Ocean Tidal Movements

Everyone knows that the Moon's gravity causes the tides in the oceans. Just as many people know that the movements of those tides are incredibly complex, and often seeming to be illogical. This discussion is intended to try to make some sense of why different aspects of the patterns occur.

We must first note that the ocean water which must produce the tidal bulge must be MOVED from other parts of the oceans, to be "piled up" at that one spot at that moment.

This first makes clear one very incorrect assumption that most people seem to have, that the tides only exist because of the MOTION of the Earth or Moon. Not so! If the Earth and Moon could somehow be "bolted in place", there would still be that same tidal bulge on the FRONT side of the Earth (toward the Moon). And fairly simple and obvious logic shows that there would ALSO be the same tidal bulge on the BACK side of the Earth (opposite where the Moon is)! The motions have nothing to do with WHY there are tides! However, the reality is that the Earth and Moon MUST constantly move, such as the Moon orbiting the Earth, such that it does not shortly gravitationally fall down to hit us!

It just SEEMS that the rotational motion "must" be causing the tides, so nearly everyone seems to believe that. Even many web-pages that claim to provide an explanation for the tides, present such wrong explanations!

Next, we must realize that all that water is rather massive, and that massive bulge of water has to be at one location at 6 am and 1/4 of the way around the Earth six hours later at noon! So there HAS to be some (apparent) lateral motion of water to transfer all that water across the oceans. (Remember that the water itself is actually trying to STAY IN PLACE, lined up with the Moon, and it is actually the EARTH and the rest of the oceans which are rotating underneath that tidal bulge.) In fact, we can see that it should be as much as that 6300 miles (1/4 of the way around the Earth's Equator) in each six hour period, or slightly over 1,000 miles per hour!

This is not quite actually true, mostly due to frictional losses in the ocean against the ocean bottom. But the speed of the tidal bulge across an open ocean can be around 800 mph. This brings up another seemingly obvious question! If you are in a small boat in the middle of the ocean, and an 800 mile wave comes along, how come there is not massive destruction?

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The reason is because the surface water speed does not have to be very great, while still transferring an enormous mass of water. Have you ever sent a wiggle down a long slinky? The WAVE moves pretty fast, but the metal of the slinky coils only moves a very small distance before returning to where it was to start with. A Tsunami is actually rather similar. An earthquake can move a large area of ocean bottom laterally, which suddenly leaves no room for the water that used to be there. What happens is that the water over the local region gets "stacked up" by a foot or two (because water cannot be compressed). However, this is not a situation that is stable, and the water must soon "seek its own level". The way it does that is to be as a wave (of that excess water) which may only be one or two feet high, but that wave travels at incredible speed, often over 500 mph. (for the same reason that other large ocean waves travel that fast.) In this way, the energy which needs to get distributed does so. The interesting part of this is that, in the open ocean, boats may hardly even notice that a Tsunami wave passes by. Even though the WAVE might pass by at 500 mph, the water does not. And since a Tsunami wave is VERY long, and only that one or two feet high, there is actually very little sense of any significant wave even passing by! The people in such a boat MAY sense that their boat gets lifted up a foot or two and then a few seconds later lowered back down. THAT is all they would experience from a tsunami.

For the ocean tides, the length of the waves (wavelength) is even longer, and still the height of the tide waves are only around one to two feet in the open ocean. So even though you might be in a small boat in the middle of the ocean and the tide goes by, you probably will not even notice it! The ONLY effect would be where your boat was gradually lifted up about a foot or two over a period of several hours, and then gradually lowered back down that foot or two. In the meantime, you would have experienced many thousands of wind-generated surface waves that would have rocked your boat around. So even though you actually would have experienced the passage of the tide wave at maybe 800 mph, you would have had no possible way to realize it!


This presentation was first placed on the Internet in May 2007.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago