What an “attractive” question! The Moon and the ocean share a mutual gravitational attraction; so, the Moon helps tides happen. When you first think about this, it makes perfect sense that the Moon’s gravity tugs on the ocean enough to make a small bulge form in the sea below the Moon. As Earth turns, ocean shores pass through the bulge, and we observe tides.
Tidal tug-o’-war
The oceans account for a tad more than 0.02 percent of Earth’s total mass. So, one might describe this as a small bulge. You can’t really see it in, say, a picture from space. But, of course, it’s enormous. The world ocean carries about 1.4 quintillion tons (1.4 x 10(18) tons, a billion-billion) of seawater. Those trillions of cubic kilometers of seawater are sloshed around easier than you can make a ripple in a bathtub. And, if you’re sitting on an ocean shore, the tides are pretty easy to detect.
The Moon does some tidal tugging, but so does the Sun. When the Moon and Sun are in a line with Earth, the tides are higher. We call those spring tides. When they’re at right angles in outer space, the tides are lower. We call those neap tides.
The Moon is much, much smaller than the Sun, about a hundred millionth of the Sun’s mass. Its gravitational pull would be diminished by that enormous fraction, but the Moon is also much, much closer–about 400 times closer to Earth than the Sun. So, the Sun’s tidal influence is about half (46 percent) of the Moon’s.
Observe a lamp or streetlight. When you get twice as far away, the light is a quarter as bright. So it goes with gravity. (Gravity goes with 1 over the distance between objects squared.) The two enormous masses of the Sun and the Moon, positioned as they are at enormous distances, conspire to make tide times a bit complicated, but very predictable.
It’s high tide somewhere
As you think about this tidal bulge for a few more moments, it may surprise you to realize that a second bulge in Earth’s ocean forms on the side of the world farthest from the Moon. Along with gravity, Earth is spinning. (Uh … thanks, Bill.) That spin causes the ocean to bulge as well. With no other influence, the ocean’s bulges would be biggest near the equator and thinner near the North and South poles. But the bulge, due to Earth’s spin, is in the grip of the Sun and Moon’s gravity. As the saying goes, it’s five o’clock somewhere. Well, the ocean is bulging high-tide-wise somewhere all the time as well–and on opposite sides of Earth. And the effect is more noticeable the farther you go from the equator, where the slopes of the bulges are steeper.
And there’s more: The Moon orbits Earth all right, but the plane of its orbit is not the same as the plane of Earth’s equator. Instead, the Moon’s orbit is tilted. So think about this: Most of the time, one tidal bulge forms north of the equator, and the other tidal bulge on the other side of the world is south of the equator.
Furthermore, the Moon is going around Earth in the same direction that Earth is spinning. Earth has to turn an extra little bit, about 12 degrees of arc, to have the Moon in the same position relative to any given place on Earth. So, the tides nominally lag behind our 24 hours of daylight by about 50 minutes. This all adds up and has to be taken into careful account by algorithms created to predict tides, which are known as tide tables.
What’s a tidal pool?
If you’ve ever wandered on the shore, if you’ve gone beach combing, as it’s called, you’ve come across entire ecosystems that came to be because of the tides. In these intertidal zones, the beach is high and dry part of the day. At other times, the whole zone is underwater. So animals and plants that live there on the beach and in the tidal pools have developed ways to stay wet inside when it’s dry outside. Sand crabs burrow. Barnacles close their shells up tight. When the tide is high and their world is wet, clams are filter feeders. They siphon water through their digestive systems, gathering tiny food particles from the flow. When the tide goes out, many clams close up tight. With no food available and no longer submerged, their shells protect them from predators. Clams came to be over billions of years, so they probably wouldn’t bother with all this, if the Moon hadn’t been up there tugging up the tide for quite some time. Shore birds come looking for a meal when the tide’s out. For a few hours, they can see creatures of the sea exposed to the Sun. See?
The tides we measure on shore are affected by other factors as well: local seafloor shapes, runs of rivers, and which direction–west, east, south, or north–a beach faces. But it all starts in outer space. How mysterious it must have all been long ago, without belief in inverse square laws and massive distant objects beyond Earth. The Moon’s gravity tugs up tides in the sea–part of the graceful dance of our spinning worlds…. Tides are science.
Here are something needed to explain:
from http://www.es.flinders.edu.au/~mattom/I … ure11.html
As the earth revolves around the gravitational centre of the sun/earth system, the orientation of the earth´s axis in space remains the same. This is called revolution without rotation (see the animation for further explanation).
The tide generating force is the sum of gravitational and centrifugal forces. In revolution without rotation the centrifugal force is the same for every point on the earth´s surface, but the gravitational force varies (Figure 11.1). It follows that the tide generating force varies in intensity and direction over the earth’s surface. Its vertical component is negligibly small against gravity; its effect on the ocean can be disregarded. Its horizontal component produces the tidal currents, which result in sea level variations (Figure 11.2).
The gravitational force exerted by a celestial body (moon, sun or star) is proportional to its mass but inversely proportional to the square of the distance. The Sun’s mass is equivalent to some 332,000 Earth masses, while the mass of the Moon corresponds to only 1.2 percent of the mass of the Earth. The mean distance Sun -Earth is 149.5 million km, the mean distance Earth – Moon only 384,000 km. If the gravitational force of the Sun and Moon are compared, it is found that the Sun’s enormous mass easily makes up for its larger distance to Earth, to the extent that the gravitational force of the Sun felt on Earth is about 178 times that of the Moon. As a result the Earth’s orbit around the Sun is not seriously distorted by the Moon’s movement around the Earth.
However, as is evident from Figure Figure 11.1, tides are not produced by the absolute pull of gravity exerted by the Sun and the Moon but by the differences in the gravitional fields produced by the two bodies across the Earth’s surface. Because the Moon is so much closer to the Earth than the Sun, its gravitational force field varies much more strongly over the surface of the Earth than the gravitational force field of the Sun. Quantitative analysis shows that the differences of the gravitational forces across the Earth’s surface are proportional to the cube of the distances Sun – Earth and Earth – Moon. As a result the Sun’s tide-generating force is only about 46% of that from the Moon. Other celestial bodies do not exert a significant tidal force.
A detailed webpage having animations: http://home.hiwaay.net/~krcool/Astro/moon/moontides/