= Why Two High Tides a Day? = ---- Long before Newton, it was obvious that the moon exerted a force on the oceans. After all, day in and day out, year after year, ever since the oceans formed, there have been two high tides a day, once with the moon at its zenith and once again nearly 12 hours later. When the concept of universal gravitation was introduced, it quantified the forces involved and neatly accounted for the ocean tides. Today, anyone can tell you that the tides are caused by the "gravity" of the moon. Or are they? The oceans form a thin, incompressible, but easily deformable layer on most of the earth's surface. One might expect that the moon's gravitational force would pull the entire ocean layer towards it, distending the layer (producing a high tide) in the direction of the moon on the near side of the earth, while simultaneously compressing it (producing a low tide) on the opposite side of the earth (see Figure 1, not to scale). This model is half right and half wrong: half of all high tides are generated in exactly this way, but instead of there being a low tide on the far side of the earth, there is also a high tide. There must be some other explanation.
Indeed there is. The motion of the earth-moon system, taken in isolation, is a classical two-body problem where each body exerts an attractive force (gravity) on the other. Solving the equations of motion, one finds that the two bodies rotate about their common centre of mass, like a rigid asymmetric dumbbell spinning around an axis perpendicular to the bar. As shown in Figure 2, the centre of mass of the earth-moon system turns out to be inside the earth, about three-quarters of an earth radius from its center, along the line joining the earth and moon.
The earth, therefore, orbits the center of mass in a tight circle (almost) while the moon orbits in a large one. This orbital motion causes the earth to experience a centrifugal (pseudo-) force, which distends the ocean layer in the direction away from the moon, not unlike what happens to the water in a pail when you swing it in a circle. Combining this orbital effect with the direct gravitational pull of the moon explains the simultaneous high tides on opposite sides of the earth: on the near side the direct pull dominates and causes the oceans to bulge in the direction of the moon; on the far side the centrifugal effect dominates and causes the oceans to bulge in the direction away from the moon. As the earth spins on its axis, a given seaside location will experience a high tide when the moon is at its closest, and then another one about 12 hours later when it is at its furthest. Reality, of course, is never quite so simple. Although the sun is much farther away from the earth than the moon, it is also much more massive, so its gravitational pull on the earth is relatively large (almost half of what the moon exerts). It therefore plays a significant role in determining both the timing and strengths of the tides. So, are the tides caused by the "gravity" of the moon? In large part, yes. But the gravity of the earth is as much responsible for the two-body rotation as that of the moon. The tides are caused by the combined gravitational forces of the moon, earth, and sun. Here is a simpler explanation: Looking down from the North Star, the Earth rotates anticlockwise by about 180 degrees in 12 hours. During that same period of time, the moon travels on its orbit around the earth clockwise by about 180 degrees. The result is that the moon crosses the same earth longitude (but not necessarily the same latitude) every 12 hours, or twice every day. This results in 2 local high tides. The 2 low tides happen between times.
a diurnal tidal pattern.and 2 high tides and 2 low tides in one day is semi-diurnal.Neap tidea high tide is where the water is really high and a low tide is where the water is low and you have to be carfullThe highest of the high tide is called a spring tide and the lowest of the low tide is called a neap tide.
The moon's gravity pulls the water away from the earth on the side closest to it. Also the earth is pulled away from the water on the far side. These are the high tides. The sides that are perpendicular from the moon are the low tides. Because the earth spins once in a day whereas the moon takes a month to orbit this means there are 2 high and 2 low tides each day.
Yes, normally in a 24 hour day there are 2 high and 2 low tides. I think that what you are asking might be can there be a high tide and then at the change of tide, a second high tide instead of a low. that answer is also yes. I do a lot of recreational fishing in salt water and have experienced it several times, although mostly 2 consecutive low tides. Tides are caused by the gravitational pull of the moon but can also be severly affected by wind. such as strong east wind blowing water out on low tide, then continuing to blow it out when tide is supposed to be coming in (high tide).
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