The Standard, but Incorrect Version
Most people who have been near the sea know there are tides, with the sea's surface rising and falling in harmony with the moon and the sun. We are taught in school that tides are caused by the gravitational pull of the moon and to a lesser degree, the sun, on the sea.
There are diagrams like the one at left to demonstrate this relationship.
The diagram is easy to understand. The gravitational attraction of the moon bulges up the sea on the side of the earth nearest the moon and, for symmetry, a smaller bulge develops on the other side. As the Earth spins under the moon once every 24 hours, any one point will have two high tides (one lower than the other) and two low tides.
The sun has a similar, but smaller effect. During full and new moon, when the sun and moon are lined up, the tides are highest (spring tides). When the sun and moon are at right angles to each other, at the quarter moons, they tend to work against each other and the tides are lower (neap tides).
All very reasonable and logical. But completely wrong.
The behavior of the ocean does not follow the diagram. In fact, the sea acts just the other way, and when the moon is overhead it is low tide, not high tide.
This is because the whole planet is influenced by the gravitational effect of the moon and the planet is a liquid with the continents floating like icebergs on the surface.
The moon distorts the liquid sphere of our planet only a tiny amount, less than 500 mm, and the resultant earth tides are measurable even in the middle of the largest continent (When the moon is over Moscow, the land rises about 300-mm). The tides rise and fall about the same amount in the middle of the ocean with no land around.
When the land rises, the sea also rises, but not as much. When the moon passes directly overhead, the land rises more than the sea, and the sea drains away. The difference in rise between the land and the sea determines the local tidal height.
There are a multitude of reasons the ocean level changes more close to land than in the open sea. Perhaps the two primary reasons are:
1. Land masses are less flexible than water and their structure distorts the smooth curve of the planetary bulge while the liquid ocean follows the bulge exactly.
2. Land masses are closer to the moon than the sea surface and thus the gravitational effect is greater.
Most small ocean islands (the exposed summits of submerged mountains) have tides of about one meter either side of the mean sea level. The smaller and more remote the island, the less the tide range.
Far from the influence of the continents, the tide is always low (or near low) when the moon is directly overhead of a small island.
There are exceptions to this rule. Tahiti, for example has only one tide per day. This is because the huge massive continent of Eurasia is on the opposite side - thus dampening the gravitational bulge.
This diagram exaggerates the rigidity of the land and the gravitational bulge of the planet. The actual change in the radius of the planet is less than 0.0000046%.
The larger continents cause a more drastic dampening effect on the distortion of the planetary crust.
Continents flex and lift in different ways depending on their shape and tectonic structure (especially their elasticity). When the gravitational effect causes the continent to warp or tilt, ocean tides can exceed 10 or even, in rare cases, 20 meters as the edge of the land lifts out of the sea (see diagram at left).
Although the effect of gravity is instant (or perhaps at the speed of light), and changes the molecular patterns of all the planet right to the core (land and water alike), the resultant movement of water relative to a rising or sinking land mass takes time.
The time of high and low tide, therefore, varies considerably from the sun and lunar gravitational effect depending on the local topography. Where there is, for example, a large coastal plateau or lagoon, the ocean may require two or more hours to flow away from the rising land. There will be strong tidal currents in these areas, continuing an hour or two after the actual high or low tide has passed.
People have continued to live happily with the standard theory of tides because, quite simply, the calculations to predict tides work very well. Fishermen, sailors, surfers and the many people who want to know the tide heights from time to time are perfectly satisfied with the existing predictions.
If the formula works, most people think, the theory must also be spot on. So why, if the standard theory of tides is wrong, does the formula to predict tide heights work so well?
Tide tables accurately predict the tide level in any particular location years in advance. I have a tide program that will accurately predict the tides in any particular location for any time in the future.
The tide predictions are based on a formula that gives the position of the moon and the sun relative to any given set of coordinates on Earth at any given time. This part can be done with perfect accuracy, and it isn't too complex. But by itself it is not very useful.
To be accurate for a particular location, the formula must be corrected for effects on the tides from a number of factors. These include:
The shape of the coast
The shape of the depth contours off shore
Local water currents
Various other conditions such as changes in salinity and water flow from river estuaries.
The upshot of all this is that the tidal formula accurately predicts the relationships of Earth, Sun and Moon and then calibrates the results to particular localities using fudge factors based on accurate measurements of tidal conditions over many years. Once you know the "offset" for any given locality, all you really need to do is add or subtract this from where the tide should be given the position of the sun and the moon.
It does not matter if your expected tide position is six hours out of whack because this is simply tacked on to the local fudge factor.
The standard theory of ocean tides has nothing to do with the fact that the tides can be predicted with great accuracy using the existing formula.
For starters, the theory of tides was devised before we realized the planet was itself a big liquid sphere and before we understood much about how gravity works. And even before we knew anything about the deep ocean.
The original idea was simple. Solid Earth, liquid sea. Sea lifts up when moon passes over, with lots of rushing tidal currents in and out of coastal passes.
To get around the obvious complication that the tide happens to be low when the moon is overhead in many of the island areas of the world, the scientists of the day developed an elaborate idea of a tidal wave that is propagated around the planet. Then they imagined there must be some kind of frictional force between this wave and the ocean floor in deep water. They needed that to slow down the tidal wave. One accepted theory has it that the gravitational effect of the moon actually pushes the sea due to friction. Any residual time deficit, after these wavy, frictional deviations, was written off to local factors influencing the tidal wave.
But there isn't any tidal wave and the bulging of the planet does not propagate like an ocean wave and there is no friction of any sort between the sea floor and ocean tides or between gravity and the ocean.
Gravity, as most of us discovered in our infancy when we first tried walking, does not have a time lag. Except in cartoons and in tidal oceanography. Gravity happens right now, and it happens to the whole planet all at the same time. The bulge created by the moon is a collective change in molecular motion created by the gravitational effect of the sun and the moon. The greatest change in molecular motion is on the part of the planet nearest to the sun and moon - the surface. But the gravity effect of the moon and the sun distort the planet everywhere, right to the core and on to the opposite side of the planet.
As the planet rotates under the moon, the molecular effect remains at its maximum directly under the moon. But this is not a wave propagating and has none of the characteristics of one. If the moon vanished instantly, the effect would vanish in the very short time it would take the planet to assume a more uniform, unbulged, shape. Waves propagate from one area to another, gravity bulges don't. The gravity bulge from the moon remains perfectly stationary under the moon. The gravity bulge from the sun stays perfectly stationary under the sun.
Waves also generate wave trains with specific features of multiple wave propagation. The tidal effect is a singular egging of the planet.
Further, there is no friction of water movement over the deep sea because the tidal effect does not move water around the planet every day. It does not directly move water anywhere. In the open sea, when the moon passes over, the water stays where it is, moving in whatever direction it happened to be moving.
The only effect of the moon is that the water molecules alter their motion slightly, along with all the rest of the molecules of the planet, so the whole mass elevates a miniscule amount (relative to the diameter of the planet).
The massive movement of water we observe near ocean passes or from estuaries and lagoons is only indirectly caused by the gravitational effect. It is not a tidal wave rushing in and out of the harbor. The harbor is lifting up and descending and the water is, quite naturally, sloshing in and out as this happens.