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Does the moon cause the flux and reflux of the tides?

  1. Dec 30, 2009 #1
    I assume wind is also a factor but would the gravitational pull of the moon be the main force behind the oceans tides? To be honest I have no idea what even causes the wind so go easy on me if that was a dumb question.
  2. jcsd
  3. Dec 30, 2009 #2


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    Gravity from the moon and sun does influence tides, and wind (and atmopheric pressure) does also. One has probably also heard of the 'storm surge' in reference to hurricanes. The combination of high wind and low air pressure can add many feet (meters) to water (sea level).

    Wind is induced by differences in air densities and the rotation of the earth. Warm air rises (because it is lighter) and cool air sinks (it is denser). Moisture is another factor in air density. The H2O water molecule is lighter than O2 or N2, so moist air is lighter than dry air. The masses of air also response to the rotation of the earth (Coriolis effect).

    About tides - http://co-ops.nos.noaa.gov/education.html


    Last edited by a moderator: Apr 24, 2017
  4. Jan 1, 2010 #3
    Water can certainly be driven inland by some titanic weather events, but in terms of regular and periodic tides, it's primarily the moon and sun. Atmospheric pressure certainly plays a role, but only in shaping the tides, not in their creation. Winds need to be sustained at a high level to drive water inland, and even then, while it is devestating to people and their welfare, it's not a vast event really.

    Remember to think of the earth's atmosphere and oceans as the fluid systems they are. Convection is a constant, as is the coriolis force as Atronuc mentioned. If the Earth somehow didn't experiene Coriolis force, there would still be wind, but cyclonic winds on a large scale would be rare or impossible.

    Finally, it's not just the "pull" of the moon "lifting" water... it's interaction between two fairly large masses (remember the moon is about 1/4 the mass of the Earth... no slouch), and the sun plays its role on a slower scale reflecting the period of our orbit around it.
  5. Jan 1, 2010 #4

    D H

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    It is indeed the "pull" of the moon "lifting" water. It has nothing to do with orbits per se. Suppose the Moon's orbital angular momentum with respect to the Earth suddenly vanished. That would have zero instantaneous effect on the tides.

    The reason the tides exist is because the Moon (and Sun) have a nearly spherical gravitational field. This means that the gravitational acceleration by some object on the Earth's surface toward the Moon (or Sun) is not the same as the gravitational acceleration of the Earth as a whole toward the Moon (or Sun). The difference between the acceleration at some point on the surface and the acceleration at the Earth's center of mass is the tidal acceleration. These tidal gravitational forces exerted by the Moon and Sun are the primary sources of the ocean tides.

    BTW, the Moon's diameter is about 1/4 that the Earth. It's mass is considerably less than that of the Earth. The Moon's mass is about 0.0123 that of the Earth.
  6. Jan 1, 2010 #5

    Jonathan Scott

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    The tricky thing is understanding why there is a high tide on the opposite side to the moon as well as on the near side.

    That's because on average the small acceleration on the earth due to the moon acts at the earth's centre, but the near side of the earth is closer than the centre, so gets accelerated a little more strongly towards the moon, and the far side is further away than the centre, so it gets accelerated a little more weakly towards the moon than the earth as a whole. This creates bulges on both sides. See the Wikipedia article on http://en.wikipedia.org/wiki/Tide" [Broken] for more details.
    Last edited by a moderator: May 4, 2017
  7. Jan 1, 2010 #6
    I stand corrected. Hmmm... it seems that New Years parties can really take a toll. My apologies. As for orbits, I wasn't proposing that action of orbiting gave rise to the tides, but rather that orbits and relative positions of the various bodies creats the periodic tides we know. When I'm talking about orbits here, it's about the periodic nature of the tides.
  8. Jan 6, 2010 #7
    The way that I visualize it, is to consider a rubber balloon filled with water on the end of piece of string. Now if you spin the balloon around you head, then it deforms into an egg-shape whose "points" represent the tides.

    One of the pointed ends of the oval balloon coincides with where the string is attached, and represents the gravitational force of the Moon on the Earth's water. On the opposite side of the balloon, is a bulge due to the centrifugal spin of the balloon.
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