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Why does the ocean get bigger gravitational pull of the moon? Why low and high tides occur throughout a day?
Question Date: 2013-01-29
Answer 1:

The pull is NOT greater on oceans. The pull of the moon on near side of the earth is a little greater than the pull of the moon on the center of the earth. The land and water move towards the moon when the moon is over head. Meanwhile 12 hours ON THE OTHER SIDE OF THE EARTH, the land (and ocean) gets a slightly SMALLER tug from the moon, RELATIVE to the center of the earth. Hence, relative to the center of the earth, the ground and the oceans MOVE AWAY from the center of the earth, to create a high tide there, AT THE SAME TIME WHEN THERE IS HIGH TIDE, 12 HOURS or 180 degrees in longitude AWAY.

Answer 2:

Throughout the day, the oceans are constantly moving, rising and falling. This change in the level of the water is called the tide. And the tide is controlled not only by the moon, but also by the sun, the way the earth rotates, and more. The most important factors affecting tides are gravity and rotation - and as the moon rotates around the earth because of gravity the moon is said to `control´ the tides.

But to understand how the moon´s gravity and rotation cause water to rise and fall as tides we need to understand a bit about water. Water molecules (every individual molecule that makes up the ocean and every body of water) are attracted to each other in a way that makes them stick together (think about dew droplets on a plant leaf in the morning; the water forms this droplet because the different molecules of water stick together). This property of water is called `cohesion´ and because of this property of water all water molecules want to stick together and then they act like a single body. So rather than every water molecule in the ocean acting in different ways, all of the molecules stick together and act together.

As the earth rotates, the force of the rotation pulls the ocean water away from the earth´s surface (you can test this by holding your arms our and spinning as quickly as you can - do your arms feel like they want to fly off of your body?). Now we know that the ocean water molecules all stick together which means that every molecule in the ocean pulls away from the earth´s surface. This creates a mound of water around the center of the earth (the earth´s equator) and a thinner layer of water at the arctic and the Antarctic poles. If the earth was all alone in its rotation then we would just have a mound of water at the middle of the earth and less water at the north and south poles; however, because the moon is very close to the earth, its gravity has an effect on this bulge. So when you watch the water level rise in Santa Barbara (as the tide rises) it means that the rotation of the earth is carrying California away from the moon because the beach that we are watching is being rotated toward the part of the earth where the ocean has a water mound. As we move right in to the bulge of water we have a high tide. Then as the earth rotates and carries Santa Barbara away from that water mound, the tide falls and we end up in a low tide. So why do we have two high and two low tides every day? We have two high and two low tides every day, because there are actually two mound of water on opposite sides of the earth. It takes the earth 24 hours to complete one full rotation and in that 24 hours we pass through these two mound of water (2 high tides) and two low points in the water (2 low tides). And that mound of water is caused by the gravitation force of the moon on the earth as the spot where you are on the earth rotates past the moon.

Answer 3:

The oceans aren't pulled on any harder than the land, but the oceans, being liquid, are able to slosh where gravity pulls them which the land, being solid, is more resistant to.

Tides occur twice a day because the far side of the Earth from the moon is experiencing less force than the midway-point, which effectively causes it to stretch in the other direction.

Now, that said, the sun has a large effect on tides, too.

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