The Tides
What are the tides? The level of the ocean rises and falls at constant intervals. This occurance happens two times per day. However,everyday the tidal periods are approximatly 50 minutes later.
What causes the tides? It is the force of the moon's gravitational pull is what causes them. The moon produces a much more intense tidal force than the sun even though the sun has a much greater force of attraction.
Because the moon's gravitational pull is stronger on the water that is closer to it, it's pull will be more powerful at this point than the far sides. This makes the water bulge out from the earth in two places. It moves away from the other point because it trails behind as the water speeds up when pulled by the moon's gravity. In a sense, the water is left behind.
Spring Tides are when the moon and the sun are both alligned with the earth on either side and the tides are at their highest point. This is because the sun's force on the Earth essentially PULLS the water towards it while the moon PULLS it the other direction. When the sun is not in line with the moon in the first quarter and the last quarter of the year, the tides are at their lowest. This is called Neap Tides.
We can clearly see the strong gravitational force exerted by the moon on the Earth. As a result of the more intense force, the two bulges are created. One in the front because of the moon's gravity. And one in the back because of the Earth's speed and because it is so thin in the middle. We on Earth see these bulges as recurring rising and fallings of waves.
Picture from Our Satellite: THE MOON
For further information we recommend you visit this site on Lunar Tides