible - assuming an ideal lens of arbitrary size - to concentrate this collimated light down to a smaller area?
- Blaz (age 36)

You could always pick a subset of the light scattered off the moon that happens to be nearly collimated, and then focus that nearly collimated beam. The point is that since the light is scattered over a broad range of angles, the  collimated light is just a fraction of the initial light. Better collimation means picking a smaller fraction. So that limits the brightness of the focussed spot.

Mike W.

(published on 06/05/2016)

Follow-Up #10: what can science know?

To continue on a debate whether concentrated Moonlight can start a fire or freeze things, here's what I have to say. Shut up and conduct an experiment about it. This guy Mike here is absolutely discouraging people from doing their own research and conducting their own experiments. What happened to the good old principles of peer review and repeating experiments? What Mike does here is representing Science as religion where you just have to blindly believe what you are told under hundreds of assumptions. This is cancer and it sickens me. So the correct answer to the question "Can moonlight start a fire?" is WE DON'T KNOW. I want to see different independent people conducting this experiment and preferably I'd conduct one myself and only then I can come to a reasonable conclusion.
- Erich (age 27)
Tartu, Estonia

It's odd that you think that I'm pushing the view that science knows all the answers. This site has many answers that amount to either "we don't know" or even "nobody knows". Whenever possible, we try to give ideas for home experiments, including in some of the answers above.

We refuse, however, to take the extreme position that science knows nothing at all. Some things are known. Whether focused moonlight can start fires in ordinary materials or freeze anything at all are two of those things. Still, it's a great idea for people to try the experiments themselves. Anyone trying experiments should try to do them with at least a little care, unlike the extremely sloppy videos from Flat Earthers. 

Mike W.

p.s. When I find my old copy of Lewis and Randall's text on chemical thermodynamics I'll include a much more eloquent quote on the philosophy.  

Aha- I couldn't find the book but did find the line: "Let this book be dedicated to the chemists of the newer generation, who will not wish to reject all inference from conjecture or surmise, but who will not care to speculate concerning that which may be surely known."

(published on 07/18/2016)

Follow-Up #11: temperature of focussed moonlight

I think that people are getting very confused between two quite separate issues. One is the law of conservation of energy. It seems quite obvious to most people that even though moonlight is quite weak, the total amount hitting the earth, added together and focused (somehow) should be enough to start a fire. The second issue is with the use of a lens. People generally don't have such an intuitive grasp of optics (including me). Is it not true that the whole argument here hinges on the use of a lens as opposed to some other means of focusing energy. For example, if I covered the earth in solar panels capable of instantaneously converting moonlight to energy, then surely I would be able to start a fire with the resulting energy source. If the moon produces 0.00146 W per m squared, and the moon light hits roughly half the surface of the earth at any one time (250 trillion m squared) then we should get around 365 billion watts of energy.Now the lens argument does usually assume we are talking about classical lenses. What if we used holographic lenses capable of focusing light with a dependence on the incident angle of the light sources?i.e.
- Darren (age 36)
Cambridge, UK

The fundamental issue is not one of optics, and certainly not one involving details of lens design. It's a question of thermodynamics.

Take some light from the Sun, e.g. the light in a region of a cubic km. It's got some energy and some entropy. The maximum compression of that light to a small region will of course conserve the amount of energy and, by the second law of thermodynamics, not decrease the entropy. If your device is any sort of lens or mirror, then the spectrum of frequencies and hence the number of photons won't change. So it's just a question of how much you can focus the energy without the reduction of volume decreasing the net entropy of those photons. The reason you can do a good job of focusing is that the light initially has a very narrow range of directions. A good lens system will trade the increase of entropy from an increase in the directional spread for a decrease from reducing the spatial spread. The focused light can be much more dense, close to the temperature of the Sun's surface.

If you take the same sunlight and let it bounce off the surface of the Moon, the directions it's traveling in get all scrambled up, no longer confined to a narrow range. The entropy of the light increases a lot. The focused light that you could have gotten before has less entropy than this scattered light, so the Second Law of Thermodynamics (entropy never decreases) says that you can't get to that state unless you use some active device that's making entropy somewhere else. But if you had a device like that (e.g. a match) you wouldn't need the moonlight.

Why was the point about not changing the photon spectrum important? As one of our readers pointed out, a device like a photovoltaic cell can convert the energy to forms that are intrinsically higher-entropy (e.g. thermal jiggling of electrons and atoms in a wire) and use that to get a small region very hot without decreasing net entropy.  As long as you're just steering the photons around, however, thermodynamics says you won't get them concentrated enough to start a fire,

Mike W.

(published on 05/28/2017)

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