Why does the moon rotate at the same rate that it revolves around the earth?

I have just discovered that the moon rotates on its axis once every 27.3 days and it revolves around the earth once every 27.3 days as well. Not only this but they are in the same direction meaning that the same surface of the moon is facing the earth at all times.

Is it just me or does this seem like an astronomic coincidence!?

Or is there a reason for why they ended up happening at the same time?


7 Answers

  • 9 years ago
    Best Answer

    There's a bit more to it than simple "tidal lock". The Moon is believed to have been formed by an early collision (30 - 50 million years after Earth's formation over 4.5 billion years ago) with another smaller planetesimal dubbed Theia, if you're into names. The glancing blow absorbed the iron core of the smaller Theia and sent a large chunk of surface debris into space in what became an arc of accreting mass. This arc of mass was the genesis of why the Moon ended up with a rotation equal to the revolution around the Earth. It also explains rather nicely why the Earth has a majority molten iron core and the Moon does not have much of an iron core.

    All this talk about the ocean tides comes much later in the scheme of things, and has much more to do with why the Moon is receding from the Earth at the rate of about one and a half inches per year while the earth's rotation is slowing by about 7 millionths of a second per year.

    The Earth's surface temperature was too hot to allow water condensation until around 3.8 billion years ago. Somewhere between that time and 2.5 billion years the oceans began forming. So ocean tides don't play a factor until nearly 2 billion years after the Moon's formation.

    BTW, if you get the free download "Virtual Moon Atlas", the software can demonstrate for you a phenomenon called "libration" in which the Moon actually "rocks" or "wobbles" ever-so slightly, revealing about 59% of its surface to Earth over time. To the casual observer, it's not noticeable, but it does happen. This wobble is believed to be a "ringing" remnant of the collision and an indication that the Moon's center of mass isn't exactly in the center of the Moon.

    Source(s): Louisville Astronomical Society http://www.scientificpsychic.com/etc/timeline/time...
  • 9 years ago

    This is not a coincidence. It is actually a natural outcome of having a smaller body orbit a larger one. In fact, every single moon that orbits a planet in our solar system does exactly the same thing as our moon. This concept is known as tidal locking (by that I just mean the process that causes a moon to rotate in the same time it takes to orbit) and this type of rotation is known as a synchronous rotation.

    To understand how tidal locking occurs, the Wikipedia article has a good description.


    I can try to summarize it more simply though. Let's talk about the earth and the moon. As you may know, the earth has tides because of the pull by the moon. The oceans swell up where the moon is overhead, as well as on the opposite side of the planet. Conversely, the moon also has tides created by the earth. The rocky material itself bulges slightly due to earths gravity. Now that bulge has to always face the earth. So if the moon rotates such that different parts of the moon face the earth, the bulge will travel around the moon and either slow it down or speed it up to the point that the same side always faces the earth. This way the bulge doesn't have to move at all.

    Source(s): Astrophysics Major
  • 9 years ago

    It isn't really that much of a coincidence.

    Nearly EVERY moon in the solar system of comparable size to the Moon of Earth is tidally locked to its own host planet, and has the same phenomena of rotating at the same rate that it orbits its primary.

    It is a connection of the tidal bulge being created by the non-uniform gravitational field of the host planet squishing and squashing on the moon (not necessarily the Earth's moon). This bulges its surface toward and away from the host planet, and that bulge is always tidally torqued back to the condition of it being aligned with the line between the bodies.

    More and more tidal distortion of the moon ultimately converts the original rotational kinetic energy into thermal energy. Io has extreme volcanism because it is an inner moon of the giant Jupiter, and experiences an extreme tidal distortion and heating due to this.

    This happens to Earth due to the tidal distortion from the moon, but to a MUCH lesser extent. Earth has its oceans that can slosh around instead of skid to a grinding halt. We definitely notice this when we visit the beach and experience the tides. No it is not God getting in to and out of the bath twice a day. It is the Moon squishing and squashing on Earth's oceans as Earth rotates. The ultimate fate of Earth is to someday be tidally locked to the moon, and for the Earth-Moon system to resemble the Pluto-Charon system as a binary planet that is mutually tidally locked (but that won't happen for billions of years, and it is in fact predicted that the sun will have entered a substantially different state, that could possibly engulf the Earth instead).

    There are some more interesting examples where tidal locking isn't successful to cause a complete synchronicity of rotation and orbit. Mercury is the prime example of the next-order-up case of tidal resonance...3:2 spin-orbit resonance. It used to be thought that Mercury was tidally locked to the sun...but we now know (and have known for 4 decades) that that isn't the case. It instead makes exactly 3 rotations for every 2 orbits. If you were an observer on Mercury, you would experience a daylight cycle that lasts for 176 days, which is exactly 2 orbits of Mercury (each themselves lasting 88 days).

  • arslan
    Lv 7
    9 years ago

    The moon is tidally locked to the Earth, and has a 1:1 spin-orbit resonance with the Earth.

    No, this is not a coincidence, tidal locking is a common thing to happen when a relatively massive body orbits very close to a relatively small body, as is the case with the moon and the Earth.

    Similarly, Mercury is also tidally locked with the sun, but its spin-orbit resonance is 3:2

    That means for every 3 times its spins on its axis, it makes 2 revolutions around the sun.

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  • Paul
    Lv 7
    9 years ago

    Yes there is a reason for it. The moon is gravitationally tidal locked to the earth.

    That means there's a trasfer of rotational energy from the earth to the moon which will continue until they are facing each other. The moon is slowing down the rate at which the earth spins on its axis and the earth is speeding up the rate at which the moon orbits the earth.

    check this out for more information.


  • Gary H
    Lv 6
    9 years ago

    No, it's not a coincidence. It was caused by 4 billion years of Earth's tidal effect on the moon.

    By the same token, the moon's tidal effect on Earth is doing the same thing to Earth, but at a much MUCH slower rate due to the relative masses.

  • Anonymous
    9 years ago

    yes, it's a coincidence. It was caused by 8 billion years of Earth's tidal effect on the moon

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