Spring tides occur during the new or full moon, and neap tides occur at half-moon. The extremes of both high and low tides are affected. (Recall that the tidal forces cause bulges on both sides.) Figure 13.22(c) shows the relative positions for the smallest tides, called neap tide s. During spring tides, Earth, the Moon, and the Sun are aligned and the tidal effects add. Depending upon the positions of the Moon and Sun relative to Earth, the net tidal effect can be amplified or attenuated.įigure 13.22 illustrates the relative positions of the Sun and the Moon that create the largest tides, called spring tide s (or leap tides). However, as we show later in an example, the tidal effect of the Sun is less than that of the Moon, but a significant effect nevertheless. ![]() The gravitational attraction of the Sun on any object on Earth is nearly 200 times that of the Moon. In addition to the Moon’s tidal forces on Earth’s oceans, the Sun exerts a tidal force as well. The actual timing of the tides is complicated by numerous factors, the most important of which is another astronomical body-the Sun. Since the Moon also orbits Earth approximately every 28 days, and in the same direction as Earth rotates, the time between high (and low) tides is actually about 12.5 hours. In 6 hours, the near and far locations of Earth move to where the low tides are occurring, and 6 hours later, those locations are back to the high-tide position. But Earth rotates (in the direction shown by the blue arrow) approximately every 24 hours. Relative to the Moon, the bulges stay fixed-along the line connecting Earth and the Moon. If Earth were not rotating and the Moon was fixed, then the bulges would remain in the same location on Earth. Why does the rise and fall of the tides occur twice per day? Look again at Figure 13.22. These forces would diminish to zero as you approach Earth’s center. ![]() In this figure, the tidal forces are shown at the ocean surface. This defines the baseline sea level and here we consider only the much smaller tidal bulge measured from that baseline sea level.)įigure 13.23 The tidal force is the difference between the gravitational force at the center and that elsewhere. We saw earlier that Earth bulges many kilometers at the equator due to its rotation. (Note that the change in sea level caused by these tidal forces is measured from the baseline sea level. In Figure 13.23, this difference is shown at sea level, where we observe the ocean tides. The tidal force can be viewed as the difference between the force at the center of Earth and that at any other location. The net force on Earth causes it to orbit about the Earth-Moon center of mass, located about 1600 km below Earth’s surface along the line between Earth and the Moon. Tidal variations of the oceans are on the order of few meters hence, this diagram is greatly exaggerated. It is the difference between the gravitational force from the far side to the near side that creates the tidal bulge on both sides of the planet. This is why a tidal bulge appears on both sides of Earth.įigure 13.22 The tidal force stretches Earth along the line between Earth and the Moon. The magnitude at the center of Earth is between these values. Hence, the gravitational force is greater on the near side than on the far side. One side of Earth is closer to the Moon than the other side, by a distance equal to Earth’s diameter. In Figure 13.22, we are looking “down” onto Earth’s North Pole. Let’s consider the effect of the Moon first. This is caused by a combination of Earth’s rotation about its axis and the gravitational attraction of both the Moon and the Sun. ![]() If you live on an ocean shore almost anywhere in the world, you can observe the rising and falling of the sea level about twice per day. ![]() They are responsible for the internal heat that creates the volcanic activity on Io, one of Jupiter’s moons, and the breakup of stars that get too close to black holes. These same tidal forces are present in any astronomical body. Ocean tides are the result of gravitational tidal forces. But the work of Newton is considered to be the beginning of the true understanding of the phenomenon. The origin of Earth’s ocean tides has been a subject of continuous investigation for over 2000 years. Describe how tidal forces affect binary systems.Describe how neap and leap tides differ.Explain the origins of Earth’s ocean tides.By the end of this section, you will be able to:
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