The movement in which water alternately rises and falls is called a tide. Tides are primarily caused by the gravitational pull of the moon and, to a lesser extent, the sun. The Earth's rotation also influences the timing and height of tides. As the Earth rotates, different parts of the Earth move through the moon's gravitational field, resulting in variations in the sea level. High tide occurs when the sea level rises to its highest point, while low tide occurs when it falls to its lowest point.