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How does the Moon control the tides?
Question Date: 2003-04-29
Answer 1:

Although it is often asserted that the moon "controls" the tides, this is really an oversimplification of the tidal system. In fact there are many factors which determine the tides, including the moon, the sun, the rotation of the earth, the geomorphology of the ocean basin, and the location of the particular spot where you're measuring the tide along that basin. All of these factors interact in a complex way to determine the specifics of the tide's characteristics at each location on Earth.

However, it is true that the basic global phenomenon of tides (ignoring local particulars and peculiarities) can be explained as a consequence of rotation and gravity, with the moon being the factor that tips the rotation/gravity system into one which produces tides.

Before you can understand how the moon's influence helps to create tides, you must first understand a little about the properties of water. Water molecules are attracted to each other and tend to stick together. (This is the reason why two droplets of water merge quickly into one when they touch each other, and why a single drop of water will sit as a little ball on the table rather than spreading out into a thin film). This property of water is called "cohesion". Because water is cohesive, it tends to act together like a single body or mass. You can think of it kind of like a weak blob of Jello: when you push or pull on one part, the rest of it moves too.

As the earth rotates, it is subjected to a centrifugal force which pulls the ocean water molecules away from the earth's surface. (This is just like when you spin around very fast, and your arms want to fly away from your body). Because the ocean water molecules stick together, this caused the entire ocean to bulge outward in the center, creating a kind of "mound" of water encircling the earth. Because of this, the world's oceans bulge around the equator, and get thinner toward the poles. (This means that the surface of the ocean is higher in the middle than on the northern and southern edges!)

If the earth simply rotated on its own axis, then even though there would be this bulge there would be no tide. However, because the moon is very close to the earth, its gravity exerts an effect on the rotational system of the earth, causing it to also rotates in tandem with the moon on a different axis. This rotation is like the rotation of a baton, with the earth and moon each representing on of the balls on the end of the baton. Because of this additional rotation, there is an additional centrifugal force exerted on the side of the earth that is away from the moon (on the "outside" of the two baton balls). Thus, the centrifugal force is greatest on the side of the earth that is away from the moon, and there is a greater bulge in the water on that side.

Now, remember that the earth also rotates on its own axis. Say that you are sitting at the beach in Santa Barbara watching the water level for the whole day. As the rotation of the earth carries California away from the moon, you would see the water begin to rise, because the spot you are watching is moving into the part where the ocean is bulging; as your beach moves into this bulge of rising water, it gets flooded a little bit, and you might have to move your blanket higher up to avoid getting it wet!

This explains why there is a high tide once a day. However, you have probably learned that there are usually two high tides every 24 hours. Where does the second one come from? The answer to that question is also that it is a consequence of the moon's gravity affecting the earth, but in a different way.

Whereas the first high tide is due to the fact that the moon and earth are linked into a rotating tandem by their gravity, the second high tide is a result of the moon's direct gravitational pull on the earth's oceans. When a region of the ocean is directly under the moon, the moon's gravity pulls the ocean away from the earth, increasing the size of the water bulge there. This is the second high tide!

Thus, there are actually two bulges in the oceans on the earth: one bulge on the same side as the moon (caused by the moon's gravitational pull), and the other on the opposite side (caused by the centrifugal force of the earth-moon tandem rotation). As California passes under the moon, the water along the California coast is pulled into a bulge, causing a high tide. As California moves away from the moon, the gravitational pull gets less and the water starts to go back down. However, as California passes to the opposite side of the earth the water starts getting pulled up onto shore again by the centrifugal force, causing a second high tide, and then as California moves away from this spot it goes down again, for the second low tide.

So, as you can see, it is clearly due to the moon that we have our tides. However, tides vary all over the world.

Answer 2:

Tides are created by the moon (and to some degree actually, the sun) because of the moon's gravity and the rotation of the earth. The moon has a large mass that exerts a large gravitational force or 'pull' on the earth. However, because of the shape and size of the earth, the portion of the earth's oceans that are facing the moon at any time are closer to the moon than the center of the earth, and therefore will feel a greater pull, and will be pulled outward towards the moon forming a bulge, or high tide. Likewise, the portion of the earth's oceans that are on the opposite side of the earth from the Moon are farther away from the moon than the earth's center, and the center of the earth will feel the greater pull and be pulled away from the ocean on the side opposite the moon. This forms a second bulge or high tide on the opposite side of the earth. As the earth rotates on its axis, the tides will move on the surface of the earth so that the high tide remains fixed on the side directly facing, and directly opposite the moon.

Many other factors can affect the path taken by tides in each ocean basin, such as the shape of surrounding landmasses, but the dominant effect is from the moon's gravitational pull.

Answer 3:

The tides are generated by the gravitational pull of the moon on the Earth. It is tempting to think that the moon pulls on the ocean, which makes it rise slightly - and this rising of the ocean is interpreted as a high tide by folks on the Earth. In this picture, as the Earth rotates under the moon, the high tide would appear to move - and you would see a high tide every 24 hours. It turns out, however, that high tide occurs every 12 hours! So this theory of the tides is definitely wrong.

