Why does the moon always have a dark side?

Discussion in 'Digital Cameras' started by Richard Henry, Apr 7, 2004.

  1. Because there is only one sun?

    Actually the "dark" side can be lit up quite a bit from reflected Earthshine
    when the angles are right.
    Richard Henry, Apr 7, 2004
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  2. The Moon is lit by the Sun. The Sun is off to one side of the Moon.
    One side lit, one side dark. You would need two suns to light both
    sides at the same time.
    John Popelish, Apr 7, 2004
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  3. Richard Henry

    vhl Guest

    Its because by some fluky chance, the moon rotates at the
    same rate as the earth does so the near side is always
    facing the earth, so even when its in shadow, its still
    the same side. We can see more than half the sphere as
    we can see it from different parts of the earth.
    vhl, Apr 7, 2004
  4. Richard Henry

    Squishua Guest

    Over the course of a lunar day (29.5 earth days) all sides of the moon
    receive sunlight. If you are operating under the assumption that one
    particular side of the moon *never* recieves light, you are mistaken.

    The "dark side" of the moon refers to the "forbidden" side that is
    never viewed directly from earth (or at least has not been viewed
    directly for eons) because the same side of the moon always faces us.



    (remove the capital letters from my e-mail address to contact me)
    Squishua, Apr 7, 2004
  5. Richard Henry

    Obake Guest

    one side of the moon is heavier then the other. the heavy side was/is
    attracted by earth's gravity and over the many many years has stabilized so
    only one 'side' of the moon faces the earth.
    Obake, Apr 7, 2004
  6. Richard Henry

    Nacho Guest

    This is completely off-topic, but it is not true. The point is tidal
    forces. Tidal forces creates intense deformations in the moon when it
    rotated, time ago. This deformations uses energy, obtained from its
    rotation. So it slowly stopped. It could have been stopped in any other

    Best regards.
    Nacho, Apr 7, 2004
  7. Any light seen from the Moon is relected light. Since the only
    significan t source of the light is from the Sun and there is only one
    Sun, then only one side of the Moon is lit up by the Sun.

    Bob Kolker
    Robert J. Kolker, Apr 7, 2004
  8. Richard Henry

    Sam Wormley Guest

    We (posters) should add that only half of the spheroid is illuminated,
    a opposed to more than half, because the illumination source is far
    enough away to approximate a point source.
    Sam Wormley, Apr 7, 2004
  9. It looks to me that all but 2 of the responses were "straight".
    Richard Henry, Apr 7, 2004
  10. Richard Henry

    Uncle Al Guest

    Why does the Earth always have a dark side?
    Uncle Al, Apr 7, 2004
  11. Richard Henry

    Chris Brown Guest

    Actually, he's quite right - although tidal forces are involved, your
    explanation is trivially wrong:
    The problem with this is that the Moon hasn't actually stopped rotating at
    all. It rotates on its axis once every 28 days. This is the same as its
    orbital period, so that we always see the same side. If it had stopped, we'd
    see opposite sides at 14 day intervals.

    The reason the same side faces us is because the orbital and rotational
    periods coincide. The reason they coincide is indeed because tidal forces
    slow the rotation down, but, and this is the important bit, the Moon's
    centre of mass is offset from its centre of rotation, like a bicycle wheel
    that has a weight on one side. Over the N billion years that the Moon has
    been orbiting the Earth, tidal forces have slowed the rotation until it
    reached the optimal rotational speed, which is the speed where the Moon's
    centre of gravity is always as close as possible to the centre of the Earth,
    i.e. always facing down.
    Chris Brown, Apr 7, 2004
  12. Richard Henry

    briggs Guest

    Under the tidal forces associated with the Earth's gravity, the
    moon naturally assumes a slightly elongated shape -- something
    like an egg. Those same tidal forces naturally pull the long axis
    into alignment with the earth.

    When the moon was rotating at a different speed than it revolved around
    the Earth, it was something like a gushy soft-boiled egg -- always a
    little elongated, but with various pieces of the lunar crust continuously
    rotating into and out of the bulges. Since the moon is (or was) neither
    completely rigid nor completely fluid, the viscosity would cause the
    bulges to lag a little. And this would, in turn, cause a tidal torque,
    eventually bringing the moon's rotation rate into lockstep with its
    orbital period about the earth.

    A moon that was merely heavy on one side would continue to spin
    out of synch forever (*), much like an unbalanced frictionless
    bicycle wheel. It takes a gushy satellite to efficiently lock up with
    its partner.

    I've never learned this out of a textbook in so many words, but it
    seems pretty obvious.

    John Briggs

    (*) the rotation of an unbalanced satellite can eventually damp due to
    other effects, but the time frames are much longer than the induced
    tidal drag scenario described above.
    briggs, Apr 7, 2004
  13. Richard Henry

    Ian Stirling Guest

    No, it's not.
    Somewhere around an extra half a degree, or some extra 10Km of illumingation
    round the edge of the illuminated area.

    Anyway, the correct answer is that there has been a dark side of the
    moon since 1973, as that's when the album was released.
    Ian Stirling, Apr 7, 2004
  14. Richard Henry

    Jim Hutton Guest

    But it does NOT have a dark side.
    How anthropocentric!
    Jim Hutton, Apr 7, 2004
  15. Richard Henry

    Jeff Guest

    Damn gravity. ;)
    Jeff, Apr 7, 2004
  16. Richard Henry

    Jeff Guest


    It does in fact orbit the earth. It doesn't rotate WRT to earth, that's
    why we see the same side.

    What does Mars have to do with this?
    Jeff, Apr 7, 2004
  17. Richard Henry

    Peacenik Guest

    For the same reason the earth always has a dark side.
    Peacenik, Apr 7, 2004
  18. For the same reason that the Earth always has a dark side.
    Richard Schumacher, Apr 8, 2004
  19. Richard Henry

    Steve Yeatts Guest

    There is no dark side of the moon, really. Matter of fact, it's all dark.

    Pink Floyd
    Steve Yeatts, Apr 8, 2004
  20. The same reason the Earth always has a dark side. The side facing away from
    the Sun is dark because it is night there duh!

    Llanzlan Klazmon The 15th, Apr 8, 2004
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