Somebody suggested that the side of the moon that always faces the earth is as dark as the far side of the moon, but is that really the case? Doesn’t earth-shine make the earth-facing side of the moon overall brighter? Or does the absence of a moon atmosphere negate any possible gain from earth-shine?
Why would the side of the moon that faces earth be as dark as the far side of the moon?
7$\begingroup$ Your question is unclear. Are you asking about the albedo of the moon on the near and far sides, or are you talking about the amount of illumination received on those sides? If the latter, then obviously Earthshine only affects the nearside. The amount of Earthshine received by the moon differs according to the phase of the Earth as seen from the moon. A Full Earth will produce more earthshine than a New Earth from the moon's point of view. $\endgroup$– chasly - supports MonicaFeb 7, 2021 at 15:25
6$\begingroup$ Matter of fact, it's all dark. $\endgroup$– hobbsFeb 8, 2021 at 5:17
2$\begingroup$ This should be indeed made more clearer as for it can be interpreted in different ways leading to different answers equally correct. $\endgroup$– AlchimistaFeb 8, 2021 at 13:33
2$\begingroup$ @Alchimista. thx for the constructive comment. Have tried to make the question clearer. Have learned a lot from the already received answers. $\endgroup$– ConstantthinFeb 9, 2021 at 14:03
$\begingroup$ Question looks good now, has +13/-0 score and has several good answers, so voting to leave open! Next time maybe we can just leave a comment about improvement without the close votes? $\endgroup$– uhohFeb 10, 2021 at 6:20
Somebody suggested that the side of the moon that always faces the earth is as dark as the far side of the moon, but is that really the case?
That is not the case.
I'll look at two different wavelengths: visible and radio.
Doing astronomy from the surface of the Earth when the Moon is full is much more difficult than is doing astronomy from the surface of the Earth when the Moon is new. While much of this difficulty arises due to the Earth's atmosphere, some is the direct result of the presence of a largish, well-lit object that has same angular size as does the Sun. The full Earth as seen from the Moon is, on average, over 40 times brighter than is the full Moon as seen from the Earth. Reading a newspaper on the Moon at night under a full Earth would be a piece of cake compared to reading a newspaper on the Earth at night under a full Moon.
Radio (and microwave)
These are the wavelengths where an observatory on the far side of the Moon would truly shine. The Earth radiates significant amounts of electromagnetic radiation in the radio and microwave wavelengths. There are a few places on the Earth where radio towers and microwave towers are absolutely forbidden, such as the area around the Murchison Radio-Astronomy Observatory in Australia, the area around the Itapeting Radio Observatory in Brazil, and the area around the National Radio Astronomy Observatory (Green Banks, West Virginia) in the US. These Earth-bound radio quiet zones are still plagued with radio and microwave radiation reflected by the atmosphere and by radiation from satellites orbiting the Earth.
Radio telescopes on the far side of the Moon would be shielded from the large amounts of electromagnetic radiation in those wavelengths emitted by the Earth, and possibly even from the electromagnetic radiation in those wavelengths emitted by satellites orbiting the Earth. Moreover, radio telescopes on the far side of the Moon would not be hindered by the Earth's atmosphere. The transparency of the Earth's atmosphere at radio wavelengths ends at the very long wavelengths that would be useful in studying the early universe, and it ends at the upper microwave that would be useful in studying molecular gas clouds.
3$\begingroup$ +1. It would be good if you could include a few references or links in your answer too. $\endgroup$ Feb 7, 2021 at 10:53
2$\begingroup$ The last point about lack of atmosphere of course applies to the whole Moon, not just the far side. And of course there'd be problems in setting up a telescope on the far side of the moon, in that in order to be of any use, it would need to relay signals off of something else in order to send any data back to Earth. The ideal place might be somewhere in the belt between the near and far sides, which periodically has a direct line of sight back to Earth to send data home, and in between is shielded from all of our noise. This would eliminate the need for an orbiter to be the go-between. $\endgroup$ Feb 8, 2021 at 15:16
1$\begingroup$ I am sure that there will be an orbiter and possibly even a space station in lunar orbit, but if you want a land based sight you would choose the "peaks of eternal light" at the north and south poles where there is contact with Earth (and the far side) as much as 90 percent of the time (the south pole has nice craters with water ice). $\endgroup$ Feb 8, 2021 at 20:49
$\begingroup$ @JackR.Woods I think if you're putting up a permanent, unmanned telescope on the Moon, you might not bother with an orbiter. The only reason we needed that in Apollo was because we had to bring the people (and samples) back. Eliminating the orbiter would greatly simplify things; you don't need fuel, rocket, reentry shielding for the return trip, you don't have to maintain stable orbit (also requires some fuel). That's a lot of weight (and thus cash) savings. And I don't think you'd want 90% coms time because of aforementioned radio interference, just enough to send back data periodically. $\endgroup$ Feb 9, 2021 at 14:58
In addition to user-LTK's very good answer, it is also true that the mare which we see on the Near Side are largely absent on the far side. The basaltic flows of the mare are darker than the regolith of other areas, contributing to a higher albedo, brightness, on the Far Side.
The far side of the Moon has sometimes been called the "dark side of the Moon", mostly because we can't see it from Earth. Apart from partial libration, we never see most of the far side of the Moon from Earth, we can see about 59% of it as it wobbles slightly as it orbits a somewhat elliptical orbit.
