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How is it that the sun affects our tides?

  1. Jun 27, 2010 #1
    I've read that along with the moon, the sun also has an effect on our tides. I understand how the moon would have an effect on tides, but I don't quite get why the sun would.

    According to my physics book, when something is in orbit, it's really just in free fall around the thing it's orbiting. Here's what confuses me: if the earth is in orbit around the sun, then isn't the entire earth already being acted on by the sun's gravity? And therefore the oceans wouldn't be affected differently in relation to the rest of the earth, since it's all falling together?

    Think about if you were in an elevator on the top floor of a skyscraper and the cable snapped, sending you into free fall. On the inside of the elevator, since gravity is accelerating everything at the same rate, from your perspective nothing would accelerate towards the floor of the elevator unless acted on by a force different from the earth's gravity. Objects would just seem weightless, and there would be no pull from gravity towards the floor, since in reality the floor would be accelerating along with everything inside. So why is it that when the earth is in free fall around the sun, that the sun would have an additional pull on the earth's oceans, if they too are in free fall along with our planet?

    I'm guessing that it has something to do with the earth being so large in diameter that different parts of it are being pulled on with noticeably more force by the sun's gravity than others (since they're closer)? This concept also confuses me as to what the implications of this are.

    Hopefully I've articulated why I'm puzzled by this well enough that people who know why this is the way it is can at least understand how I could be confused. I appreciate any responses.
     
  2. jcsd
  3. Jun 27, 2010 #2
    I think your guess is right. But don't forget the "centrifugal bulge" on the side of the earth facing away from the sun at any moment. Not taking into consideration the earth's annual trip around the sun, it seems to me that there should be two "solar tides" every day. But they are dwarfed by the lunar tides.
     
  4. Jun 27, 2010 #3

    K^2

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    Only a point-mass can be in total free-fall around another body. To say that any real object is in free-fall is merely an approximation.

    The gravity depends on the distance. On the Sun-side it's a bit higher than at the Earth's center. On the opposite side, it's a bit weaker.

    The centrifugal force balancing gravity out is a bit weaker on the Sun-side, and a bit stronger on the opposite side. As a result, the net force on the Sun-side is towards it, and on the opposite side away. The effect is stretching the planet.

    This effect is called "Tidal Force". It's a very general phenomenon, not specific to tidal waves on Earth. Any time an object is in gravitational field, in addition to net effect on center of mass, there are tidal forces pulling on the object in radial direction, and squishing it in transverse direction.
     
  5. Jun 27, 2010 #4

    russ_watters

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    Remember, in both cases, both objects are orbiting a common center of gravity....

    ...but this has nothing to do with tides...
    Yes, that's what tides are. You can see it in the equation for tidal force.
     
  6. Jun 27, 2010 #5

    russ_watters

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    The orbital motion and centrifugal force have nothing to do with tides. Tides are caused by one side of the earth being closer to the other, resulting in a force pulling the earth apart.
     
  7. Jun 27, 2010 #6

    russ_watters

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    Tidal force does not squeeze in the transverse direction - there may be a squishing force as a biproduct of the internal mechanics of an object, though.
     
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