Ocean Tides   E-mail     Print     PDF

	Enlarge Image FIGURE 1. The gravitational attraction between Earth and the Moon (dimensions and size of the tidal bulge are exaggerated). (a) In this instance, the Moon is parallel to Earth's equatorial plane. Points X and X' indicate the change in position of a point on Earth's surface after half a lunar day (12 hours, 25 minutes). This position produces 2 equal (or semidiurnal) tides per 24 hours, 50 minutes at point X or X'. (b) In this instance, the Moon is inclined relative to Earth's equatorial plane. This position will produce 2 unequal (or mixed) tides at point X or X' per lunar day.
	Enlarge Image FIGURE 2. This image represents how the four phases of the Moon, as well as the Sun, produce the spring and neap tides. Neap tides occur when the trough of the solar tide is aligned with the crest of the lunar tide, or when the trough of the lunar tide is aligned with the crest of the solar tide. Spring tides occur whent he crests of both lunar and solar tides are aligned. The view is from the North Pole and the tides are greatly exaggerated. The red arrow indicates the direction of Earth's rotation.
	Enlarge Image FIGURE 3. The three principal types of tides: diurnal, semidiurnal, and mixed.

Excerpted from "Introduction to Oceanography" by David A. Ross, WHOI

The bodies of interest here are the Earth, Sun, and Moon. Earth and the Moon are about 400,000 km (nearly a quarter of a million miles) apart. Earth and the Sun are about 150 million km (about 93 million miles) distant. The mass of the Sun is about 27 million times that of the Moon. The Moon, however, is very roughly 400 times closer to the Earth than the Sun, so its nearness gives it a greater effect. The bottom line is that the gravitational effect between the Moon and the Earth is about twice that of the Sun.

The gravitational attraction among Earth, the Moon, and the Sun helps keep these bodies in their orbital relations to one another. However, without some counterbalancing force to gravity, Earth, the Sun, and the Moon would all be pulled against each other. This counterbalancing force is inertia, which is the tendency of a moving object to continue moving in a straight line. It is this force (sometimes called centrifugal force) that holds water in a bucket when you swing the bucket in an overhead arc. It is inertia that makes an automobile tend to go straight when you are trying to make a turn at high speed.

Without inertia, Earth, the Moon, and the Sun would crash together quickly. Without gravity, they would fly apart into the universe. Because these forces are in balance, these bodies have maintained their present orbits for millions of years. This is an overall balance, for either gravity or inertia may prevail briefly at various positions of Earth and the Moon in their orbits.

Gravitational attraction affects everything on Earth—solid earth, atmosphere, and water—but the results on the first two cannot be observed by the unaided eye. The effect on the oceans, however, is obvious: the daily tides, which are low, Earth-spanning wave forms that vary widely in height when they reach a coast, depending on the nature and location of the coastline.

The Moon
Because the Moon is so much closer to Earth than the Sun, its gravitational effect on the tides is about twice that of the Sun. Let us first consider the simple case of the gravitational attraction between Earth and the Moon (Figure 1). The gravitational attraction is strongest on the side of Earth that happens to be facing the Moon, simply because it is closer. This attraction causes the water on this “near side” of Earth (N) to be pulled toward the moon. Inertia is also active here, attempting to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the Moon, causing a “bulge” of water on the near side toward the moon.

On the opposite of Earth (far side,” F), the gravitational attraction of the Moon is less because it is farther away. Here, the inertial force exceeds the gravitational force, and the water tries to keep going in a straight line and thus moves away from the Earth, also forming a bulge.

Thus, the combination of gravity and inertia creates two bulges of water. One forms where Earth and Moon are closest, and the other forms where they are farthest apart. Over the rest of the globe, the two forces are relatively in balance.

Because water is fluid, the two bulges stay essentially aligned with the Moon as Earth rotates. Thus, a coastal area on Earth may experience two tidal highs when it passes through the bulges (the Moon side and the opposite side) within this time period. Of course, two lows are experienced when the shore is outside of the bulge area. As we shall see shortly, three possible tidal patterns actually exist.

It takes Earth about 24 hours to rotate once, relative to the Sun. But, because the Moon is moving with respect to Earth and Earth is spinning, it takes Earth a little longer to complete a rotation relative to the Moon—24 hours and 50 minutes. Thus, two daily tides occur 12 hours, 25 minutes apart.

The Sun
The gravity of the Sun also exerts a strong tidal influence on the ocean. The tidal bulge produced by the Moon, due to the Sun’s greater distance from the Earth. Remember that although the gravitational force between the two bodies varies directly with their mass, it also diminishes inversely with the square of the distance between them.

The effect of the Sun becomes especially important when the Sun and Moon happen to align with the Earth. The combined gravitational attraction of the two bodies produces a very strong tide that “springs forth” onto the coast, and thus is called a spring tide. (Spring tides have nothing to do with the season of the spring.) Spring tides occur roughly every 14 days, at the new Moon and full Moon (Figure 2).

Relatively weak tides, call neap tides, occur when the Sun and the Moon form a right angle with the Earth and essentially are opposed to each other. This also occurs about every 14 days, at first-quarter Moon and last-quarter Moon. The tidal range (the maximum water height at high tide, minus the minimum water height at low tide) is greater than the average during spring tides and less than the average during neap tides.

The tidal range worldwide averages between 1 and 3 m (3.3 to 9.8 ft.), but can attain 20 m (about 66 ft.) in some areas, such as the Bay of Fundy in Nova Scotia or the Gulf of California. Such exceptional tides are generally due to the geographic position and geometry of an area. A long V-shaped or U-shaped basin (such as the two examples cited), facing into the direction of an incoming tide, can generally compress and focus the incoming water to form higher tides.

When the tide is rising—water is coming in toward the shore—the current is called a flood tide. Eventually the incoming water attains high-tide level and starts to withdraw or fall. The current produced by the falling current is called the ebb tide.

There are three basic tidal patterns. Most areas have two high tides and two low tides a day. When the two highs and the two lows are about the same amplitude, the period is called a semidaily or semidiurnal tide (Figure 3). If the highs as well as the lows each differ in height, the pattern is called a mixed tide. Some areas, such as the Gulf of Mexico, have only one high tide and one low tide each day, which is called a diurnal tide. The U.S. West Coast tends to have mixed tides, whereas a semidiurnal pattern is more typical of the East Coast. Figure 1 shows why some areas have semidiurnal tides and others have mixed tides.

Nevertheless, this small rate can become significant over billions of years of geologic time.About 400 million years ago, the number of days in a year was close to 400, and the length of these days would have been about 22 hours, because the time it took Earth to make one complete revolution around the Sun has not changed significantly. (Scientists are able to determine these figures by counting the daily growth rings preserved in ancient coral--similar to the way trees have annual growth rings.)

There were other effects of a change in the speed of Earth’s rotation. It would have affected the Moon, because these two bodies are closely linked to gravity. If Earth spun faster, the Moon did too. For the forces of gravity and inertia to balance, Earth and the Moon must have been closer together. With the Moon closer to Earth, tides would have been much stronger than today, because of stronger gravitational attraction.

These stronger tidal currents would have created biological and geological conditions different from those common today. Some scientists have suggested that the increased tides due to the proximity of the Moon may have provided the impetus for the evolution of hard-shelled organisms.Soft-shelled organisms, living in shallow-water conditions, probably would have had difficulty surviving in such a rigorous environment. Geologically, vast inland seas, flushed once or twice daily by high tides, would have covered many of the low-lying areas of the world.