Tides are caused by the gravitational pull of the Moon and, to a lesser extent, the Sun. The Moon's gravity creates bulges in the Earth's oceans that result in high and low tides about every six hours. There are two types of tides: spring tides, which occur during a full moon or new moon and have the largest difference between high and low tides, and neap tides, which occur when the Sun and Moon are at right angles to each other and have a smaller difference between tides.