Tides

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Tides

• What causes them?

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What causes tides?

• The high and low tide are caused by the
  gravitational forces between the earth and the
  moon. However, the source of the real effect
  takes some explaining to grasp.
• The incorrect way of thinking is that the moon
  attracts all the water to itself, therefore
  causing a high tide on the side of earth close to
  the moon, and a low tide on the side far from the
  moon. However, if this was the case, there would
  be a high tide once per day. But there are TWO
  high tides a day. The reason is, the part of the
  earth both AWAY from the moon, and CLOSE to the
  moon BOTH get high tides...

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• Since the earth is big object, the gravitational
  forces fails to deform the earth (much). However,
  it succeeds quite well in deforming the oceans --
  which are not as rigid.
• Since the waters on the moon side are attracted
  more strongly than average, they tend to bulge
  TOWARDS the moon, hence causing a high tide. The
  waters on the opposite side of the moon, since
  they are attracted less strongly than average,
  tend to 'lag behind' the rigid earth, and bulge
  AWAY from the moon, which in this case, is also
  AWAY from the earth, again, causing a high tide.
• Low tide occurs at about right angles to the
  moon, where the force on the waters match the
  average pull of the moon on the earth closely.

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One more time in English.

• Tides are periodic rises and falls of large
  bodies of water. Tides are caused by the
  gravitational interaction between the Earth and
  the Moon. The gravitational attraction of the
  moon causes the oceans to bulge out in the
  direction of the moon. Another bulge occurs on
  the opposite side, since the Earth is also being
  pulled toward the moon (and away from the water
  on the far side). Since the earth is rotating
  while this is happening, two tides occur each
  day.

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In this diagram, you can see that the moon's
gravitational force pulls on water in the oceans
so that there are "bulges" in the ocean on both
sides of the planet. The moon pulls water toward
it, and this causes the bulge toward the moon.
The bulge on the side of the Earth opposite the
moon is caused by the moon "pulling the Earth
away" from the water on that side. If you are on
the coast and the moon is directly overhead, you
should experience a high tide. If the moon is
directly overhead on the opposite side of the
planet, you should also experience a high tide.
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Spring Tides

• The sun also causes a similar (like the moon),
  but much weaker effect. When the sun, moon, and
  Earth are all lined up (at the new and full
  moons) the gravity of the two are combined and
  results in spring tides.
• Spring tides, high tides are higher than normal,
  and low tides are lower than normal. Despite
  their name, spring tides occur twice a month. Not
  just during SPRING!!

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Neap Tides

• During the moon's quarter phases the sun and moon
  work at right angles, causing the bulges to
  cancel each other. The result is a smaller
  difference between high and low tides and is
  known as a neap tide. Neap tides are especially
  weak tides. They occur when the gravitational
  forces of the Moon and the Sun are perpendicular
  to one another (with respect to the Earth). Neap
  tides occur during quarter moons.

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Tide Video Clip I
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Tide Video Clip II