Why is only one side of the Moon visible from Earth?

  • Why do we only ever see the same side of the moon?

    If this is to do with gravity are there any variables which mean we might one day see more than we have before?

    To my understanding (I was told in an Astronomy class) the tidal locking can only occur when the body is solid, i.e. the moon does not have a liquid core now.

  • TildalWave

    TildalWave Correct answer

    9 years ago

    The reason for this is what we call tidal locking:

    Tidal locking (or captured rotation) occurs when the gravitational
    makes one side of an astronomical body always face another,
    an effect known as synchronous rotation. For example, the same side of
    the Earth's Moon always faces the Earth. A tidally locked body takes
    just as long to rotate around its own axis as it does to revolve
    around its partner. This causes one hemisphere constantly to face the
    partner body. Usually, at any given time only the satellite is tidally
    locked around the larger body, but if the difference in mass between
    the two bodies and their physical separation is small, each may be
    tidally locked to the other, as is the case between Pluto and Charon.
    This effect is employed to stabilize some artificial satellites.

             Tidal locking of the Moon with the Earth

        Fig. 1: Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth. (Source: Wikipedia)

    Fig. 1, cont.: Except for libration effects, this results in the Moon keeping the same face turned towards the Earth, as seen in the figure
    on the left. (The Moon is shown in polar view, and is not drawn to
    scale.) If the Moon were not spinning at all, it would alternately
    show its near and far sides to the Earth while moving around our
    planet in orbit, as shown in the figure on the right.

                             Tidal locking of the Moon with the Earth

                             Fig. 2: Lunar librations in latitude and longitude over a period of one month (Source: Wikipedia)

    Libration is manifested as a slow rocking back and forth of the Moon
    as viewed from Earth, permitting an observer to see slightly different
    halves of the surface at different times.

    There are three types of lunar libration:

    • Libration in longitude results from the eccentricity
      of the Moon's orbit around Earth; the Moon's rotation sometimes leads
      and sometimes lags its orbital position.

    • Libration in latitude results
      from a slight inclination between the Moon's axis of rotation and the
      normal to the plane of its orbit around Earth. Its origin is analogous
      to how the seasons arise from Earth's revolution about the Sun.

    • 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.

    All quotes and images from Wikipedia on Tidal locking and Wikipedia on Libration.

    Can you write a bit more than just quoting Wikipedia?

    I might add that the tidal effects of the moon are slowing down the Earth's rotation as well, leading to "leap seconds" and the like. The energy of the lost Earth's angular momentum causes the moons distance to recede. In a few billion years the Earth will also be tidally locked with the much more distant Moon.

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Content dated before 7/24/2021 11:53 AM

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