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Assume, for just one moment, that someone has managed to break the barrier needed to turn Earth's moon into Earth Jr. In other words, give it an Earthlike atmosphere and oceans. This scenario was discussed at length in Alternate History Hub's What if the Moon Was Habitable? However, there is one oversight in that video that remains unmentioned: How would a habitable moon affect luminosity?

Today, the moon is bright, but the sun is 400,000 times brighter. With no water and a very thin atmospheric density of 100 molecules per cubic centimeter, it has a pretty low albedo, or ability to reflect sunlight off the surface and back into space. But if, somehow, we were to give Earth's moon a more Earthlike atmosphere, seas and greeneries, would that make nights on Earth brighter or darker? In either case, by how much?

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    $\begingroup$ Moon albedo: ~7%, Earth albedo:~30%. So the moon would be about 4x brighter? But I think there a number of other effects this would have, hence the short comment rather than an answer. Also, are you looking for a hard-science answer? $\endgroup$
    – IronEagle
    Commented Aug 10 at 5:36
  • $\begingroup$ Where on Earth? New York/Tokyo and the middle of the Sahara would get different results. $\endgroup$
    – L.Dutch ♦
    Commented Aug 10 at 6:18
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    $\begingroup$ @IronEagle Science-based, actually. $\endgroup$
    – JohnWDailey
    Commented Aug 10 at 7:29
  • $\begingroup$ @L.Dutch - can you explain what you mean by your comment ? Do you mean "when observing from where on Earth" .... or ?? $\endgroup$
    – Fattie
    Commented Aug 10 at 22:09
  • $\begingroup$ @Fattie precisely: in a place with high light pollution, a terraformed Moon wouldn't make the night brighter or darker. $\endgroup$
    – L.Dutch ♦
    Commented Aug 11 at 3:08

2 Answers 2

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Google search the first the average albedo of Earth is 0.3, Google search the second the moon has an average albedo of 0.14. Therefore if the moon looked like Earth it will be roughly twice as reflective and twice as bright, on average. Exactly how bright the moon looks from Earth will depend on where the open water ends up and how much cloud cover there is on the earth facing side.

Interestingly because of the way our eyes scale light input the night won't actually look twice as bright, if I recall correctly it'll look about a quarter brighter than what we're used to seeing.

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  • $\begingroup$ Human perception of light/sound is roughly logarithmic, which is why sound volume is measured in decibels, which is a power ratio; 70 decibels is 10x as loud as 60, and 80 is 10x as loud as 70. I'm not sure what the exact analog/base of the exponent is for visible light, but I think it works the same way. $\endgroup$
    – controlgroup
    Commented Aug 10 at 19:21
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    $\begingroup$ @controlgroup Oh okay if visual input also scales logarithmically then twice the light will only look about 10% brighter. $\endgroup$
    – Ash
    Commented Aug 11 at 0:28
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    $\begingroup$ I think this gives the scaling factor: en.wikipedia.org/wiki/Stevens%27s_power_law $\endgroup$
    – IronEagle
    Commented Aug 11 at 2:00
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The other answer seems to address how bright the moon would look if the moon became twice as bright.

(Answer: "if the moon became twice as bright, it would look twice as bright")

However the question seems to be asking:

would that make nights on Earth brighter or darker

Totally set aside terraforming etc.

Let's say for whatever reason the full moon suddenly became, say, twice as bright.

The moon is only apparently as large as the sun.

You've already pointed out that the sun is about 1/2 million (!!!!) times brighter than the moon.

It's immediately, totally, obvious then that the moon becoming 2x (or even, say, 100x) brighter would make utterly no difference in terms of the question

would that make nights on Earth brighter or darker

ie, you cannot read a newspaper by the light of a full moon currently; if it was suddenly 10x as bright (far less 2x as bright) there would obviously be no perceptual difference at all in terms of:

would that make nights on Earth brighter or darker

its out by a factor of tens of thousands -- !!! So, simply, "no difference".

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  • $\begingroup$ I would argue that there is a perceptible difference between a half-moon and a full moon currently. Also, human perception of light, like human perception of sound, is exponential. $\endgroup$
    – IronEagle
    Commented Aug 11 at 1:59
  • $\begingroup$ "a perceptible difference between a half-moon and a full moon currently" For sure - but it's tiny. My point was that the other answer the question would that make nights on Earth brighter or darker (answer: no or only a minuscule amount, as with the current situation when the moon if full/half). The other answer is about how bright the moon looks which is totally different from whether night on Earth is brighter or darker. $\endgroup$
    – Fattie
    Commented Aug 11 at 2:45
  • $\begingroup$ @Fattie the other answer claims that the night on Earth would be about 10-25% brighter (the last para combined with the comment) $\endgroup$
    – justhalf
    Commented Aug 11 at 9:46
  • $\begingroup$ (You're right, thanks, but, that being said) If the light from the full moon doubled I have to say it is extremely dubious that nights on earth would be 10% brighter, but (a) I may be wrong and (b) the whole idea of "percent brighter" is very dubious / meaningless? if the full moon was twice as bright, what F stop difference would you need to take an equivalent photo of a newspaper page? IDK. $\endgroup$
    – Fattie
    Commented Aug 11 at 14:12

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