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B Questions about Lunar Phases

  1. Mar 10, 2016 #1
    My first question is: Shouldn't the lunar phases be thrown off by Earth's orbit of the sun? As in, after 6 months of orbit, since we are on opposite sides of the sun, the phases would be reversed so that the moon being in the same position relative to the Earth as it was before the 6 months passed would have the opposite phase from what its phase was on the other side of the sun.

    Second: Despite how far away the moon is, shouldn't there be a slight difference in what portion of the moon we see at different points on Earth? From the north pole vs. south pole, shouldn't people in the north pole see a slight shift "upward" in what part of the moon is visible to them, while people in the south pole see a shift "downward" in what part of the moon is seen?
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  3. Mar 10, 2016 #2


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  4. Mar 10, 2016 #3

    Ken G

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    The answer is yes on both counts. Your first question points to the difference between a "lunar month" (the one we actually use), and a "sidereal month" (the time it takes for the Moon to revolve once around once relative to the stars). Your second question is true-- different places on Earth see a slightly different part of the Moon, so the appearance of the phases does vary a little as the hours tick by and your location spins with the Earth. But the effect is small, so the tendency is not to pay it much attention-- it's hard enough just to explain the more basic things. You like to look past that, and ask about it-- that's science.
  5. Mar 10, 2016 #4
    Thanks! So has it actually been documented how slightly different parts of the moon are visible at different places on Earth?
  6. Mar 10, 2016 #5


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    Almost certainly. :wink:
  7. Mar 10, 2016 #6


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    not so much about different places on earth, but about oscillations of the moon itself

    have a read of this.....


  8. Mar 10, 2016 #7


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    From that article:

    • Diurnal libration is a small daily oscillation due to the Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's and the Moon's centers, allowing the observer to look first around one side of the Moon and then around the other—because the observer is on the surface of the Earth, not at its center.
    That's pretty much what the OP was asking about.
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