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Some Misconceptions C lose to H ome

We are exposed to more information about the solar system (the Sun and everything that orbits it, namely the planets, moons, asteroids, meteoroids, and comets) than about more distant space objects. Most of us observe solar system objects like the Moon, shooting stars, comets, and planets in the night sky, and astronomical observatories and spacecraft provide tantalizing news snippets about these and other phenomena in our cosmic neighborhood. This torrent of information carries many opportunities to develop misconceptions about astronomy. In fact, over two thirds of astronomical misconceptions pertain to objects in our solar system, with the rest centered around more exotic, distant objects in space, like black holes, galaxies, and quasars.

On one hand, it seems obvious that we manufacture so many incorrect beliefs about the solar system—after all, it is comprised of the most accessible elements of astronomy. On the other hand, it seems odd that the solar system is a source of so many wrong ideas—after all, astronomers understand so much more about its objects than about objects more distant and strange. As we will see in the next two chapters, the reasons for our misconceptions are nearly as varied as the features in the night sky.

LLoosen Your Asteroid Belt

The late 1960s and early 1970s were NASA's heyday. Successful missions to the Moon, Venus, and Mars laid the groundwork for Pioneer 10 to be launched on March 2, 1972, en route to Jupiter and beyond. To get to these distant bodies, the 570-pound, io-foot-diameter spacecraft had to cross the gulf between Mars and Jupiter, a region containing the asteroid belt. What dangers did it face in the belt, and how did NASA equip it to cope with them?

Except for NASA scientists and a few other astronomers who studied asteroids, most scientists recognized the asteroid belt only as a relatively obscure region of the solar system. Astronomers did know that the asteroids are primarily rocky and metallic debris left over from the formation of the solar system 4.6 billion years ago. Some asteroids, like Ceres, are spherical and up to one quarter the diameter of our Moon. But the vast majority are just a few meters across.

The Belt According to George Lucas

Before answering the question of the dangers that might have befallen Pioneer 10, let's consider the image of the asteroid belt in the popular mind today. This image dates from 1980, when the film The Empire Strikes Back was released. In it, we were treated to the memorable scene of the Millennium Falcon dodging asteroids to evade the pursuing Empire spacecraft. The asteroid field shown in the movie allegedly existed far from our solar system. Nevertheless, virtually overnight, most of the people on Earth conceived of our own asteroid belt as strewn with asteroids so big and so close to each other that you could practically jump from one to another.

George Lucas's portrayal of closely packed asteroids served an important cinematic purpose, but it also created a belief in the minds of countless millions (even billions) of people that is absolutely wrong. Consider what would happen if the Star Wars scenario were correct: each of those closely packed asteroids would be gravitationally attracting all the asteroids around it. If they were as large as portrayed in the movie, within a few thousand years of forming the belt, the asteroids would have smashed each other into dust or smashed into each other to form a single large body (which is how the planets in our solar system actually did form), or they would have had many near misses. Such near misses often have the effect of speeding one body up while slowing the other down. So an asteroid belt that didn't pulverize itself or form one larger body would quickly evaporate because after the near misses, the higherspeed asteroids would fly away from the rest. Therefore, clumps of asteroids as shown in the movie should not be found in the real asteroid belt.

But this is just a scientific theory. There might be other effects not accounted for here, and George Lucas could be right after all. What is the observation-based reality of the asteroids?

Passage to Jupiter

Let's return to 1972 and Pioneer 10. Most astronomers who studied the asteroid belt back then were convinced by scientific arguments like the one above that the asteroid belt must be nearly empty. But they were human like the rest of us and prone to asking "what if?" questions, like "What if there are zillions of pieces of debris in the asteroid belt that we can't yet see that might damage sensitive equipment or perhaps completely destroy the spacecraft?"

Pioneer 10 didn't have the technology to detect or avoid asteroids. Hurtling through the asteroid belt, it was essentially a very expensive interplanetary bullet. Its twin spacecraft, Pioneer 11, developed to travel to Saturn, was held in storage until the scientists and engineers were certain that Pioneer 10 had survived the belt. For seven grueling months, all they could do was watch and wait. Finally, Pioneer 10 emerged on the other side of the asteroid belt completely unscathed. On April 5, 1973, Pioneer 11 was launched.

Our scientific understanding of the asteroid belt has advanced a great deal since those prehistoric days. Observations and refined theories confirm that Star Wars-like clumps of big asteroids do not exist, although we have discovered pairs of asteroids, one orbiting the other. Indeed, the asteroid belt is so empty that whenever we send spacecraft into it, we actually go out of our way to make sure that they do encounter asteroids, so we can study them further.

Even though astronomers have known since 1973 that the asteroid belt is nearly empty space, since 1980 almost everyone else has thought of it as chock full of matter. The reason is very simple: since the 1973 press releases announcing Pioneer 10's successful passage through the asteroid belt, there has been virtually no scientific mention of the belt in the media. However, numerous films and television shows have portrayed it as Star Wars did. And besides, the word "belt" in "asteroid belt" evokes the image of a solid, or nearly solid, collection of matter. In reality, asteroids are typically separated from each other by several million kilometers.

The Seasons of Your Discontent

Our practice of protecting our beliefs also creeps into perhaps the most notorious misconception in astronomy. Many, perhaps most, people have an incorrect model in their minds of what causes the seasons. This by itself doesn't make this issue notorious. That happened because the crew filming the 1988 documentary Private Universe asked Harvard faculty and graduating Harvard seniors to explain why the seasons change. Here these folks were, capped and gowned, feeling justifiably proud of their abilities and accomplishments, although the vast majority of them (twenty-one out of twenty-three) gave explanations of the seasons on film that were dead wrong.

