The pull of gravity of the Moon causes the tides on Earth.
Tides may seem simple on the surface, but the ins and outs of tides confounded great scientific thinkers for centuries they even led Galileo astray into a bunk theory.
Today people know that the gravitational pulls between the earth, moon and sun dictate the tides. The moon, however, influences tides the most.
The moon's gravitational pull on the earth is strong enough to tug the oceans into bulge. If no other forces were at play, shores would experience one high tide a day as the earth rotated on its axis and coasts ran into the oceans' bulge facing the moon.
However, inertia -- the tendency of a moving object to keep moving -- affects the earth's oceans too. As the moon circles the earth, the earth moves in a very slight circle too, and this movement is enough to cause a centrifugal force on the oceans. (It's centrifugal force that holds water in a bucket when you swing the bucket in an overhead arc.)
This inertia, or centrifugal force, causes the oceans to bulge on the opposite side facing the moon. While the moon's gravitational pull is strong enough to attract oceans into a bulge on the side of the earth facing the moon, it is not strong enough to overcome the inertia on the opposite side of the earth. As a result, the world's oceans bulge twice once when they are on the side of Earth closest to the moon, and once when they are on the side farthest from the moon, according to the Wood's Hole Oceanographic Institution in Wood's Hole, Mass.
Geography complicates the tides, but many places on Earth experience just two high and two low tides every 24 hours and 50 minutes. (The extra 50 minutes is caused by the distance the moon moves each day as it orbits Earth).