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Phases of the moon

  1. Aug 26, 2013 #1


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    amateur question.
    referring to the following link
    Lunar phase - Wikipedia, the free encyclopedia
    (particularly the video on the bottom right)
    I understand what's going on except one thing, why don't the phases of the moon change overnight?
    is it the scale of the video that is probably confusing me? It seems to me that over the course of a night you should be able to see more or less of the moon at different times.
    Maybe you can, it's just that the difference is smaller than I would think because of the distance?
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  3. Aug 26, 2013 #2


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    But they do change. They progress roughly by 1/28th of the full cycle per day.
    The bottom-right animation might not be the best for visualising this. Try the top-right one. This one include a clock and a huge Moon face.
  4. Aug 26, 2013 #3


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    The change is simply to subtle to notice. You'd have to take a picture or make a drawing at the beginning of the night and compare it to another at the end. There is also the problem that unless the moon is full it is only visible for part of the night.
  5. Aug 26, 2013 #4


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    You can easily see the change in 24 hours, particularly when it is close to a new moon. Just look at the moon on successive evenings after a new moon.

    As Drakkith said the moon is only visible for a long period round about full moon, and that is when the change in shape is least visible.

    With a telescope you can see changes in a time span of one or two hours, by tracking the "sunrise" over the edge of craters or lunar mountains. See http://www.weasner.com/co/videos/Copernicus-051111.html
    Last edited: Aug 26, 2013
  6. Aug 27, 2013 #5


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    Click on this this link
    We can't see the far side of the moon since it is tidally locked, but this site explains why we can see about 59% of the moon's surface.
    Perhaps what you are asking is that as the earth rotates, we traverse a distance of one earth diameter and should see a different view of the moon from one monrise to moonset. That is called diurnal libration.
  7. Aug 27, 2013 #6
    Do you mean, "Why don't we see all of the phases in one day?" or "Why don't we see the phase change from night to night?"

    The answer to the first is - because the Earth spins faster than the Moon orbits. We spin around and come back under the Moon, and it has changed in phase some, but only by the amount it has advanced in its orbit - about 1/28th of a circle.

    The answer to the second is - We do. It's noticeable, but you have to be looking for it.
  8. Aug 29, 2013 #7


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    everyone else, too.
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