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Author Topic: Does a full moon reflect the sunís heat as well as itís light?  (Read 13227 times)

Offline John Chapman

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A full moon can reflect quite a bit of light. As a multi-drop delivery driver who used to work nights, I know that you can quite easily read a map under a moon that is still many days away from being full. I have no idea if anyone here can put a figure on it but I would guess that strong moonlight is at least 5% as strong as sunlight. Maybe 10!

So does the moon also reflect a significant amount of the Sunís heat? Apart from reflection, and even in a sliver of a moon, does the moon ever absorb heat elsewhere during the day and radiate some of it to us at night when the sun is no longer shining on it?
 


 

Offline techmind

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Moonlight is waaaaay dimmer than full sunlight, although it can be deceptive because our eyes respond more approximately logarithmically.

A simple geometric consideration (the moon is illuminated by much the same strength of sun as the earth, and might be perhaps 50-60% reflective), yet the earth is a small fraction of the region of space illuminated by the moon... to convince you of that.


For some figures, I refer John to http://en.wikipedia.org/wiki/Lux
(Lux is a linear scale, by the way)

According to that, full-moon illumination is 0.25 to 1 lux, while full sunlight is of the order 100000 lux.


You're not going to get any significant energy from a solar photovoltaic device exposed to moonlight, nor will you get "moonburn" if you lie out in the moonlight for too long. ;D

 

Offline John Chapman

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Wow. That's really surprising. Very interesting chart not least because the perception is completely different. At least it is for me.

The point you made about only a very small proportion of the light which is reflected from the moon lands on the Earth is something that hadn't previously occured to me.

Thank you techmind. Informative as always.
 
 

Offline LeeE

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The magnitude (visual brightness) of the Sun is around -26, compared with the full Moon at about -12.  However, this isn't a linear scale and the Sun is actually nearly 450000 times brighter.

A lot of it is to do with the way the irises in our eyes open wider when it's dark, to allow more light in, and contract when it's bright to reduce the amount of light reaching the retina.

Heat is really just a measure of how quickly the molecules in a medium are moving and as there's a lot of near vacuum between us and the Sun we don't really get direct heat from the Sun.  What we do get though is Infra-Red radiation, which is associated with heat.  IR radiation will certainly be reflected by the moon.
 

Offline John Chapman

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Thanks LeeE

Yes, your comparison corresponds roughly with techmind's 0.25 lux for the moon and 100,000 lux for the sun. It's amazing how well the human eye can adapt to low light conditions. I would never have imagined there was anything like that extent of difference.


IR radiation will certainly be reflected by the moon.


Presumably the amount of heat reflected is also restricted to the same one 450,000th of sunlight?
 
 

lyner

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does the moon ever absorb heat elsewhere during the day and radiate some of it to us at night when the sun is no longer shining on it?

Don't forget, the Moon's day is a month (the length of time it takes to rotate once). That means that the bit that has just passed the terminator has been roasting for 14 days. I should imaging it would take some while to cool down - several days?. I guess a thermograph of the Moon should show a definite blurring of the trailing edge of the illuminated bit, where the shadow starts.
 

Offline lightarrow

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IR radiation will certainly be reflected by the moon.
Presumably the amount of heat reflected is also restricted to the same one 450,000th of sunlight?
Infrared radiation is certainly reflected less than visible radiation, from a powder of that kind, but I woudn't know how much.
 

Offline LeeE

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Umm... I can't recall ever coming across anything specifically about the Moon's IR reflectivity, but as IR is only just outside the visible light range (unless we're talking about the far IR) I'd expect it to be similar to visible light.  I think SC may have a good point about IR being emitted from areas in shadow.
 

Offline lightarrow

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Umm... I can't recall ever coming across anything specifically about the Moon's IR reflectivity, but as IR is only just outside the visible light range (unless we're talking about the far IR) I'd expect it to be similar to visible light.  I think SC may have a good point about IR being emitted from areas in shadow.
I think IR is reflected less because of two reasons: 1. IR has a longer wavelenght and so penetrates better every small object as a grain of powder; 2. at the surface of a powder's grain, let's say a silicate crystal but it's not essential, the reflectivity increases with frequency because so does the refraction index. However I admit they are not conclusive proves at all, just clues.
 

Offline LeeE

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They're good arguments, but I think the difference may be smaller then you might think; if it comes down to wavelength Vs. particle size then I think that both visible and IR wavelengths are going to be of a similar magnitude when compared with the particle size.

I don't know either, but it would be handy if any astronomers reading this, who have done the experiment, could let us know  :)
 

Offline lightarrow

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They're good arguments, but I think the difference may be smaller then you might think; if it comes down to wavelength Vs. particle size then I think that both visible and IR wavelengths are going to be of a similar magnitude when compared with the particle size.

I don't know either, but it would be handy if any astronomers reading this, who have done the experiment, could let us know  :)
I've found something. In the graph at the end of this documents seems that the Moon's reflectance generally increases with wavelenght:
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1269.pdf
 

Offline LeeE

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Good find.
 

Offline HankRearden

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Heat doesn't reflect.....so no
 

Offline AllenG

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Heat doesn't reflect.....so no

Incorrect.

Infrared light reflects just fine.
« Last Edit: 29/07/2009 08:12:04 by AllenG »
 

Offline LeeE

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Heat is a property of a medium, and a heated medium can generate IR, but IR isn't heat, so heat can't be reflected.
 

Offline AllenG

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Oh wow, yeah, it's been a while.  Sorry.
 

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