What causes two high tides and two low tides in a day?

Answer 1

= 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.

Answer 2

The moon's gravitational field pulls on it. When the moon is on one side of the earth, it pulls the sea water slightly towards the moon while the opposite side of the earth will bulge slightly away from the earth too. The other sides will experience low tide as some water are used to bulge away from the earth on the other sides.

Answer 3

Tidal forces are caused by the differential pull of gravity on a planetary body or moon.

Consider the Earth-moon system. The Earth and moon are orbiting around their common center of mass, which is a point that lies somewhere inside the Earth since the Earth is so much more massive than the moon. So since we are in orbit around this center of gravity, we don't feel the moon's gravitational force, much like how astronauts in orbit don't feel the gravitational force of the Earth, even though it is pulling on them all the same.

However, because the Earth is a large body, the pull of the moon is stronger on the side of the earth facing the moon and weaker on the opposing side. The pull of the moon at the center of the Earth falls in the middle. The effect of this differential pull is that the water on the near side wants to pull away from the Earth, and the Earth wants to pull away from the water on the far side. This net effect is a force that tends to stretch the earth and oceans, creates two tidal bulges, hence two high tides per day and two tides per day. The water of course is much more deformable than the Earth itself, so it is the oceans we see forming the tidal bulges. Low tide occurs at the location on the earth at right angles to the Earth - moon line, and high tide occurs along the Earth-moon line.

For more information on this, please see the link below:

• http://www.badastronomy.com/bad/misc/tides.html
• http://www.jal.cc.il.us/~mikolajsawicki/Tides_new2.pdf

Answer 4

The moon's gravitational force pulls on water in the oceans so that there are "bulges" in the ocean on both sides of the planet. The moon pulls water toward it, and this causes the bulge toward the moon. The bulge on the side of the Earth opposite the moon is caused by the moon "pulling the Earth away" from the water on that side.

If you are on the coast and the moon is directly overhead, you should experience a high tide. If the moon is directly overhead on the opposite side of the planet, you should also experience a high tide.

During the day, the Earth rotates 180 degrees in 12 hours. The moon, meanwhile, rotates 6 degrees around the earth in 12 hours. The twin bulges and the moon's rotation mean that any given coastal city experiences a high tide every 12 hours and 25 minutes or so.

Answer 5

There is a low tide and high tide because the moon is farther and closer to the earth at certain points. The moon controls the tides.

Answer 6

The moons gravitational pull on the water.

Answer 7

because it is