2.2 Revolution about the Sun Leads to Changes during the Year

Constellations and the Zodiac

About 9000 stars are visible to the naked eye—spread out across the sky. As Earth orbits the Sun, our view of these stars changes. To follow the patterns of the Sun and the stars, early humans grouped together stars that formed recognizable patterns, called constellations. People from different cultures saw different patterns and projected ideas from their own cultures onto what they saw in the sky. Constellations are creations of the human imagination.

If you noted the position of the Sun in relation to the stars each day for a year, you would find that the Sun traces out a great circle against the background of the stars (Figure 2.13). On September 1, the Sun appears to be in the direction of the constellation Leo. Six months later, on March 1, Earth is on the other side of the Sun, and the Sun appears from our perspective on Earth to be in the direction of the constellation Aquarius. Recall that the Sun’s apparent path against the background of the stars is called the ecliptic (the blue band in Figure 2.13). The constellations that lie along the ecliptic through which the Sun appears to move are called the constellations of the zodiac.

Today, astronomers use an officially sanctioned set of 88 constellations to serve as a road map of the sky. Constellations visible from the Northern Hemisphere draw their names primarily from the list of constellations that the Alexandrian astronomer Ptolemy compiled 2000 years ago. Constellation names in the Southern Hemisphere come from European explorers who visited the Southern Hemisphere during the 17th and 18th centuries. Every star in the sky lies within the borders of a single constellation. This makes it possible to catalog all the stars using the names of their parent constellation. Following the Greek alphabet, astronomers call the brightest star in a constellation alpha; the second brightest, beta; and so forth. For example, Sirius, the brightest star in the sky, lies within the constellation Canis Major (meaning “big dog”). So Sirius is also called “Alpha Canis Majoris,” indicating that it is the brightest star in Canis Major. Appendix 7 includes sky maps showing the constellations.

Earth’s Axial Tilt and the Seasons

To understand why the seasons change, we need to consider the combined effects of Earth’s rotation on its axis and its revolution around the Sun. Many people believe that seasons change because Earth is closer to the Sun in summer and farther away in winter. How can we test that hypothesis? We might predict: if the distance between Earth and the Sun caused the seasons, then all of Earth should experience summer at the same time of year. But the United States has summer in June, whereas Australia has summer in December. In modern times, we can directly measure the distance between Earth and the Sun, and we find that Earth is actually closest to the Sun at the beginning of January, by about 5 million km. Yet, in the Northern Hemisphere, we have winter in January. We have just falsified the hypothesis tying seasonal change to distance in two ways, so we need to look for a different explanation that accounts for all the available facts.

Earthbound observers can see that as the year passes, the Sun is above the horizon longer in summer than in winter. We often say “the days are longer in the summer”, meaning there are more daytime hours, and fewer nighttime ones. The Sun is also higher in the sky as it crosses the meridian in summer than in winter. Observing more closely, the Earthbound observer finds that the Sun appears to move along the ecliptic, which is tilted 23.5° with respect to the celestial equator.

AstroTour: Earth Spins and Revolves

Looking at Earth from space, Figure 2.14 shows that as Earth moves around the Sun over the course of the year, its rotation axis always points in the same direction, toward distant Polaris. This axis is tilted 23.5° from the perpendicular to Earth’s orbital plane, causing the ecliptic to likewise be tilted 23.5° from the celestial equator. As it orbits, Earth is sometimes on one side of the Sun and sometimes on the other. As a result, Earth’s North Pole is sometimes tilted more toward the Sun, and other times the South Pole is tilted more toward the Sun. This accounts for the changing altitude of the Sun as it crosses the meridian, and the changing length of daytime and nighttime. When Earth’s North Pole is tilted toward the Sun, an observer on Earth views the Sun north of the celestial equator. Therefore, for observers in the Northern Hemisphere, the Sun is above the horizon more than 12 hours each day, thus making the daytime longer than 12 hours. Six months later, when Earth’s North Pole is tilted away from the Sun, an observer in the same place views the Sun south of the celestial equator and the daytime is less than 12 hours.

In the preceding paragraph, we specified the Northern Hemisphere because seasons are opposite in the Southern Hemisphere. Look again at Figure 2.14. Around June 20, while the Northern Hemisphere is enjoying the long days and short nights of summer, Earth’s South Pole is tilted away from the Sun. It is winter in the Southern Hemisphere; less than half of the Southern Hemisphere is illuminated by the Sun, and the days are shorter than 12 hours. On December 21, Earth’s South Pole is tilted toward the Sun. It is summer in the Southern Hemisphere, so its days are long and its nights are short.

