THE SUN RISES AND SETS because Earth rotates on its axis. That is only the most obvious of the solar cycles.

Even in the daytime, the sky is filled with stars, but the glare of sunlight fills Earth’s atmosphere with scattered light, and you can see only the brilliant sun. If the sun were fainter and you could see the stars and the sun at the same time, you would notice that as the day passed the sun and the stars moved westward. If the sun rose among a group of stars, it would remain among that group of stars all day and eventually set among the same stars.

If, however, you watched carefully as the day passed, you would notice that the sun was creeping slowly eastward against the background of stars. It would move a distance roughly equal to its own diameter between sunrise and sunset. This motion is caused by the orbital motion of Earth around the sun. In January, you would see the sun in front of the constellation Sagittarius (Figure 3-1). As Earth moves along its circular orbit, the sun appears to move eastward among the stars. By March, you would see it in front of Aquarius.

Through the year, the sun moves eastward against the background of the stars following a line called the ecliptic, the apparent path of the sun against the stars. If the sky were a great screen, the ecliptic would be the shadow cast by Earth’s orbit. That is why the ecliptic is often called the projection of Earth’s orbit on the sky.

Earth circles the sun in 365.25 days, and consequently the sun appears to circle the sky in the same period. That means the sun, traveling 360° around the ecliptic in 365.25 days, travels about 1° eastward in 24 hours, about twice its angular diameter. You don’t notice this motion because you cannot see the stars in the daytime, but the motion of the sun has an important consequence that you do notice—the seasons.

The seasons are caused by a simple fact: Earth’s axis of rotation is tipped 23.5° from the perpendicular to its orbit. As you study The Cycle of the Seasons on pages 26–27, notice two important principles:

1. Earth’s axis of rotation is inclined 23.5°, and that makes the sun move into the northern sky in the spring and into the southern sky in the fall. The four seasons of the year are defined by the location of the sun along the ecliptic.
2. The seasons are caused by the changes in solar energy that Earth’s northern and southern hemispheres receive at different times of the year. Because of circulation patterns in Earth’s atmosphere, the northern and southern hemispheres are mostly isolated from each other and exchange little heat. When one hemisphere receives more solar energy than the other, it grows rapidly warmer.

Now you can set your friends straight if they mention two of the most Common Misconceptions about the seasons. First, the seasons don’t occur because Earth moves closer to or farther from the sun. Earth’s orbit is nearly circular. Also, it is not easier to stand a raw egg on end on the day of the vernal equinox! Have you heard that one? Radio and TV personalities love to talk about it, but it just isn’t true. It is one of the silliest misconceptions in science. You can stand a raw egg on end any day of the year if you have steady hands. (Hint: It helps to shake the egg really hard to break the yoke inside so it can settle to the bottom.)

The seasons are so important as a cycle of growth, harvest, death, and rebirth that it is not surprising that people have strong feelings about them. Ancient peoples saw the motion of the sun around the ecliptic as a powerful influence on their daily lives. The ancient superstition of astrology is based on the cycle of the sun around the sky. You have probably heard of the zodiac, a band around the sky extending 9 degrees above and below the ecliptic. The signs of the zodiac take their names from the 12 principal constellations along the ecliptic. Astrology was once an important part of astronomy, but the two are now almost exact opposites—astronomy is a science that depends on evidence, and astrology is a superstition that survives in spite of evidence (Window on Science 3-1). The signs of the zodiac are no longer important in astronomy. The zodiac itself is of interest only because it is on the path followed by the sun, moon, and the five visible planets as they move around the sky.

The planets of our solar system produce no visible light of their own, and they are visible only by reflected sunlight. Mercury, Venus, Mars, Jupiter, and Saturn are all easily visible to the naked eye, but Uranus is usually too faint to be seen, and Neptune is never bright enough. Pluto is even fainter, and you need a large telescope to find it.

All the planets of the solar system move in nearly circular orbits around the sun. If you were looking down on the solar system from the north celestial pole, you would see the planets moving in the same counterclockwise direction around their orbits, with the planets farthest from the sun moving the slowest.

When you look for planets in the sky, you always find them near the ecliptic because their orbits lie in nearly the same plane as Earth’s orbit. The planets whose orbits lie outside Earth’s orbit, move slowly eastward along the ecliptic as they orbit the sun.¹ Mars moves completely around the ecliptic in slightly less than 2 years, but Saturn, being farther from the sun, takes nearly 30 years.

Mercury and Venus have orbits inside Earth’s orbit, and that means they can never move far from the sun in the sky. As seen from Earth, they move alternately eastward and then westward, and they circle the sun. To find one of these planets, you need to look above the western horizon just after sunset or above the eastern horizon just before sunrise. Venus is easier to locate because it is brighter and because its larger orbit carries it higher above the horizon than does Mercury’s (Figure 3-2). Mercury’s orbit is so small that it can never get farther than 27°50’ from the sun. Consequently, it is hard to see against the sun’s glare and is often hidden in the clouds and haze near the horizon.

By tradition, any planet visible in the evening sky is called an evening star, even though planets are not stars. Similarly, any planet visible in the sky shortly before sunrise is called a morning star. Perhaps the most beautiful is Venus, which can become as bright as magnitude −4.7. As Venus moves around its orbit, it can dominate the western sky each evening for many weeks, but eventually its orbit carries it back toward the sun, and it is lost in the haze near the horizon. In a few weeks, it reappears in the dawn sky, a brilliant morning star.

Building Scientific Arguments

If Earth had a significantly elliptical orbit, how would its seasons be different?
Sometimes as you review your understanding it is helpful to build a scientific argument with one factor slightly exaggerated. Suppose Earth had an elliptical orbit so that perihelion occurred in July and aphelion in January. At perihelion, Earth would be closer to the sun, and the entire surface of Earth would be a bit warmer. If that happened in July, it would be summer in the northern hemisphere and winter in the southern hemisphere, and both would be warmer than they are now. It could be a hot summer in Canada, and southern Argentina could have a mild winter. Six months later, at aphelion, Earth would be a bit farther from the sun, and if that occurred in January, winter in northern latitudes could be frigid. Argentina, in the southern hemisphere, could be experiencing an unusually cool summer.

Of course, this doesn’t happen. Earth’s orbit is nearly circular, and the seasons are caused not by a variation in the distance of Earth from the sun but by the inclination of Earth in its orbit.

Nevertheless, Earth’s orbit is slightly elliptical. Earth passes perihelion about January 3 and aphelion about July 5. Although Earth’s oceans tend to store heat and reduce the importance of this effect, this very slight variation in distance does affect the seasons. Now use your scientific argument to analyze the seasons. Does the elliptical shape of Earth’s orbit make your winters warmer or cooler?

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Active Figure 3-1
Earth’s motion around the sun makes the sun appear to move against the background of the stars. Earth’s circular orbit is thus projected on the sky as the circular path of the sun, the ecliptic. If you could see the stars in the daytime, you would notice the sun crossing in front of the distant constellations as Earth moves along its orbit.

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Figure 3-2
Mercury and Venus follow orbits that keep them near the sun, and they are visible only soon after sunset or before sunrise when the brilliance of the sun is hidden below the horizon. Venus takes 584 days to move from the morning sky to the evening sky and back again, but Mercury zips around in only 116 days.

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The Cycle of the Seasons
pages 26-27

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Window on Science 3-1
Astrology and Pseudoscience: Misusing the Rules of Science