Figure 1.7 Tides created by the moon. The arrows (top) show the strength of the gravitational force from the Moon at different places on the Earth. The resulting differences in force creates the tides, as shown below.
mass, located about 1,000 miles below the Earth's surface in a line between the Earth and the Moon. Like two dancers holding each other and whirling around, this orbital motion creates a centrifugal force on Earth's oceans, acting away from the Moon. At the same time, the Moon's gravitational pull attracts different parts of the Earth toward it by different amounts (figure 1.7). Combining the outward centrifugal force with the inward gravitational force, the tides occur as ocean water is dragged toward or away from the Moon.
There is symmetry in these interactions. Different ocean water drains from halfway around the globe either toward the Moon or away from it, simultaneously creating equivalent high tides on two sides of the Earth (and two low tides in between them). This is consistent with my observations on Mount Desert Island that there are two high tides and two low tides each day. The high tides occur when the Moon is high in the sky or when it is beneath our feet. The low tides occur when the water near us is being pulled along the Earth's surface toward or away from the Moon—when the Moon is near the horizon.
The Sun's gravity also has a significant effect on Earth's tides. After all, the Sun is 27 million times more massive than our Moon. However, the greater distance between the Sun and the Earth—nearly 400,000 times the distance between the Moon and the Earth—significantly diminishes the Sun's tidal effects here. Combining the effects of the Sun's greater mass and greater distance leads to the result that the Moon creates tides about twice as high as those created by the Sun.
We are now in a position to understand why the height of the high tide changes throughout the cycle of lunar phases. When the Earth, Sun, and Moon are in a straight line, at either new moon or full moon, the Sun and Moon pull on the oceans in the same direction. Note that they do so even when on the opposite sides of the Earth because of the symmetry of tides created by each body. At new and full moon, the range from high tide to low tide is especially high. These are the spring tides.
When the Sun and Moon are pulling on the Earth in perpendicular directions, say when the Moon is just rising at noontime (when the Sun is highest in our sky), the tides they create partially cancel each other out. This occurs because the Sun and Moon are pulling the oceans in competing directions at the same time. It is most noticeable at the first and third quarter lunar phases. Tides on these days, when the range between high and low tide is especially low, are called neap tides. We can now see how the changing distance from the Moon to the Earth is only part of the tidal story: when the Moon is closest to us at neap tide, the tidal range is still lower than when the Moon is farthest from us at spring tide.
Finally, let's put the Earth's rotation back in the picture. The Earth is spinning eastward roughly thirty times faster than the Moon is orbiting around us. This motion pulls the high tide from beneath the Moon. The Moon's gravity pulls the water in the high tide closest to the Moon back toward it. This means that the high tide flows westward over the Earth's surface due to the Moon's attraction. The high tide is therefore not directly under the Moon. Furthermore, the water is stopped by the islands and continents. This process does two things: first, the high tide nearest the Moon pulls the Moon ahead in its orbit (figure 1.8). This gives the Moon extra energy, causing it to spiral away from the Earth. It is presently receding at a rate of nearly two inches (four centimeters) a year. Second, the westward-moving high tides push on the eastward-spinning continents, thereby slowing the Earth down. Based on this physical and geological evidence, geologists have determined that when the Earth formed, it was spinning five to six times faster than it is now. The day was originally somewhere between four and six hours long. The Earth's rotation rate is presently slowing down by about a thousandth of a second per century.
Learning this often generates the question, Will the Moon ever leave the Earth completely? It will not, because the Moon moves away in proportion to the Earth's slowing rotation rate. Eventually, perhaps some 20 billion years from now, the Earth will be spinning at the same rate as the Moon is orbiting it. Thereafter, the Moon will remain at a fixed distance from Earth with a high tide directly between the centers of the two bodies. The Moon will then appear fixed over one side of the Earth, never to be seen on the other side. This extrapolation into the future is moot, however, because the Sun will have stopped shining long before this and, in all likelihood, will have swallowed the Earth and Moon in the process.
Fifty Commonly Cited Incorrect Beliefs About Astronomy
On every topic in astronomy, indeed on nearly every topic of science, people hold myriad incorrect beliefs. The common, incorrect ideas about astronomy I have explored in this chapter literally describe the tip of the iceberg. Over the past decade, my introductory college astronomy students provided me with lists of incorrect beliefs they had
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