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Two tides per day


pic: Tauranga Beach

Why two tides per day?
Over most of the earth there are two tides, one higher than the other. Yet there is only one moon, passing once above us. So why? The answer is up in space and deep within the earth, well away from the actual sea. The main tide is one in which the body of the earth extends towards the moon in a bulge as the moon passes overhead. Sometimes called the Earth or Land Tide, it has a period of about 12 hours. Stress forces from sun and moon are distributed throughout the entire volume of the earth. Although we cannot see the land tide because there is nothing to compare it to, the NZ land surface rises and falls 8 inches per day as the moon passes overhead. Australia moves vertically about 50cms per day and Indonesia about 60cm.

Earth tides are the reason for soil in the ground, as the constant heaving of the land over thousands of years grinds up rocks like cutting knives. By the seashore the same process enables shells to be ground into fine sand over millions of years. It is why ground and grind are connected. Otherwise we might ask, how do soil and sand get formed? The earth tides have been known since 1903. Petroleum engineers who monitor pressure in large underground reservoirs of petroleum watch the effects of these earth tides caused by the moon. The liquid-filled cavity in the rock below them is stretched and squeezed as the tides deform the solid earth, and the pressure rises and falls on their gauges twice each day. Surveyors too, know that a motorway bridge between a hill and an estuary, such as the Waterview Bridge (below) on State Highway 16 between Pt Chevalier and Te Atatu in Auckland, NZ, will twist and tilt each day (about 4cm and back) because the high side has more land mass that will move, compared with the lesser land in the lower estuary.

On the globe the vertical and east-west movement is maximum at 45° latitude and is zero on the equator and at the poles. As both are at 43deg latitude both north and south, Japan and Canterbury are close to maximum amplitude. It is one reason that Japan receives the most earthquakes worldwide, about 80% of all seismic events, and why NZ gets the second largest number.

Movement due to earth tides is both sideways and vertical. The tidal forces affect the entire earth, and although the movement of the solid earth is in the order of centimeters, there is so much earth and so little water that the water effect is magnified and more visible. The tidal bulges are very small, seemingly insignificantly small, compared to the radius of the earth. Yet over the huge area of one of the oceans, the tidal bulges alone still raise a huge amount of water. It can be shown that for a small land rise, the water-rise is large on a beach or bay.

Imagine a round bucket 10cms wide. Further imagine water in that bucket to a depth of 5cms. The formula for volume of a cylinder is v = Pi x r x r x h. If the radius of the container is 10cm and the height 5cm, then the volume is 3.142 x 100 x 5 = But if we raise the height by just 1cm then the volume becomes 3.142 x 100 x 6 = 1885.2 The difference is 20%. Applying this to the sea and land, for every cm rise in land there may be a 20% increase in the amount of water entering a bay, although the actual tide height will depend on shape of coastline, latitude and other factors. If we suppose a 2cm land rise then we have a 40% water increase. This translates to a high tide difference of 2.0m from just over half a metre for low tide, which is about the average in NZ.

We have the two tides because of the stretching of the earth and also the moving away from the water by the land, on both sides of the world at once. From the time the moon is above our heads, in 12 hours time the moon is beneath our feet, because the earth has rotated 180deg away from the moon. This would be the normal situation without tidal pulls from the moon

(1)        water + earth       -[middle of earth]-      water+earth                         

Now add the moon below in (2), at the side of the earth as seen from space. The water+earth on the moon's side try to move towards the moon along the earth-moon line. The land moves more because there is more of it. The water doesn't move much because there is less of it. The result, as the earth moves towards the moon and pulls away from the water, is low tide. On the other side of the earth this also happens, as the water+earth are pulled by the moon and therefore attempt to move through the middle of the earth towards the moon. But being further from the moon, the pull is less, and this becomes the lesser low tide of the day on the opposite side of the earth at that same time.

(2)              water.....+earth-[middle of earth]-     earth ..+water   [moon]

(3) In 12 hours time the situation is reversed because the same point on earth 12 hours ago is now, due to earth's daily rotation, on the other side and away from the moon, although the illusion is that the moon has moved.

[moon]   water...+earth-     [middle of earth]-earth ...+water                         

In all cases the earth pulls away from the water. The high tide in the water is due entirely to a low tide in the land. It has nothing to do with the earth's rotation or centrifugal forces. Old school-science books that told us that were incorrect. These bulges distort the shape of the solid earth, and also distort the oceans. It is not the end of the story because the oceans don't cover the entire earth uniformly. But there are land masses and ocean basins in which the water is mostly confined as the earth rotates and these create lags. The sun also has tidal pulls, but less than half that of the moon. These all incur variations that are smaller than the overall effects of the lunar gravitational forces.

Simple logic dictates that whatever is joined to land, i.e. water above and air even further above, must also be tidal. Otherwise some break in interfaces must be demonstrated and of course there is none. The atmosphere is much more fluid and compressible so its surface moves kilometers, in the sense of the contour level of a particular low pressure in the outer atmosphere. 'Tidal' means governed by 'time' and pattern. Patterned means cyclic. Cycles can be predicted, but just as meteorology has moved little from where it was 150 years ago predicting confidently one day ahead, so, too, western earthquake scientists seem slow to accept what eastern science has known for aeons - that Land Tides are governed by the moon and may therefore be predictable.

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