Why would the side of the moon that faces earth be as dark as the far side of the moon?

  • 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?


    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.

    Matter of fact, it's all dark.

    This should be indeed made more clearer as for it can be interpreted in different ways leading to different answers equally correct.

    @Alchimista. thx for the constructive comment. Have tried to make the question clearer. Have learned a lot from the already received answers.

    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?


  • 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.



    • Visible

      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.




    +1. It would be good if you could include a few references or links in your answer too.

    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.

    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).

    @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.

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Content dated before 7/24/2021 11:53 AM

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