The way this really works is a little tricky. It is helpful to try to draw a picture of what I will try to describe for you. It might also help to go get a book on the solar system that has a picture of this drawn for you so you can follow along. It's not that it's too hard to understand how the tides work; it's just that what is happening is very geometrical and hard to describe without pictures. So if you don't understand, it is definitely my fault.

You probably heard that the gravitational force gets weaker as you get farther away. This means that, things close to the moon get pulled with a higher force than things farther away from the moon. It is helpful here grab a piece of paper and draw a circle to represent the Earth, inside a larger circle that represents the surface of the ocean. Now next to those two concentric circles, draw a smaller circle to represent the moon. As you can see from this diagram, the part of the ocean closest to the moon gets pulled with a stronger force than the Earth, which gets pulled with an even stronger force than the part of the ocean farther away from the moon.

If the gravitational force causes things to shift so that they are closer to the moon, the part of the ocean closest to the moon gets pulled farther than the earth which gets pulled farther than the far side of the ocean. The result is that the oceans look like they have been pulled away from the Earth on BOTH the side closest to the moon and the side farthest from the moon, implying that high tide occurs on opposite ends of the Earth at the same time. Now, from the point of view of people on the Earth, the high tide appears to move as the Earth rotates, with a high tide occurring every 12 hours.

As often happens in science, the details are a little more complicated. The basic picture, however, is correct.

Answer 4:

The tides are due to the gravitational pull of the Moon and the Sun on the Earth. Because the gravitational force decreases as the distance squared, the part of the Earth that is closer to the Moon or the Sun is pulled a little extra and a bulge is created. In addition, the other side of the Earth is pulled a little less than the center of the Earth so another bulge is created on the back side. As the Earth rotates about its axis, these bulges move along the surface of the Earth and causes what we call tides.

Answer 5:

The tidal effect occurs because of the relative closeness of the Earth and Moon. The easiest way to picture the tidal effect is to consider the gravitational effect of the Moon on the earth. In particular consider the differential gravitational tug between a point on the surface of the Earth on the opposite side of the Earth- moon line and a point at its antipode (that is, on the Earth surface directly under the moon). A picture is worth a 1000 words but email media does not allow this. But if you look up tidal effects in almost any geology or astronomy book you will see the picture. At any rate,the near-side bulge on the earth is a direct consequence of the greater gravitational attraction by the moon compared to the center of the Earth. after all, the near side sub-lunar point is closer to the moon than the center of the earth.. The bulge at the opposite side results from the weaker attraction of the far side Earth point compared to the center... A lot of people have trouble seeing this, but if you draw a diagram with force vectors of the appropriate lengths(qualitatively) and then add the opposite of the force of attraction between the moon and the center of the Earth, you will see that directly below the moon the residual force or the tidal force points directly towards the moon whereas on the opposite side the net force points directly away. The ocean responds almost instantaneously to these forces because the viscosity of water is so low.

Answer 6:

The moon (or any gravitational body, for that matter) exerts agravitational pull that is stronger the closer you are to it. Since the Earth has a physical size, the gravitational pull on the Earth is stronger on the side facing the moon than it is on the side facing away from the moon.

Because the Earth is being pulled on more strongly on one side than from the other, it is effectively being stretched by the difference in the magnitude of gravitational force from the moon. Most of the Earth, of course, is solid (it is made of rock), but the oceans are not, and, as a result, they are much easier to stretch than the solid Earth. Therefore, the liquid is pulled toward the moon more strongly on the side facing the moon, causing it to accumulate there, while the side opposite the moon, not feeling the moon's pull as strongly, stays on the side opposite the moon. Because the ocean is still held by the Earth's gravity as well, it does not come off the surface, and instead sea level on the side of the Earth facing the moon and on the side opposite the moon rises. At the same time, sea level falls due to a relative lack of water halfway around the Earth from the moon. This change in sea level is what we call tides.

Also of note: the Sun has about the same tidal influence on the Earth as the moon does.

If you are good at math, here is the equation that determines gravity:
F = G*m*M/(r2),
F = Force due to gravity,
G = a constant which has to be looked up
m = the mass of the moon
M = the mass of the Earth
r = the distance between the Earth and Moon.

The Earth has a diameter of 12,800 kilometers. I think that the moon is about 100,000 kilometers away, though I could be wrong on that.

Answer 7:

The moon affects the tides depending on its position around the earth. For example, if the sun, the moon and the earth are all lined up in a straight line (180 degree angle) then they form tides call SPRING TIDES (they do not necessarily occur during spring). The Earth's tides are about the same whether the Sun and Moon are lined up on opposite sides of the Earth (full moon phase) or on the same side (new moon phase). During spring tides you will notice higher and/or lower tides than normal. When the earth, sun and moon form right angles (occurs when the moon is at the first quarter or last quarter phase) to each other they form tides called NEAP TIDES. Neap tides are generally weak tides.

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