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). 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), 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. Figure 13.22 The tidal force stretches Earth along the line between Earth and the Moon.
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