Water on the Move The Ebbs and Flows of the Sea

The Water PlanetOceans in MotionLife in the SeaScientist at SeaResources


Spring tide: the Earth, sun and moon are alignedTide-generating forces are a result of the gravitational attraction between the Earth, sun, and moon. It was not until Sir Isaac Newton (who lived from 1642-1727) discovered the law of gravity that the effect of the sun and the moon on the tides was fully understood. All surfaces of the Earth are pulled toward the moon and sun. This force has little effect on land masses, but it does have a very great and obvious effect on the water of the Earth's oceans. Twice each month the tidal range reaches a maximum and these large tides are called the spring tides. Halfway through the monthly cycle the range is much smaller, and these weak tides are called neap tides.

Neap tide: the sun and moon are at right angles to the EarthAs the moon rotates around the Earth, it pulls the water on the nearest side of the Earth outward into a bulge. A similar bulge on the opposite side of the Earth is caused by the water being thrown outward by the planet's spin. These two bulges travel around the globe, producing two high tides each day. During time of the new moon and full moon, when the sun and moon are in a straight line, their gravitational pulls combine and produce spring tides; at this time the high tides are very high and the low tides are very low. When sun and moon are at right angles from the Earth, during the quarter phases of the moon, the gravitational pull on the oceans is less producing a smaller difference between high and low tide known as a neap tide.

Some locations have much bigger tides than others. Tidal ranges are usually small in the middle of the ocean but can be very large where tidal waters are funneled into a bay or river estuary. Hawaii has hardly any tidal range at all while the water in the Bay of Fundy, in Canada, has a range of about 40 feet.

Of Tides and Time

Tide data can be charted to reveal rhythmic patterns over time. Gather your own tide data and see if you can make predictions on when the next new moon will occur.


Science Learning Network | email: sln@mos.org | © 1998 The Museum of Science