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Thread: Which side of the moon get more light?

  1. #1
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    Which side of the moon get more light?

    The near side of the moon gets all the earthglow but also all the sun-blocking lunar eclipses. Which factor dominates, and as a result, does the near or far side of the moon get more sunlight?

  2. #2
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    I would think the amount the near side loses due to eclipses is negligible, thus all areas (with the possible exception of some polar regions) receive nearly identical amounts of sunlight, 14 days of light then 14 days of darkness.

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    Quote Originally Posted by ronin View Post
    I would think the amount the near side loses due to eclipses is negligible, thus all areas (with the possible exception of some polar regions) receive nearly identical amounts of sunlight, 14 days of light then 14 days of darkness.
    It may be negligible for most practical purposes, but as a mathematical curiosity it is real and can be calculated. Very slight does not mean zero.

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    Lunar eclipses - even partial ones - only occur every few months. And they only last about 4 hours.

    Let's be generous and say that's a total of 6x4=24 hours in a year that the Moon is even partially eclipsed.

    Anytime the Moon is not eclipsed, it is receiving some Earth glow (since the Moon would not be directly behind the Earth).

    So, in a year, the Moon is receiving about ((365x24=) 8760-24) = 8736 hours of Earth glow, versus about 24 hours in a year when it is blocked by the Earth and receives no direct - or reflected - sunlight.

    That's 99.97% of the time spent in Earth glow and 0.03% of the time in darkness.
    Last edited by DaveC426913; 2018-May-21 at 10:50 PM.

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    Quote Originally Posted by DaveC426913 View Post
    Lunar eclipses - even partial ones - only occur every few months. And they only last about 4 hours.

    Let's be generous and say that's a total of 6x4=24 hours in a year that the Moon is even partially eclipsed.

    Anytime the Moon is not eclipsed, it is receiving some Earth glow (since the Moon would not be directly behind the Earth).

    So, in a year, the Moon is receiving about ((365x24=) 8760-24) = 8736 hours of Earth glow, versus about 24 hours in a year when it is blocked by the Earth and receives no direct - or reflected - sunlight.

    That's 99.97% of the time spent in Earth glow and 0.03% of the time in darkness.
    The light from full Earth is only about 1/10,000 that of the Sun, and is much less most of the time. It appears to me that it will be roughly even.

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    The moon is not dark even when it is in complete eclipse. I mean, we can see it, the Blood Moon.

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    Quote Originally Posted by kzb View Post
    The moon is not dark even when it is in complete eclipse. I mean, we can see it, the Blood Moon.
    Right, so you'd also have to account for the light that bends through the Earth's atmosphere and hits the moon. And what about starlight?
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    Quote Originally Posted by VQkr View Post
    The near side of the moon gets all the earthglow but also all the sun-blocking lunar eclipses. Which factor dominates, and as a result, does the near or far side of the moon get more sunlight?
    Eclipses dominate, to the extent that Earthshine is negligible in comparison. So the "dark side" gets more sunlight.

    One way to see this is, instead of having a single Moon going around, imagine a chain of Moons that completely surround Earth at one time. The Moons are close enough together to conjure in our minds a continuous ring, but not so close that we need to worry about how one Moon occults another, we ignore that. Your question then must have the same answer for that case as well, because it just folds it all into one time. So the question is now, does placing the Earth in the center of that circle increase or decrease the light received by the ring of Moons? If it increases it, the Earthglow wins, if it decreases it, the eclipses win.

    What this device shows is that the size of the Earth is important. The amount of light removed from the ring of Moons depends on the diameter of the Earth, because that diameter eclipses the ring of Moons. But the amount of light added to the ring of Moons depends on the surface area of the Earth, which scales with the square of the diameter. So if we imagine shrinking the diameter, the Earthglow is penalized more heavily than the eclipse effect.

    To see that in the net, the Earth must reduce the light the Moon gets, take the limit as the Earth fills the entire orbit of the ring of Moons. Then the Earth occults half that orbital ring, so eclipses reduce the Earth-facing insolation to the factor 1/2 (ignoring minor effects like the angular size of the Sun or atmospheric refraction). Meanwhile, the other half of the orbit finds the Earth-facing insolation increased by the giant Earth filling the sky. If the Earth scattered back all of the Sunlight that reaches it, and did so isotropically over the sky, this would take all the light removed by eclipses from the back side of the Moon ring and give it to the Earth-facing sides of the Moons on the front side of the ring. So the net result of the Earth's presence would largely cancel. But the Earth has a low scattering albedo-- it takes something like 90% of the light that hits it and absorbs it, turning it into infrared light. Hence, if we are only counting visible light, the net effect of the Earth is to reduce the light hitting the Earth-facing side. If we shrink the Earth back down to its correct side, this penalizes the scattered light much moreso than it does the eclipsed light, because of the above argument about scaling with diameter.

    So there are two separate reasons why the eclipse effect dominates by a huge factor (though it is a tiny effect). One is that the Earthglow is mostly infrared rather than visible, and most likely you are only counting the visible because you are talking about "sunlight" (rather than reprocessed infrared). The second is that the Earth is way smaller than the radius of the orbit of the Moon.
    Last edited by Ken G; 2018-May-22 at 02:49 PM.

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    I’m mildly surprised that no one has said “the outside”

    Less frivolously, since the Earth is between the nearside of the Moon and the Sun when the nearside is in sunlight and they Moon is between the Earth and Sun when the farside is illuminated, I think the farside gets greater amounts of sunlight.

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  10. #10
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    Interestingly, the argument also works for the Earth, though it might seem even more surprising there. Start with a Moon that is the size of its distance from Earth, and move it around Earth. Eclipses would be half the time, the other half the Moon would fill the sky and be almost as bright as the Sun, except reduced significantly by the low scattering albedo. So even then, the eclipses would dominate, and Earth would get less sunlight due to the Moon. Now shrink the Moon down, and again the eclipses are less affected than the moonlight, until the Moon is the size of Earth (after which any further shrinking affects both equally). So this shows that the amount of moonlight we receive over any long period of time is way less than the amount of sunlight we lose from solar eclipses. That does seem surprising-- it is very common to see better by moonlight, but I would never have witnessed a total solar eclipse if I hadn't traveled to one. Still, that little bit of moonlight we get when it is otherwise dark turns out to pale in comparison to the lost light of eclipses, both partial and full, the latter of which can actually drop the temperature measurably.
    Last edited by Ken G; 2018-May-22 at 03:26 PM.

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