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Quick one about moon

  1. Aug 15, 2008 #1
    Hello to all,

    Just a question related to a thought I’ve had for some time now…

    Most astronomy interested people now about the earth-moon relationship including tides, locked rotational periods and stuff like optical illusion about moon size as it moves about in the night sky.

    Well, for me, another moon related particularity (at least that’s what I make of it) is the fact that, during a solar eclipse both moon’s and sun’s diameters, as they appear to us, are identical, the moon perfectly overlapping the sun. This fact has certainly helped to verify relativity predictions, so here’s my question…

    Is this a fortunate physical/natural fact or is it just some optical property I’ve been too lazy to reason out or search for…


    Thank you and best regards,

    VE
     
  2. jcsd
  3. Aug 15, 2008 #2

    russ_watters

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    It is just a happy coincidence, and it won't exist forever. The moon is receeding at 1.5" per year and a hundred million years or so there will be no more total solar eclipses.
     
  4. Aug 15, 2008 #3

    DaveC426913

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    I am not aware of this.
     
  5. Aug 15, 2008 #4

    DaveC426913

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    The coincidence has not gone unnoticed. It's actually the only attraction that draws ET tourists to Earth. (That, and the largest ball of elastic bands in the galaxy, situated somewhere in South Dakota).
     
  6. Aug 15, 2008 #5

    Janus

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    One thing that should be noted is that is is not an "exact" fit. Both the Moon's And Sun's apparent diameters vary due to the eccentricity of both the Moon's and Earth's orbits.

    The Sun's apparent diameter can vary from 31.5 minutes of arc to 32.5 minutes of arc, and the Moon's from 29.8 minutes to 33.41 minutes. As a result, depending on the time of year, and at what point the Moons is in its orbit during the solar eclipse, the moon can be larger, smaller or very close to the size of the Sun.

    During those eclipses when the moon is smaller than the Sun, you won't get a total eclipse, as there will always be a part of the Sun visible. When the disk of the Moon is centered over the disk of the Sun, this will appear as a ring bordering the Moon. This ring is called an annulus, and these types of eclipses are called "annular" eclipses.
     
  7. Aug 15, 2008 #6

    tony873004

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    I saw one of these in 1992 in San Diego. Here's a picture I took:
    [​IMG]

    In addition to total and annular eclipses, there's also hybrids. These eclipses start out annular, but since the part of Earth that experiences the midday eclipse is 4000 miles closer to the sun than the other parts of the world, the moon is now large enough to barely cover the sun, and the eclipse turns total.
     
  8. Aug 16, 2008 #7
    Thank you all for your responses...

    Your quite right Dave, it's the solar eclipse itself and not the apparent similar diameters that helped during the may 29, 1919 eclipse experiment.


    Regards,

    VE
     
  9. Aug 18, 2008 #8
    Also regarding eclipses, during a lunar eclipse, when the moon is in the deepest darkness of the earth's shadow, a star in Pegasis, some 110 light years away, will appear to the naked eye and will be blue. (If you don't believe me, I have photos made with a handheld camera to show you.)
     
  10. Aug 18, 2008 #9

    russ_watters

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    Huh? First, 110 ly isn't terribly far away (or meaningful given the wide variation in star brightnesses). Second, the sky isn't any darker during an eclipse than at night, so the number of stars you see is the same. Third, Pegasus isn't always in the sky druing a lunar eclipse.

    I'd still like to see a good eclipse pic, though...
     
  11. Aug 18, 2008 #10

    tony873004

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    In most eclipses, only the brightest stars are visible. It's not dark like night. It's dark like 30 minutes after sunset. I was in Hawaii for the 1991 eclipse. Although it was partly cloudy, and I couldn't see the sun/moon during totality, there was pleanty of open sky. I didn't see any stars. If I had to walk back to my car at the height of totality, I would not have needed a flashlight. It's my understanding though that viewers in Mexico had a much darker eclipse, as their sky was cloud-free. Still, I doubt it was any darker than an urban sky during a full moon. Lots of light bleeds in from the horizons.
     
  12. Aug 18, 2008 #11

    russ_watters

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    tony, isly_ilwott flipped on us and went to lunar eclipses...
     
  13. Aug 19, 2008 #12
    My apologies. I suppose I should have started another thread.




    True, the sky is not darker, but when the moon goes dim from lack of sunlight, one can see objects in the field of vision surrounding the moon solely because the moon is not blindingly bright compared to them. As the moon comes back into the light, it precludes sighting of the dimmer objects. On that particular night, several of us strained to see the moon through wispy cloud cover that waxed and waned almost predictably. When the clouds were thinnest, the blue dot appeared (to the naked eye) fixed in a position above and to the left of the moon, while another object appeared (not so brightly) to the left and below the moon. I was told by my astronomy-stricken neighbor that the blue dot was a star in Pegasus and the other object was Saturn.

    I'm not sure I would describe the pictures as "good", but I will post them in a new thread when I can convince a certain person to let me into his photobucket album.
     
  14. Aug 19, 2008 #13

    russ_watters

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    Oh, ok - well in any case, all you would have had to do to see that star and Saturn not washed-out by the moon is wait two days so the moon has moved out of the way.
     
  15. Aug 19, 2008 #14
  16. Aug 21, 2008 #15

    HallsofIvy

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    He is referring to the measurement of the bending of light around the sun which had to be done during total eclipse in order measure the light passing close enough to the sun.
     
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