Tides - Characteristics

Tides are the periodic rise and fall of the ocean waters. They are caused by the gravitational pulls of the Moon and (to a lesser extent) Sun, as well as the rotation of the Earth.

The Sun and Moon pull on the Earth, the water, even you! But gravitational attraction depends on distance and mass. For example, you have very little mass and you're very close to the Earth, so the Sun and Moon can't just yank you off the planet. The Sun is extremely massive, but it is an average of 93 million miles (150 million km) from Earth, compared with about 238,866 miles (384,400 km) from here to the Moon. And since the Moon is nearly 400 times closer to our planet, its influence on our oceans is twice as strong as the Sun's.

The key to tides is the varying strength of the Moon's gravitational pull on different parts of the globe. The Moon pulls most on the water nearest to it, creating a high tide bulge of water. On the opposite side of the planet, about 7,926 miles (1,2760 km) away, the Moon's pull is much weaker and the water is left to form another high tide bulge. Low tides are found halfway between the highs. The rotating Earth carries us through these regions of high and low water.

Within a small body of liquid, such as a pond or bowl of soup, there are no tides because the whole body of water is the same distance from the Moon, feeling an equal gravitational pull.

The timing of tides is determined by the Earth's rotation and the Moon's orbit around the Earth. As the Earth rotates once about its axis in 24 hours, the Moon is moving 1/30th of the way around in its orbit. It takes a given location on Earth about 50 minutes to "catch up with" the orbiting Moon, so a particular tide returns in approximately 24 hours and 50 minutes.

When the Moon, Earth, and Sun fall in a straight line, which we call syzygy (siz-eh-gee), we notice the greatest difference between high and low tide water levels. These spring tides occur twice each month, during the full and new Moon. If the Moon is at perigee, the closest it approaches Earth in its orbit, the tides are especially high and low.

When the Sun and Moon form a right angle, as when we see a half moon, their pulls fight each other and we notice a smaller difference between high and low tides. These are called neap tides.

Factors such as the path the Moon takes around the Earth, our planet's tilt, even the water's depth, and the ocean floor affect tides. Therefore, not all coasts experience two high and two low tides each day.

The difference in the height of the water surface between the high and low tides is the tidal range. Tidal ranges can be measured in inches, like those in Lake Superior, Michigan, or in feet or yards. In fact, the Bay of Fundy, a V-shaped Canadian inlet in Nova Scotia, has the greatest tidal range known--up to 50 feet! In areas with large tidal ranges, boats anchored at high tide are often left stranded on the dry beach at low tide.

As the sea level rises and falls, it generates a tidal current that flows horizontally. Tidal currents caused by the dropping water level (as the tide "goes out") are called ebb currents. The rising tide generates flood currents. Tidal currents are especially strong where the ocean is connected to an estuary or bay, and boats sometimes have to wait for a current in to enter or leave a harbor.