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B The way the moon 'behaves', coincidence or 'something more'?

  1. Apr 18, 2016 #1
    Even though the moon is much smaller than the Sun, the ratios between their distances to Earth is such that they look the same size in the sky. Further, we have a tidally locked moon with life to observe it all.

    Now, I'll admit this does raise some superstition in me, especially since religions described the moon as the "Night's Sun" that went over our skies... Nonetheless, this relationship quite amazing.
    Last edited: Apr 18, 2016
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  3. Apr 18, 2016 #2


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    The moon is tidally locked. There's nothing even remotely magical or amazing about it.
  4. Apr 18, 2016 #3
    Oh I see, I didn't know that. I read somewhere that it was a "rare coincidence" and didn't bother researching any further.
  5. Apr 18, 2016 #4
    As for 'they look the same size', this is only approximate.
    Depending on the details of where the Earth is in orbit around the Sun, and the Moon around Earth this can vary.
    So sometimes it's possible for the Moon to eclipse the Sun, but there is no 'total' eclipse.

    Gradually the Moon is becoming more distant from Earth, so in the future this kind of 'annular' eclipse will be the usual case. Annular_eclipse_-ring_of_fire-.jpg
    Last edited: Apr 18, 2016
  6. Apr 18, 2016 #5


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    Eclipses are a phenomenon of present times. In the remote past, the moon was significantly closer to earth (not that any organisms were present to observe eclipses), and the moon is currently moving away from earth, so that in the distant future, what we call eclipses will not longer occur with the same totality.

  7. Apr 18, 2016 #6


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    Well, I suppose if a pop-science writer wanted to stretch the concept a bit, it's not too "normal" to have a situation where you have a tidally locked moon AND have that moon the exact right distance to cause full solar eclipses AND have sentient beings around to observe it. As has been pointed out, that has certainly not always been the case here on Earth.
  8. Apr 18, 2016 #7
    Yes I agree, but I meant that the website just said that the moon's rotation around the earth and itself was a mystical coincidence. Now I see that basically over time tidal friction caused it to slow down into a synchronous orbit (tidal locking).
  9. Apr 18, 2016 #8
    There's nothing mystical about co-incidence, (in this case the apparent similar size of the Sun and Moon)
  10. Apr 18, 2016 #9


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    Well, there's science.

    Then there's pop science, wherein there is usually a grain of truth hidden amidst stupidity

    Then there's trash science, which is apparently what you stumbled across.
  11. Apr 18, 2016 #10
    and then there is stuff that is not science at all, but is pure gobbledegook.
  12. Apr 18, 2016 #11

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    98% of regular moons are tidally locked.
    58% of regular moons can completely occult the sun.
    What is so unusual here?
  13. Apr 19, 2016 #12
    Yeah, but only one fits so nicely. Titan could easily occult the distant sun, but that's because it would completely dominate the sky in comparison, and the further you get out, the easier it is to cover up the sun. I'll bet tiny little Nix out there in the Pluto system can do it.

    It's a coincidence that it so nearly fits, it's tidally locked, and that there is life to observe it, AND they both dominate the sky.
  14. Apr 19, 2016 #13
    thank you.
  15. Apr 19, 2016 #14


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    It depends on what you consider as "nearly fits", The moon's angular size ranges from being nearly 10% smaller to 8% larger than the Sun.
    Assuming a constant rate of recession for the Moon, it will be ~800 million years before it recedes far enough that's its greatest angular size will be smaller than the the Sun's least angular size ( assuming no changes in Earth's solar orbit.) and just annular eclipses will be possible. Prior to ~960 million years ago, the Moon was always larger than the Sun. So this gives a range of ~1.76 billion years in which the Moon and Sun could be of exactly the same angular size during an eclipse, out of the 5.3 billion year age of the Earth at the time total eclipses no longer happen. That's 1/3 of that time span.
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