4 ms·

I love xkcd, but this is completely wrong. It is well known that the spectral temperature of the moon is about 4000K. See for example : http://www.lumec.com/n

I love xkcd, but this is completely wrong. It is well known that the spectral temperature of the moon is about 4000K. See for example : http://www.lumec.com/newsletter/architect_06-10/the_sun_the_moon.html http://www.lumec.com/newsletter/architect_06-10/the_sun_the_... or https://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature https://physics.stackexchange.com/questions/244922/why-does-... . That is the maximum temperature that you can achieve with light from the moon, no matter how concentrated. 4000 K is plenty hot enough to start a fire.

saagarjha 8y ago
Randall has a response to this argument in the article.
- thatcherc 8y ago
  Does he though? Here's all I see - > "But wait," you might say. "The Moon's light isn't like the Sun's! The Sun is a blackbody—its light output is related to its high temperature. The Moon shines with reflected sunlight, which has a "temperature" of thousands of degrees—that argument doesn't work!" > It turns out it does work, for reasons we'll get to later. But the rest of the article is about etendue, and I don't see how the issue of reflected light is addressed (though possibly the answer is implied with an etendue argument I missed). I'm very curious now - it seems to me that if the Moon was a perfect mirror, you should be able to start a fire with it. Maybe the Moon's low albedo is the reason? Fun fact about the Moon: its albedo (roughly the fraction of incident light it scatters) is about the same as asphalt - not very reflective at all! [0] It just looks bright to us because the Sun is so tremendously bright. [0] - https://www.lcas-astronomy.org/articles/display.php?filename=albedo_effects&category=observing https://www.lcas-astronomy.org/articles/display.php?filename... , also learned this in an astronomy class
deleted 8y ago
[deleted]
dr_orpheus 8y ago
Color temperature is a representation of the shape of the spectrum as compared to a blackbody of the same temperature. It is not necessarily representative of the surface emitting the light. Because the moon absorbs some of the wavelengths of light more than others it shifts the spectrum to represent a blackbody object of a lower temperature. So depending on what wavelengths are absorbed the color temperature can actually be shifted up or down. For another example, the color temperature of standard incandescent bulbs is ~2700K, and this is similar to blackbody radiation. However, LED lights can have the same color temperature, but the surface of the LED is not at 2700K because the light emitted is being created through a different process than blackbody radiaton. That said, I am still wrapping my head around the xkcd explanation because blackbody radiation of the moon at 100K would not be enough but there is also reflected sunlight at a .16 albedo.
ummonk 8y ago
Spectral temperature is irrelevant. It's effective temperature that matters. https://en.wikipedia.org/wiki/Effective_temperature https://en.wikipedia.org/wiki/Effective_temperature

saagarjha 8y ago

Randall has a response to this argument in the article.

thatcherc 8y ago
Does he though? Here's all I see - > "But wait," you might say. "The Moon's light isn't like the Sun's! The Sun is a blackbody—its light output is related to its high temperature. The Moon shines with reflected sunlight, which has a "temperature" of thousands of degrees—that argument doesn't work!" > It turns out it does work, for reasons we'll get to later. But the rest of the article is about etendue, and I don't see how the issue of reflected light is addressed (though possibly the answer is implied with an etendue argument I missed). I'm very curious now - it seems to me that if the Moon was a perfect mirror, you should be able to start a fire with it. Maybe the Moon's low albedo is the reason? Fun fact about the Moon: its albedo (roughly the fraction of incident light it scatters) is about the same as asphalt - not very reflective at all! [0] It just looks bright to us because the Sun is so tremendously bright. [0] - https://www.lcas-astronomy.org/articles/display.php?filename=albedo_effects&category=observing https://www.lcas-astronomy.org/articles/display.php?filename... , also learned this in an astronomy class

deleted 8y ago

[deleted]

dr_orpheus 8y ago

Color temperature is a representation of the shape of the spectrum as compared to a blackbody of the same temperature. It is not necessarily representative of the surface emitting the light. Because the moon absorbs some of the wavelengths of light more than others it shifts the spectrum to represent a blackbody object of a lower temperature. So depending on what wavelengths are absorbed the color temperature can actually be shifted up or down. For another example, the color temperature of standard incandescent bulbs is ~2700K, and this is similar to blackbody radiation. However, LED lights can have the same color temperature, but the surface of the LED is not at 2700K because the light emitted is being created through a different process than blackbody radiaton. That said, I am still wrapping my head around the xkcd explanation because blackbody radiation of the moon at 100K would not be enough but there is also reflected sunlight at a .16 albedo.

ummonk 8y ago

Spectral temperature is irrelevant. It's effective temperature that matters. https://en.wikipedia.org/wiki/Effective_temperature https://en.wikipedia.org/wiki/Effective_temperature