What Doesn't Cause the Seasons?

The common explanation for the seasons originates in our everyday experience that the closer we are to a fire, the warmer we feel. The Sun is hot. Therefore, the closer we are to it, the warmer we should be.1 At its closest, the Earth is about 147 million kilometers (91 million miles) from the Sun, while at its farthest, it is 150 million kilometers (93 million miles) away. So there is not much more than a 2 percent change in distance. Couldn't this account for the change in temperature, consistent with common sense?

Two things suggest otherwise. First, if the changing distance from the Earth to the Sun caused the seasons, then we would expect the entire world to have the same seasons at the same time. I wrote this on an icy cold January 26th in the dead of Maine winter. A quick call to a friend in Perth, Australia confirmed that they were enjoying a warm summer just then. Indeed, southern hemisphere seasons are exactly the opposite of those in the northern hemisphere. Second, the Earth is closest to the Sun on January 3rd each year, in the middle of winter in the northern hemisphere—one of the coldest times of the year.

If these arguments aren't convincing, we can create a model version of Earth on which the seasons are only due to the changing distance from the Earth to the Sun to see how it compares with our world. Changes in heating just from changing distance to the Sun would occur if the Earth's rotation axis were precisely perpendicular to the plane of its orbit around the Sun, called the ecliptic (figure 1.1a). Picture a skewer inserted through the Earth's rotation axis. For the real Earth, that skewer would be tilted over at an angle of 23 1 /2° from the line perpendicular to the ecliptic. Now tilt the Earth so that the skewer is exactly perpendicular to that plane. If the Earth were

1 Of course, if you also think that the Earth's orbit around the Sun is circular, you have to reconcile our changing distance from the Sun with a circular orbit. Perhaps the Sun is not in the center of the circular orbit. In reality, observations by Tycho Brahe (1546-1601) led Johannes Kepler (1571-1630) to discover that all planets orbit the Sun elliptically.

Figure i.ia Earth's orbit around the Sun as it would appear if its rotation axis were perpendicular to the plane of its orbit.

Figure i.ib Earth with its axis correctly tilted 23 1/20. Note that the axis points in essentially the same direction throughout the orbit.

oriented this way, the Sun would rise and set in the same places every day of the year as seen from any place on the planet. Furthermore, everyone would have twelve hours of daylight and twelve hours of darkness every day. Therefore, ignoring clouds, any given place would receive the same amount of heat from the Sun every day. (Places at different latitudes would receive different amounts of heat for either this or the real version of Earth, as we will explore shortly. This fact has no bearing on the present argument, however.) On an upright Earth, no change in average temperature would occur throughout the year at any fixed place except for the change in heat we receive from the Sun due to our changing distance from it. That is, the closer we were to the "fire," the warmer we would feel. Furthermore, both hemispheres of Earth would have the same seasons at the same time.

But how much hotter would summer be than winter? If the distance to the Sun varying by 2 percent were the only cause of changing heat on the Earth's surface, then the Earth would be 7°F (4°C) warmer in summer, when we would be closest to the Sun, than in winter, when we would be farthest from it. So assuming that the seasons occur because of our changing distance to the Sun clearly creates inconsistencies with the range of temperatures that we actually experience and with the opposite seasons in the two hemispheres.

Building Reality

Two other astronomical features could conceivably cause the seasons: the Earth's rotation rate and the tilt of its axis. The rotation rate determines how long its takes the Sun to appear to go around the Earth, that is, the length of the day. If the Earth rotated at different rates during each day or at different rates at different times of the year, then the number of daylight hours could conceivably vary. The longer the Sun is "up" during a day, the more time it has to heat that part of the Earth. Perhaps the seasons are caused by the Sun being "up" fewer hours during the winter than during the summer. We know that there are different numbers of hours of sunlight at different times of the year. Therefore, we might expect (correctly) that those months during which the number of daylight hours is longest will be the warmest. But is this due to a change in the rate at which the Earth spins?

The problem with explaining the seasons by having the Earth change its rate of rotation is that such changes are not observed; they would coincide with massive, worldwide earthquakes every few hours or few months as the planet sped up or slowed down.

This leaves the possibility that the seasons are somehow related to the observed fact that the Earth's axis is tilted compared to the ecliptic. Perhaps this tilt causes the change in the number of hours of daylight throughout the year. The angle that astronomers use to describe the Earth's tilt is the angle from a line perpendicular to the ecliptic, as shown in figure 1.1b.

Contrary to intuition, the direction in which the Earth's rotation axis points does not change throughout the year. This is because the Earth's rotation stabilizes the planet like a giant top so that the axis running between the poles always points in one direction as we orbit the Sun. As figure 1.1b shows, during half the year the Earth's northern hemisphere is tilted toward the Sun, and during the other half of the year the southern hemisphere is tilted sunward.

This explanation causes many people to leap to the obvious, but incorrect, conclusion that when the northern hemisphere is tilted toward the Sun it is closer to the Sun and therefore warmer than the southern hemisphere, hence creating the seasons. This idea breaks down when we calculate the temperature difference between the two hemispheres caused by their different distances from the Sun. That difference is only about two hundredths of a degree. This shouldn't be a big surprise, since we have just seen that changing the distance to the Sun by 3 million kilometers (1.8 million miles) changes the temperature by less than 10 degrees.

What Does Cause the Seasons?

The true cause of the seasons is a combination of two effects of the tilt of the Earth's axis: duration and intensity of sunlight. Consider the northern hemisphere when it is tilted sunward. This is the interval between March 21 (the vernal equinox) and September 21 (the autumnal equi-

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