It's more accurate to say "far side of the Moon" than dark side and actually, the near side or Earth side is darker because it has more maria (spelled like the name, pronounced differently). The near side is darker on average, the far side has more craters and a thicker crust. The reason for this is because the Earth side of the Moon probably cooled more slowly, warmed by the Earth that was also quite hot after the giant impact, and took more time to cool. The far side cooled more quickly and as a result, retained more craters.
3$\begingroup$ Would it be easier or harder on average to read a newspaper on the near or the far side of the moon, though? (This is also a reasonable interpretation of the question of which side is "darker"). $\endgroup$ Feb 7, 2021 at 9:41
3$\begingroup$ By day, both sides are equally lit by the Sun. By night, the "near" side has some light from the Earth that may be enough to read. The far side by night is lit only by stars. $\endgroup$– fraxinusFeb 7, 2021 at 14:00
9$\begingroup$ Update: the light from the full (or even half) Earth on the moon is definitely enough to read. Earth is way bigger than the Moon in the sky and has more favorable albedo. And if it is less than half-Earth, you will get direct sunlight as well. It will probably never be dark enough, even in eclipses. $\endgroup$– fraxinusFeb 7, 2021 at 14:07
4$\begingroup$ "libration" not "libation". (I make this same mistake about half the time I use either word.) $\endgroup$ Feb 7, 2021 at 22:53
4$\begingroup$ Once or twice a year, when I can't quite finish a six-pack. $\endgroup$ Feb 8, 2021 at 12:17
Short answer: Yes and No.
Long answer: The question is a little ambiguous.
What do you mean by dark and light? Do you mean that:
the lunar material is lighter or darker on one side of the Moon, or
the lunar surface is illuminated by more light on one side of the Moon?
Part One: The color of lunar surface material on the two sides of the Moon.
The side of the Moon which faces the Earth has a lot more marias than the far side. Those marias are full of rocks which are much darker than the rocks in the lunar highlands. People with good eyesight can actually see the darker areas on the near side without telescopes.
So on average, the surface material on the near side of the Moon is darker and less reflective than the surface material on the far side of the Moon.
Part Two: Illumination levels on the two sides of the Moon.
The two sides of the Moon receive almost exactly the same amount of illumination from the Sun.
But the near side of the Moon which faces the Earth is sometmes eclipsed by the Earth. Two to five lunar eclipses happen each Earth year. And each time, the near side of the Moon is partially or wholly in the shadow of Earth and the sunlight is partially or wholly cut off from the near side of the Moon for up to almost two hours.
But a lunar eclipse also makes the night on Earth darker, since the Moon in eclipse reflects far less light at Earth. And overall the length of time the Moon illuminates the Earth is many times as long as when the Moon is in eclipse and does not illuminate the Earth, and so the Moon makes a total positive contribution to the the illumination of the Earth.
And similarly, the Earth makes a total positive contribution to the illumination of the Moon. Since Earth appears larger in the sky of the Moon than the Moon appears in the sky of the Earth, Earth at a specific phase illuminates the Moon much more than the Moon at the same phase illuminates the Earth.
And overall the length of time the Earth illuminates the Moon is many times as long as when the Earth is in a solar eclipse and does not illuminate the Moon, and so the Earth makes a total positive contribution to the the illumination of the Moon.
As the Earth rotates, different longitudes of Earth are illuminated by the Moon. But the Moon's rotation is tidally locked, so one side always faces the Earth. Thus the near side of the Moon is the only side which receives the extra illumination of Earthshine, and thus receives more illumination than the far side.
So the near side of the Moon is darker in the sense that it is made of darker materials on average,and the near side of the Moon is also lighter in the sense that it receives more total illumination due to receiving Earthshine.
$\begingroup$ +1. Would it be viable to compare the overall darkness of the two sides by adding together surface darkness and albedo? $\endgroup$ Feb 9, 2021 at 11:33
Dark side of he moon is not labeled dark by virtue of poor illumination. All of the moon is exposed to light from the sun. On the otherhand the Moon rotates at such a rate it's fascia is always exposed to View from Earth. Thus being synchronized with it's revolution it never shows it's "Dark side" to line of sight
The Sun lit far side of the Moon was measured during the Apollo 8 mission and a perpendicular reading of between 160-320 lumens was indicated by the Minolta 1 degree spot meter as they passed over. If the Sun is rated at 130,000 lumens at the Earth/moon distance and the Moon has an albedo of 0.12 then something is wrong somewhere. I have found no measurements for the near side, but NASA tells us that Earthshine on the Moon would provide 76 times more photons than a full moon produces on Earth, so the near side must be brighter than the far side which receives no Earthshine. See"Lighting constraints on lunar surface operations"
$\begingroup$ hmm... interesting answer! "bright" and "dark" can refer to 1) the reflective property of a surface, 2) the quantity of illumination of that surface, and 3) the combination of both together, and can have both absolute and relative interpretations. I think this answer is interesting because it points out that the near side is on average more brightly lit than the far side. Cool! However, I have a hunch that the question really wants to ask about the surface reflectivity or albedo and not about illumination. $\endgroup$– uhohApr 23, 2022 at 19:57
The same question could be asked for the Earth. If the moon is full, shouldn't it lighten up the Earth's surface? You can find your way in the light of a full moon, when walking, outside. But it's pretty dark around you. The sky is very dark blue, which already goes to show how little moonlight is coming in. I think that if you walked on the dark side facing us, the surface looks very dimly blueish (because of the Earth looking blue). But this blue won't be bright enough to make the moon shine. Only if you make a time-exposed photograph its true color will show up.