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This is a neat article, but I'd just like to point a common misconception that's made in the "How bright is the moonlight?" section. For a field that studies l

This is a neat article, but I'd just like to point a common misconception that's made in the "How bright is the moonlight?" section. For a field that studies light, astronomers and physicists have made quite a few words to describe how "bright" something is: luminosity, flux, intensity, and brightness [1]. Somewhat confusingly, all of these words have different meanings, and also have infrequently used synonyms, like irradiance. In this case, the article makes the cardinal sin of physicists which is to confuse brightness and intensity with flux. If you put a piece of film on the ground on different nights for perigee and apogee, it would be more exposed on the night the moon was closer (perigee) and less exposed on the night it was further away (apogee). This is because the moons flux at the Earth's surface is lower according to the inverse square law. However, if you have a camera that can resolve the Moon across many pixels or bits of film, you will find that it is exposed to the same level! This is because the intensity and brightness are the same, and this is ultimately due to the fact that the Moon's distance from the Sun stays approximately constant. This is counter-intuitive (and something I've asked intro astronomy classes). Think of it this way, if you put the Moon twice the distance from the Earth, its flux will decrease by a factor of four. But, you will also put all of that flux into a smaller patch of the sky: twice the distance for a circular Moon means the area of the Moon will now by four times smaller. These precisely cancel each other, and the "light density" stays the same. Now if your eye could add up all light falling on your retina (flux), you could tell a difference, but since what you will perceive is the intensity/brightness, they will look the same. [1] See the comical number of units in the table on Wikipedia https://en.wikipedia.org/wiki/Intensity_(physics) https://en.wikipedia.org/wiki/Intensity_(physics)

deleted 10y ago
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maxwelljoslyn 10y ago
Thanks for this informative reply. Today I learned something.
taejo 10y ago
> These precisely cancel each other, and the "light density" stays the same. This is very useful for a photographer! Want to correctly expose some small part of a scene? You can walk right up to that thing, use your light meter (either the one in your camera or a handheld) to set the exposure, and then walk away, recompose, change your lens... and still get the correct exposure. Until I understood this, I didn't really "get" how a hand-held light meter could be useful. There are two caveats to this. Make sure you don't cast a shadow over the subject when you're metering it; and if the subject is emitting light or has specular reflections, then it might not emit equally in all directions. On this latter point, an interesting question is why the moon appears as a disk of constant brightness, but the sun is darker around the edges (BUT DON'T LOOK AT THE SUN WITHOUT PROPER PROTECTIVE EQUIPMENT, obviously)
dr_zoidberg 10y ago
Another counterintuitive result of this effect is that if we could cut in half the distance to a beautiful nebula, we would see it just as dim (though it'd be bigger, of course). And that is why we need some huge, beautiful telescopes to peek into the depths of space.

deleted 10y ago

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maxwelljoslyn 10y ago

Thanks for this informative reply. Today I learned something.

taejo 10y ago

> These precisely cancel each other, and the "light density" stays the same. This is very useful for a photographer! Want to correctly expose some small part of a scene? You can walk right up to that thing, use your light meter (either the one in your camera or a handheld) to set the exposure, and then walk away, recompose, change your lens... and still get the correct exposure. Until I understood this, I didn't really "get" how a hand-held light meter could be useful. There are two caveats to this. Make sure you don't cast a shadow over the subject when you're metering it; and if the subject is emitting light or has specular reflections, then it might not emit equally in all directions. On this latter point, an interesting question is why the moon appears as a disk of constant brightness, but the sun is darker around the edges (BUT DON'T LOOK AT THE SUN WITHOUT PROPER PROTECTIVE EQUIPMENT, obviously)

dr_zoidberg 10y ago

Another counterintuitive result of this effect is that if we could cut in half the distance to a beautiful nebula, we would see it just as dim (though it'd be bigger, of course). And that is why we need some huge, beautiful telescopes to peek into the depths of space.