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What causes moon to change its shape.

  1. Jul 22, 2006 #1
    Ok this is really funny. This is a really simple question and I used to know it but the more complicated things I learn, the more doubt I have on my foundation. So what is it that causes moon to change its shape?
     
  2. jcsd
  3. Jul 22, 2006 #2

    arildno

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    As the moon goes around the earth, the EARTH blocks some of the sun's rays, so that these do not hit the moon. Hence, the set of reflected rays from the moon that enables us to see the moon changes, depending on where in its orbit around the earth the moon is.
     
  4. Jul 22, 2006 #3

    berkeman

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    Is that what you meant, superweirdo, or were you asking about the physical change to the shape of the moon due to tidal gravitational forces?
     
  5. Jul 22, 2006 #4
  6. Jul 22, 2006 #5

    chroot

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    Er, no. The Earth only blocks sunlight destined for the Moon during a lunar eclipse (i.e., rarely). The Moon is rarely in the Earth's shadow.

    The phases of the Moon are caused by (1) the presence of a directional light source, the Sun, and (2) the motion of the Moon around the Earth.

    A picture from Wikipedia makes it clear:

    [​IMG]

    Think about standing on the surface of the Earth, and looking directly overhead at the Moon. Then consider which parts would appear lit, and which parts would appear dark.

    - Warren
     
  7. Jul 22, 2006 #6

    arildno

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    Oh dear.
    Haven't read any line of astronomy&optics.
    Seems such a boring topic.

    :frown:
     
  8. Jul 22, 2006 #7

    selfAdjoint

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    THink of the three bodies, earth moon and sun. The sun is far enough away that we can think of it's rays as parallel lines falling from "infinity", at least for this first-cut explanation.

    So the earth turns on its axis, causing the sun to appear to rise, move across the sky, and set.

    And the moon goes around the earth once a month, and depending on what day of its orbit is in, it's more or less in line with the sun, as seen from earth. Depending on this, we see more or less of the illuminated hemisphere; the moon has day and night just as earth does, but because the moon doesn't rotate except for its once a month tour around earth, its day and night are each half a month long.

    So when the moon is close to lined up with the sun, its lit side is mostly turned that way and we see mostly dark with just and edge of light, a crescent. When the moon is on the opposite side of earth from the sun, we can see its whole lit side, a full moon. (And note at this time the moon witll rise just about when the sun sets, so if you are watching the sunset, and turn around, you should see the full moon low in the eastern sky. On the other hand, the crescent moons (new and old; in the new the crescent is turned to resemble the bow of a D, and in the old the crescent resembles a C) are close to the sun in the sky, either just ahead of it (new) or just behind it (old).

    The rest of the time we see various shapes as different amounts of the moon's night hemisphere and day hemisphere are presented to our vision.
     
  9. Jul 24, 2006 #8
    Ok what happens when moon is in b/w the sun and the earth so its blocking part of the sun and on the other side of the earth, people have night and no moon?!

    And how would the moon lit at all if earth is b/w sun and moon so blocking sunray's to go to moon, therefore, no reflection?
     
    Last edited: Jul 24, 2006
  10. Jul 24, 2006 #9

    arildno

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    Then you have lunar eclipses.:smile:
     
  11. Jul 24, 2006 #10
    arildno's explanation of why the moon changes its shape was simple however, chroot proved him wrong, can someone again plz explain me why it happens in a simpler lang. than above?

    And how would hte moon lit at all if earth is b/w sun and moon so blocking sunray's to go to moon, therefore, no reflection?
     
  12. Jul 24, 2006 #11

    chroot

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    arildno's explanation was wrong. The phases of the moon have nothing at all to do with shadows.

    Look at the picture I provided again:

    [​IMG]

    Imagine that the moon is at the position at the far left of diagram, labeled "Full." Imagine you're standing on the surface of the earth, near the point labeled "Midnight." Imagine you look directly overhead. The moon will appear directly overhead, and it will appear fully illuminated.

    Now imagine waiting a week, so that the moon has moved from the position at the left of the diagram ("Full") to the position at the bottom of the diagram ("Third Quarter"). (The complete orbit takes roughly a month.)

    Imagine you're standing on the surface of the earth, but this time near the point labeled "6 AM." You're looking directly overhead again, but this time the moon appears only half-illuminated.

    That's all that's happening. The moon, like the earth, is always half-lit by the Sun. Depending upon the moon's position in its orbit, it may look completely lit (when it is nearly opposite the Sun) or completely dark (when it is rougly between the Earth and Sun), or any phase in between. No shadows are involved.

    - Warren
     
  13. Jul 24, 2006 #12
    Hmm, thinking about:

    Absolutely, but this brings us to the question:

    Is a full moon double as bright as a half moon?
    -Yes
    -no, a little brighter
    -no, a lot brighter

    and why?

    There is more to this than meets the eye.
     
  14. Jul 25, 2006 #13
    Wait a sec., if I am at midnight, how is the sun's rays even getting to the full moon, isn't it blocked by earth?(follow th e picture) Which means, no moon! Does that ever happen?

    Ok, wait a sec., I think I got it, when that happens, it is callled total penumbral eclipse where there is no moon, is that right?
     
    Last edited: Jul 25, 2006
  15. Jul 25, 2006 #14

    Janus

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    Yes. What you need to remember is that that Moon's orbit is tilted with respect to the Earth's orbit around the Sun, Thus most of the time during a full moon, the moon passes "above" or "below" the Earth's shadow. It is only during the rare times when Moon is passing through or near enough to one of the points where its orbit crosses the Earth's orbit (called Nodes) at the same time as a new or full moon that we get either solar or lunar eclipses.
     
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