The Two Faces of the Moon (Synopsis)


You’ve got to wonder about the Moon. I’m not talking about what it looks like or what it has to teach us about Earth, but why the far side of the Moon looks so different from the side that faces us?

Image credit: NASA / JPL - Caltech / LRO.

Image credit: NASA / JPL – Caltech / LRO.

For the past 55 years, we’ve known that the far side looks so very different from the near side, but we weren’t able to figure out why. Well, thanks to Scienceblogs’ very own Steinn Sigurdsson and his collaborators, it looks like we now have the solution!



Come find out how it all works.


  1. #1 Michael Fisher
    July 8, 2014


  2. #2 Sinisa Lazarek
    July 9, 2014

    @ Micael

    will try to point you to an answer for 1&2:

    Age of the moon is thought to be around 4.5 bill. years (wiki). Evidence as i understand comes mostly from geology.

    Tidal drift and distance today can be measured directly with laser ranging and retreat today is 3.8 cm/year. Based on paleonthological and geoological evidence for tides, it was estimated that the retreat during 2.5 bill – 650 miilion years was around 1.3 cm/year. So given the distance and drift today, and average recession in the past, you can roughly quickly calculate how close it was when it formed…. (answer to 2.)

    If looking really roughly… let’s take 2cm/year as an average for total life. 4.5 billion years x 2cm = 9 bill cm or 90.000km

    90.000 km closer than today would be a rough answer to your (1). Visually looking from earth it would be maybe 15-20% bigger, but not 2000% bigger like in some “artistic” renders.

    hope this helps

  3. #3 Michael Russo
    United States
    July 9, 2014

    Ever consider the possibility the skin of the (early) moon may have migrated to the far side due to Centrifugal force?

  4. #4 Hank Roberts
    July 9, 2014

    > the Earth-Moon distance is some forty times larger
    > than the diameter of the Earth?

    Which number have I gotten approximately wrong?
    8000? 240,000?

  5. #5 Michael Kelsey
    SLAC National Accelerator Laboratory
    July 9, 2014

    @Hank #3: Neither one. The average Earth-Moon distance is almost exactly 30 times Earth’s diameter, not 40 times as Ethan wrote.

  6. #6 Sinisa Lazarek
    July 10, 2014

    @ Hank & Michael

    Michael is correct in terms of size, but both numbers @4 are wrong
    - diameter 12.700 km, distance 384.000 km

  7. #7 Sinisa Lazarek
    July 10, 2014

    ahh.. sorry.. just realized.. you were talking in miles… my bad.

  8. #8 Hank Roberts
    July 10, 2014

    Irony is dead ….

  9. #9 Jason Wright
    United States
    July 11, 2014

    Hi, Ethan. Thanks for writing up our work!





  10. #10 David Cohen
    Oakland CA
    July 12, 2014

    I would prefer a more scientific straight-forward style of writing. For example, giving up the answer straight away. It’s just easier to read and comprehend. The unfolding narrative, the questions, the wrong answers, and finally the right answer makes a good bed time story, but in a real world, little time context, sorry. It’s indulgent. Just deliver the goods.

  11. #11 Paul Cooper
    July 12, 2014

    Given the distance I doubt the Earth’s heat could have this effect. There is another recent interesting theory here:

  12. #12 Sinisa Lazarek
    July 13, 2014

    @ Michael Fischer

    actually tonight (Sunday, 13th) is “Super Moon”, the closest Moon is in it’s orbit to us. And is what you would actually see some 2 billion years ago, when moon was some 50.000km closer ;) Pretty cool for science, but a letdown in size, you will hardly notice any difference :)

  13. #13 Kitsilano
    Hagwilget BC, Ca
    July 17, 2014

    Thesis is: Earth provided a kind of blanket on the near side, the other side was exposed like an uncovered behind so it cooled off quicker.

    Good thinking.

    Was the whole Moon molten after the collision with Earth? It would have endured the greatest insult, leaving the other half, less crumpled. Maybe the Earth contact side depicts a scab

    Nice mental images but hard to select a true explanation without more evidence.