Tides of the Earth
MONDAY NOVEMBER 29, 2010
Tides of the Earth
We think we know about high and low tides, about kingtides when the moon comes closer, and when this occasion combines with a full or new moon. We think we know about neap tides, the halfway points, when the tidal variation is less, and the sea doesn't come up the beach as much or go out as far. People who have boats know when to catch the tide for coming home, so they don't get grounded in the middle of a mudflat. Parents know to watch small children at half-tides on a spring-tide day, a time of greatest rip, when an undertow can be too strong to stand up.
We are taught in school that tides are the sea's surface rising and falling, caused by the gravitational pull of the moon and to a lesser degree the sun. This pull of the moon bulges up the sea on the side of the earth nearest under the moon and, for symmetry, a smaller bulge develops on the other side. As the earth spins under the moon every 24 hours, any one point will have two high tides and two low tides. The sun has a similar, but smaller effect.
All very reasonable and logical. Pity that it’s wrong.
The behavior of the ocean does not follow the way we were taught. In fact, the sea acts just the other way, and when the moon is overhead it is more often low tide, not high tide. In Kaikoura, Otago, Adelaide and Melbourne, moon-overhead is closer to low tide. But fishermen, sailors, surfers and swimmers live happily with the standard idea of tides because the calculations to predict them work well enough. Tide tables based on a formula can accurately predict years in advance. To be accurate, the formula must be corrected for shape of the coast, shape of the depth contours offshore, local water currents and other conditions such as changes in salinity and water flow from estuaries.
The upshot is that the tidal formula predicts relationships of earth, sun and moon then calibrates the results to particular localities using fudge factors. Once you know the offsets, all you do is add or subtract this from where the tide should be, given the position of sun and moon. So why is it wrong? Well, the theory of tides was devised before we realized what the planet itself was made of, before we understood how gravity works, and before we knew anything about the deep ocean. The original idea was simple - solid Earth, liquid Ocean. Sea lifts up when moon passes over, with lots of rushing tidal currents in and out of coastal passes. Sea being fluid can move, land being solid cannot.
Now we know better. Our whole planet is influenced by the pull of the moon, and Earth has at its very centre a solid ball almost as big as the moon, then much internal liquid, being composed of a molten outer core, with the continents, which includes the land under the oceans, floating like icebergs on the surface. The moon distorts the inner liquid sphere making the resultant earth tides measurable even in the middle of the largest continent. When the moon is over the equator, the land rises about 55cm, over Moscow about 30cm and over NZ about 20cm. The land is the true tide - both the sea floor and what rises above the sea. Sea-tides rise and fall the same amount in the middle of the ocean as at the coast. The land rises and pushes the sea up; but not as much. Daily, as the moon transits overhead, the land rises more than the sea, and the sea drains away. Local tidal height is determined by difference in rise between land and sea. The massive movement of water 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. This is not what we were taught.
The closer the sea is to land, the more the ocean level changes, with the open sea changing least, because land and sea react differently to lunar gravity. While the liquid ocean follows the moon-controlled bulge exactly, land masses less flexible than water distort the curve. Small ocean islands are exposed summits of submerged mountains and have tides of about a 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 continents, the tide is usually low or near low when the moon is directly overhead.
Tidal motion lessens moving away from continents. There are exceptions. Tahiti has only one tide per day because the massive continent of Eurasia sits exactly on Tahiti’s opposite side of the earth, thus dampening the gravitational bulge. The larger continents cause the more drastic dampening on the distortion of the planetary crust. When the gravitational effect causes the continent to warp or tilt, ocean tides can exceed 10 or, in rare cases, 20 meters as the edge of the land lifts out of the sea. The reason Japan has the most earthquakes is because it is right next to a huge land mass. NZ has a lot of shakes because we are over the conjunction of two great tectonic plates.
Like water, land is always trying to move. Earthquakes are inevitable, because land finds other land in its way so seeks pathways around the blockage, as a stock car wends its way past other cars. It is time to look at the larger story of the tides in order to understand tectonics. It is time to look at the moon and sun in a deeper understanding of how they affect everything on this flexible earth. Gravity happens right now, to the whole planet at the same time. The bulge created by the moon is a collective change in molecular motion created by the gravitational of sun and moon. And the greatest change in molecular motion is on the part of the planet nearest to sun and moon - the surface.
Imagine pushing down on a soft rubber ball as you roll it along a table. The whole ball distorts. In that way the gravity effect of moon and sun mis-shapens Earth everywhere, right to the core and through to the opposite side of the planet. As with the egging of a ball, as earth rotates under the moon, the molecular effect remains at its maximum directly under the moon. If the moon vanished instantly, the effect would vanish in the very short time it would take the planet to bounce back to a more uniform, unbulged, shape. The gravity bulge from the moon remains perfectly stationary under the moon. The gravity bulge from the sun stays perfectly stationary under the sun.
So the tidal effect does not move water around the planet every day, although it appears to. It does not directly move water anywhere. In the open sea, when the moon daily passes over, the water stays where it is, moving in whatever direction it happened to be moving anyway. The effect of the moon is that the water molecules alter their motion, along with all the rest of the molecules of the planet, so the whole mass elevates by an amount relative to the diameter of the planet. The land does the same.
Although the effect of gravity is instant the resultant rising or sinking land mass takes time, and only after that is the water tide determined. The time of high and low tide varies depending on local topography. Where there is 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.
There are many little known studies that have reported a positive correlation between the earth tide and earthquake occurrence. Cantabrians have experienced thousands of land-tide aftershocks akin to ripples after a big splash, or surges after a kingtide high watermark. The changing distance separating moon and earth affects all tide heights. When the moon is at perigee, the range increases, and when it is at apogee, the range shrinks. Around equinoxes (March and September) the sun adds to the moon's tidal unfluence, and land and sea tides are usually greater. Every 7½ lunations (the full cycles from full moon to new to full), perigee coincides with either a new or full moon causing perigean spring tides with the largest tidal range. The second week in September this year was a good example. A lunation is 29.53 days. The next such dates are 20 February, 22 March and 18 April. Until then NZ should have a relatively quiet landscape.