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Sun/moon coincidence

  1. Jun 4, 2014 #1
    One of the things I never really thought about, but learned recently. Perhaps this is obvious to many. The relative sizes of the sun and the moon, and their relative distances from earth make it possible for the moon to "almost" completely block out the sun during a total eclipse, allowing us to study the sun's corona. Of course if either the ratio of the sun's diameter to the moon's, or the ratio of their distances were different, then this would not happen. I find this a remarkable coincidence!
  2. jcsd
  3. Jun 4, 2014 #2


    Staff: Mentor

  4. Jun 4, 2014 #3
    Yes, it is a remarkable coincidence, but keep in mind that it's only temporary, because the Moon is moving away from the Earth. A billion years ago, the Moon looked larger than the Sun, and a billion years from now, the Moon will look smaller.
  5. Jun 6, 2014 #4

    You may have to change your definition to match your meaning.
    A 'total eclipse' is when the silhouette of the moon entirely blocks the disc of the sun, a annular eclipse is when the silhouette doesn't quite block the sun leaving a sun sized ring. Because the earths distance to both the sun and the moon vary independently from each other, there are times when the moon appears larger in the sky, and times when the sun will appear larger in the sky giving the different type of eclipses.

  6. Jun 6, 2014 #5
    What amazes me is the fact that the moon keeps one 'side' perpetually facing the earth - meaning it exactly performs one revolution every time it circles our planet. Apart from the mind-boggling chance of this, not only happening but being maintained, what possible cause could there be for it holding this position - in spite of asteroid impacts, irregularities in orbit etc?
  7. Jun 6, 2014 #6

    D H

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    That's anything but a coincidence. It's called tidal locking. There are lots of moons that are tidally locked. If you extend the meaning of the concept, Mercury in a sense is tidally locked, too. Mercury is in a 3:2 resonance rather than a 1:1 resonance, but because of the eccentricity of Mercury's orbit, that 3:2 resonance is energetically more favorable than would be a (strictly speaking) 1:1 tidal lock.
  8. Jun 10, 2014 #7
    In addition the moon does not keep precisely the same side facing the Earth all the time. There is a "wobble", which means over time we get to see almost 60% of it from the Earth. See Libration
  9. Jun 12, 2014 #8


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    I never knew that. You learn something new every day here.
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