To understand how the combination of Earth’s axial tilt and its path around the Sun creates seasons, consider what would happen if Earth’s spin axis were exactly perpendicular to the plane of Earth’s orbit (the ecliptic plane). First, the Sun would always be on the celestial equator. Then, at every latitude (except the poles), the Sun would follow the same path through the sky every day, rising due east each morning and setting due west each evening. The Sun would be above the horizon exactly half the time, so days and nights would always be exactly 12 hours long everywhere on Earth. In short, Earth would have no seasons.

what if . . .

where the axial tilt is 25.19°? Considering only this single piece of information, would you expect seasonal variations to be larger, smaller, or about the same as seasonal variations on Earth? Explain.

The changing length of days through the year only partly explains seasonal temperature changes. A more important effect relates to the angle at which the Sun’s rays strike Earth. The Sun is higher in the sky during summer than during winter, and sunlight strikes the ground more directly during summer than during winter. To see why that is important, study Figure 2.15. During summer, Earth’s surface is more nearly face-on to the incoming sunlight. The light is concentrated and bright so more energy falls on each square meter of ground each second. During winter, the surface is more tilted with respect to the sunlight, so the light is more diffuse. Less energy falls on each square meter of the ground each second. For example, at latitude 40° north, which stretches across the United States from northern California to New Jersey, more than twice as much solar energy falls on each square meter of ground per second at noon on June 20 as falls there at noon on December 21. That variation is the main reason why summer is hotter and winter is colder.

The Process of Science Figure demonstrates how determining the causes of seasonal change requires accounting for all the known facts. The two effects of Earth’s tilted axis—the directness of sunlight and the different lengths of the night—mean that the Sun heats a hemisphere more at one time of year than another, causing summer and winter.

The Solstices and the Equinoxes

Four days during Earth’s orbit (two solstices and two equinoxes) mark special moments in the year. The day when the Sun is highest in the sky as it crosses the meridian is called the summer solstice. On that day, the Sun rises farthest north of east and sets farthest north of west. In the Northern Hemisphere, that occurs each year near June 20, the first day of summer. Figure 2.14a shows that orientation of Earth and Sun.

Six months after the summer solstice, around December 21, the North Pole is tilted away from the Sun (Figure 2.14b). That day is the winter solstice in the Northern Hemisphere—the shortest day of the year and the first day of winter in the Northern Hemisphere. Almost all cultural traditions in the Northern Hemisphere include some sort of major celebration in late December. Those winter festivals celebrate the return of the source of Earth’s light and warmth. The days have stopped growing shorter and are beginning to get longer, and spring will come again.

Between the two solstices, the ecliptic crosses the celestial equator on two days. On those days, the Sun lies directly above Earth’s equator. We call those days equinoxes, which means “equal night,” because the entire Earth experiences 12 hours of daylight and 12 hours of darkness. Halfway between summer solstice and winter solstice, the autumnal equinox marks the beginning of fall in the Northern Hemisphere; it occurs around September 22. Halfway between winter solstice and summer solstice, the vernal equinox marks the beginning of spring in the Northern Hemisphere; it occurs around March 20. In the Southern Hemisphere, the dates for the summer and winter solstices, and the autumnal and vernal equinoxes, are reversed from those in the Northern Hemisphere.

Precession of the Equinoxes

Two thousand years ago, when Ptolemy and his associates were formalizing their knowledge of the positions and motions of objects in the sky, the Sun appeared in the constellation Cancer on the first day of northern summer and in the constellation Capricornus on the first day of northern winter. Today, the Sun is in Taurus on the first day of northern summer and in Sagittarius on the first day of northern winter. Why have the constellations in which solstices appear changed? Two motions are actually associated with Earth and its axis: Earth spins on its axis, but its axis also wobbles like the axis of a spinning top slowing down (Figure 2.18a). The wobble is very slow: the north celestial pole takes about 26,000 years to complete one circle in the sky. Polaris is the star we now see near the north celestial pole. However, if you could travel several thousand years into the past or the future, the point about which the northern sky appears to rotate would no longer be near Polaris. Instead, the stars would rotate about another point on the path shown in Figure 2.18b. That figure shows the path of the north celestial pole through the sky during one cycle of that wobble.

The celestial equator is perpendicular to Earth’s axis. Therefore, as Earth’s axis wobbles, the celestial equator also must wobble. As it does so, the locations where the celestial equator crosses the ecliptic—the equinoxes—change as well. During each 26,000-year wobble of Earth’s axis, the locations of the equinoxes make one complete circuit around the celestial equator. That change of the position of the equinox, due to the wobble of Earth’s axis, is called the precession of the equinoxes.

CHECK YOUR UNDERSTANDING 2.2

If Earth’s axis were tilted by 45°, instead of its actual tilt of 23.5°, how (if at all) would the seasons be different from what they are now? (a) The seasons would remain the same. (b) Summers would be colder. (c) Winters would be shorter. (d) Winters would be colder.

AnswerAnswer

d