What Causes Tides to Go in & Out?

Lunar Tides

• As the Earth and Moon travel around the Sun, they revolve around a common point deep within the Earth. When one side of Earth is facing the Moon, lunar gravity pulls on water, causing it to bulge and become slightly deeper. At the same time, centrifugal force causes a bulge on the side facing away from the Moon. These bulges are the high tides. The formation of these two bulges causes lower water levels on the rest of the planet, which are the low tides. As the Earth spins on its axis, most places will experience two high tides and two low tides each day.

Solar Tides

• Although the Moon has the greatest effect on the tides, the Sun also causes tidal bulges. However, since the Sun is much further from Earth than the Moon, its effect is only about half that of the Moon.

Spring Tides

• When the Sun and the Moon align during new Moon and full Moon, their combined gravitational forces exert a greater pull on Earth's water. High tides are higher and low tides are lower. This is called spring tide, however it is not related to the change in season.

Neap Tides

• When the Moon is in first quarter or last quarter it is at a right angle to the Sun. The gravitational forces of the two bodies largely cancel each other and the range between high and low tides is very small. These are neap tides.

Tide Patterns

• Factors such as land masses, the Earth's rotation and the shape of ocean basins can effect how different areas experience tides. For example, some areas, such as the Gulf of Mexico and Southeast Asia, experience one high tide and one low tide. These are called diurnal tides and are about 24 hours, 50 minutes long. Areas such as the Atlantic coast of North America experience semidiurnal tides. These tides reach the same high and low levels each day and last about 12 hours, 25 minutes. In some areas, such as the Pacific Coast, there are two high tides and two low tides per day, but the tides reach different high and low levels during each cycle. Called mixed semidiurnal tides, they last about 12 hours, 25 minutes.