part one in a series of...There are many changes which take place in the world around us which happen too slowly for our (ever-decreasing) attention span to notice. Ocean tides are one such phenomenon, and this page hopefully provides a window onto understanding them.Adventures in Digital Time Lapse Photography
The tides are caused by the gravitational attraction of the oceans by the moon. There are many complicating factors, but we can simplify things somewhat if we imagine the Earth to be a perfect sphere (it's really slightly squashed,or "oblate"), and further imagine that the oceans completely cover the Earth to a uniform depth. If we make these simplifications, and calculate the gravitational potential energy at various points, we can see how tides work.
The following animation shows the relative energies of the Earth's surface and the ocean's surface at various points around the planet as the Earth rotates through 25 hours (the red dot is a stationary observer). The pictures are exaggerated for clarity: the ocean is so shallow relative to the diameter of the Earth that it would be invisible at this scale:
Since gravitational potential energy is negative, larger bars denote less energy. Since everything wants to be at its point of lowest energy, we see that on the side nearest the moon, the ocean is at lower energy when it is deeper, giving us "high tide". On the far side, the Earth is at lowest energy when it is farthest from the ocean surface, again giving us high tide. The key points here are that the oceans are fluid and so are free to move in such a way that their depth oscillates with their changing position relative to the Moon, and that the Earth is solid and so rotates about the center of gravity of the combined Earth-Moon system. The clue was there for centuries until Newton's Theory of Gravitation unlocked it: the Moon is in the same position in the sky every 25 hours, and there are two high tides and two low tides every 25 hours!
So how can we see this tidal flow? One way is to simply compare "before" and "after" pictures, such as these of Bass Harbor, Maine, at high tide:
and at low tide:
Another way is to watch a time lapse movie of the process. Here you have two choices:
This movie was taken over an hour and 50 minutes, starting approximately one hour after high tide. Notice the waves as the tide goes out: this is a "tidal bore" created by the flow through the waterway under the road:
These waves are a feature whenever tidal flow is constricted by a narrow opening, and is a dramatic indicator of the power of the moon on the waters. The "supply side" of this flow looked like this at high tide:
(notice the tidal bore running from right to left as the water flows in), and like this at low tide:
The water flow was actually against the tidal flow for the first few frames of the movie (from right to left), although it is hard to see because the surface winds were in the opposite direction.
Another illustration of tidal flow, this time at Southwest Harbor, Maine, is also available as
This movie was filmed over 4 hours and 47 minutes, from about an hour after low tide. The most dramatic changes occur during the middle half of the transition from high to low tide, or vice versa. This is analogous to the effect you notice when a basketball player or a ballet dancer seems to hang in mid-air as they leap. The laws of physics will tell you that they simply spend more time at the bottom and the top of their trajectory than they do at the middle!
©2006, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.
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