All around the world, sea waters shift and change, resulting in tides of various sizes, referred to commonly as high and low tides. These cause visible changes in the levels of water at the coastline and can, in turn, affect the surrounding land and the people living there if tidal changes are extreme. At any given time, there are multiple factors affecting tides.
The moon and the Earth are in a perpetual dance surrounding their common center of mass. This allows the moon to rotate around the Earth without running into it, even while the Earth is moving around the sun. What results from this is centrifugal force, one of the factors that affect the tides. This force will always move away from the Earth because the center of mass for the Earth is always on the opposite of where the moon is. As the moon rotates, the centrifugal force shifts, causing minute changes in the tides.
Gravity is the main factor affecting tides. The moon's gravity pulls at the Earth, which is able to hold onto everything except for water. Large bodies of water are then drawn to the moon through its gravitational pull, rising and lowering depending on where the moon is closest to the Earth. The sun's gravity also comes into play. During new and full moons, the sun lines up with the moon and the Earth to create very strong spring tides, which are higher and lower than other tides. During the quarter phases, when the three bodies are not lined up, weaker neap tides occur.
The Moon's Path
The moon's path around the earth is an elliptical one, meaning that at certain points it is closer to the Earth than at others. The difference is not a huge amount. It is only between 9 and 14 percent closer to the Earth at its closest point, called its perigee, than at its farthest point, or apogee. However, the difference between the amount of gravity the moon exerts on the Earth at its perigee than at its apogee is 30 to 48 percent greater. Therefore, when the moon is at its perigee, it creates a greater difference between the high and low tides.