Animations of models of the tides of the oceans

by Bruce Railsback, Department of Geology, University of Georgia

This is a page to present six animations of the various models or theories of the oceans' tides. The animations are click-driven: you have to click the "Next Slide" button to advance the animation. That's a primitive but effective way to let you watch a series of images in which you control the rate of advance (or reversal). Each animation begins and ends with the opportunity to return to this page. The last animation (the dynamic model of the tides) is the main point of this page, if you want to short-circuit to it.

Part I. Getting Started: Why are there two tide bulges?:

If you would say that "the tide exists because the moon's gravity pulls the ocean toward the moon", you would be half, and only half, correct. That statement would imply that there would be just one tide bulge, and thus any location on earth would experience just one high tide each day. In fact, there are two tide bulges, and two high tides each day. If you don't know why, take a look at the next two paragraphs before going on to Part II.
We usually think that the earth's gravity holds the moon in place around the earth, and that the earth travels in a roughly circular (actually slightly elliptical) orbit around the sun. In fact, the earth and moon travel as a unit, and their mutual center of gravity follows that circular (or slightly elliptical) orbit. Together, they flop along like a lop-sided dumb-bell, and/or they look like a hammer thrown through the air. Animation I shows this relationship.
Thus Earth's gravity holds the moon in place in this lop-sided dum-bell, and Moon's gravity holds Earth in place. The catch is that Moon's gravitational field, like any gravitational field, is stronger closer to the moon and is weaker farther from the moon. At Earth's center, the moon's gravity is just right to hold the Earth. On the side of the earth near the moon, there's excess lunar gravity, and the ocean is pulled toward the moon. On the side of the earth away from the moon, there's a deficit of lunar gravity, and the ocean sloshes away from the moon. Thus there are two tide bulges, as this one-jpeg single image shows. Now we're ready for the equilibrium model.

Part II. The equilibrium model or equilibrium theory of the tides:

The traditional starting point in talking about the tides is the equilibrium model, in which the solid earth spins on its north-south polar axis under two oceanic tide waves (two crests and two troughs). The crests more-or-less bulge toward the moon and away from the moon. The beauty of the model is that someone on a coastline (Observer A in the animation) sees two crests (high tides) and two troughs (low tides) every 24 hours. That's great. See Animation II, in which you're watching from directly over the North Pole.
The problem is that the model has the crests of the tide waves pass entirely around the world and thus inundate the continents (thus drowning Observer B in Kansas or Bolivia twice each day). Look at Animation II again, this time considering the fate of Observer B.

Part III. Improving on the equilibrium model of the tides:

Because the tide waves stay in the ocean basins rather than sweeping across the continents, we clearly need to improve on the previous model. The next animation retains the concept of gravitational imbalance but uses it to move the water in one ocean basin from side to side, a vision closer to reality. In this model too, any one location in or around the ocean sees two high tides and two low tides every 24 hours. We recommend that you watch the animation twice, first to focus on the dark blue ocean sloshing back and forth, and then to focus on the tide changing at the marked red point. You're again watching from directly over the North Pole. See Animation III!

Part IV. Incorporating the Coriolis Effect:

The only problem with the model above is that it implies that the tide would slosh back-and-forth from east-to-west and west-to-east across an ocean basin. The Coriolis effect will convert any seemingly straight motion across the earth surface into a curve. Thus in the northern hemisphere, as the tide turns to its right around an ocean basin, it takes a counter-clockwise circular path. (Imagine entering a room and walking along the walls of the room, always trying to turn to the right as you move forward. You'll take a counter-clockwise path around the room.) In the southern hemisphere, as the tide turns to its left around an ocean basin, it takes a clockwise circular path.
That leads us to Animation IV of tides in a simple basin on a simple planet, where you're watching from above the equator, with the moon behind you.

If you're having trouble envisioning this idea of a rotating tide, there's a one-jpeg single image and, once you've seen that, there's Animation IVa for the Northern Hemisphere

Part V. The dynamic model or dynamic theory of the tides (the true object of this page and lesson):

The result is that the tides sweep around the ocean basins, counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. As an example, the next and final animation shows the progression of the tides in the Atlantic Ocean. You're watching from above the equator, with the moon behind you, or over your right shoulder. The light blue and dark blue fields indicate the areas covered by high and low tide in an hour, so wider bands indicate faster movement of the tide. The bands curve and swell or thin in response to the depth of the ocean - the tide wave moves more slowly in shallower water and faster in deeper water.
If time and your web connection allow, we recommend watching the animation three times: first to watch the world go round and to get used to that, second to watch the tide go around the North Atlantic, and third to watch the tide go around the South Atlantic. See Animation V!

The result of all this is that, in the Northern Hemisphere, tides sweep south down east sides of continents (like the U.S. East Coast) and north up the west sides of Continents (like the U.S. West Coast). As examples, you can look at maps of times of high tide for the North Atlantic and northeast Pacific.

Sources and comments: The base maps for Animation V were generated using Online Map Creation at Das Leibniz-Institut für Meereswissenschaften an der Universität Kiel (IFM-GEOMAR). The tide positions in Animation V are from Dietrich et al., 1980, General Oceanography: New York, John Wiley & Sons, 626 p. Animations I to IVa are unabashedly northern-hemisphere centric, but persons in the southern hemisphere can just run Animations I to III and IVa backwards.

e-mail to Bruce Railsback (rlsbk@gly.uga.edu)
Railsback's GEOL 3030 (Elementary Oceanography) course web page
Railsback's main web page
UGA Geology Department web page

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