# 12.1. The Night Sky. Earth s Motions

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1 12.1 The Night Sky LEARNING TIP Skim Section Consider information gathered from the title, headings, figures, and words in bold. What do you expect to learn in this section? Astronomy is the branch of science that involves observations and explanations of events and objects that occur beyond Earth and its atmosphere. An astronomer is a scientist who studies astronomy. Different astronomers study different aspects of astronomy. Some astronomers study the Sun, some study the planets, while others study objects such as galaxies and black holes. Do you ever watch the stars at night move slowly across the sky and wonder why they all move in the same direction? Do you ask yourself questions such as, What causes the phases of the Moon? and How was our solar system formed? Do you look at the night sky when it is clear so that you can find your favourite constellation? If your answer to these questions is yes, then you are thinking like an astronomer. Earth s Motions Earth undergoes two distinct and important motions: rotation and revolution. Rotation is the spinning of an object around an imaginary line called an axis. Revolution is the motion of an object around another object (Figure 1). Figure 1 In this amusement park ride, the teacup rotates on its axis while revolving around the centre of the ride. 12A Investigation Sunrise and Sunset: Measuring the Photoperiod To perform this investigation, turn to page 404. In this investigation, you will learn how the photoperiod varies in your area. Effects of Earth s Rotation Earth takes 24 h to complete one full rotation about its axis, which is tilted 23.5 (Figure 2). If you stood above the North Pole and looked down, Earth would appear to be rotating in a counterclockwise direction. As a result of Earth s rotation, we have day and night. During rotation, the part of Earth that faces the Sun experiences daylight, and the part that faces away from the Sun experiences darkness. The number of hours of daylight between sunrise and sunset is called the photoperiod. Investigation 12A 368 Unit D Space Exploration

3 LEARNING TIP You have learned that Earth behaves much like a spinning top. As you study Figure 4, consider that one complete wobble of the spinning top (in which the top moves in a backward circle) takes Earth years to do! Precession Earth s axis has not always pointed in the same direction. In fact, Earth s axis slowly traces a circle every years. Consider a spinning top. The tip of the top does not usually point straight up. This is similar to the tilt of Earth s axis. Furthermore, the tip of the spinning top does not remain in one fixed position. It, too, traces a circle, and it tends to wobble as it spins (Figure 4). Although the angle of Earth s axis is constant, the direction in which it points slowly changes. The changing direction of Earth s axis is called precession. One effect of precession is the changing position of the North Celestial Pole. If you could extend the tip of Earth s axis north to the stars, it would point to a location astronomers call the North Celestial Pole. The North Celestial Pole is currently very close to the star Polaris. Therefore, astronomers call Polaris the pole star. About years from now, however, due to precession, the North Celestial Pole will be close to Vega, a star in the constellation Lyra. In years, the North Celestial Pole will once again be Polaris (Figure 5). (a) (b) Figure 4 Like a spinning top (a), the tip of Earth s axis slowly traces a circle in the sky (b). current North Celestial Pole Polaris years Cepheus 5000 years Draco Thuban years Deneb Figure 5 If you could transfer the circle made by Earth s axis over time onto the sky, you would see how the position of the North Celestial Pole changes over years. Can you estimate when the star Thuban will be the pole star? years Vega years 370 Unit D Space Exploration

4 Effect of Earth s Revolution The second important motion of Earth is its revolution. Earth takes approximately / 4 days to complete one revolution around the Sun. In other words, Earth rotates / 4 times as it revolves around the Sun once. Earth s orbit around the Sun is not quite a circle; it is an ellipse. An ellipse resembles a flattened circle. The Sun is not in the centre of the ellipse; it is located at one of two focal points. There is no object at the second focal point. Consequently, there is a time when Earth is closer to the Sun and a time when Earth is farther from the Sun (Figure 6). The perihelion, the point at which Earth is closest to the Sun, occurs around January 3. The aphelion, when Earth is farthest from the Sun, occurs around July 4. Note the perihelion actually occurs during the northern hemisphere s winter. Therefore, we know that the cause for the seasons has nothing to do with Earth s distance from the Sun. Earth is closest to the Sun during the summer in the southern hemisphere, which correlates with higher southern summer temperatures and more deserts. It also correlates with a colder pole Antarctica is colder than the Arctic. Sun perihelion focal point aphelion Figure 6 At perihelion, Earth is km from the Sun. At aphelion, Earth is km from the Sun. (The ellipse is exaggerated.) Earth The Seasons The cause for the seasons on Earth is often mistakenly attributed to Earth s elliptical orbit. In fact, the seasons are due to the tilt of Earth s axis (Figure 7). In the northern hemisphere, summer is warmer because the northern hemisphere is tilted toward the Sun in the summer. The Sun is higher in the sky, and there are more hours of sunlight. In winter, the northern hemisphere is tilted away from the Sun. The Sun is lower in the sky, and there are fewer hours of sunlight. LEARNING TIP Check your understanding. Use Figure 7 to explain to a partner why we have seasons on Earth. Earth s axis equator autumn winter Sun summer spring Figure 7 When places on Earth are tilted toward the Sun, those places experience summer because the Sun s rays approach from a more direct angle The Night Sky 371

5 Orion nebula belt sword Figure 8 To people in ancient times, this constellation looked like a hunter, whom they called Orion. His belt, sword, and the great nebula in the sword s sheath are indicated. 12C Investigation Using a Star Map To perform this investigation, turn to page 406. In this investigation, you will learn how to use a star map. Constellations To the Aboriginal peoples and ancient astronomers, certain patterns of stars suggested mythical figures and animals. Today, astronomers recognize 88 different patterns, which are called constellations. One of the most impressive of these patterns is Orion, the hunter (Figure 8). There are other patterns in the sky, too, that are not official constellations. These patterns are called asterisms. The Big Dipper, only a part of the constellation Ursa Major, is an asterism. As Earth rotates, the patterns of stars appear to move across the night sky. As Earth revolves around the Sun, we see different constellations. 12C Investigation Assigning people and animals to represent star patterns is the work of our imagination and the completion of shapes by our brain. Indigenous and ancient peoples have been doing this for as long as stories have been remembered or recorded. The Big Dipper is clearly a cup with a long handle (Figure 9(a)). However, finding shapes in other groups of stars often takes a great deal of imagination. Cassiopeia looks more like a W or an M than a queen sitting on her throne (Figure 9(b)). The constellation names that you are probably most familiar with are based on Greek and Arabic mythology. The major stars all have Arabic names. Did You KNOW? The Big Dipper The Big Dipper is well known across cultures. The Anishinaabe people (also known as the Chippewa or Ojibway people) observed the sky and recorded changes and new events. The changing sky is related to story telling, ceremonies, and life activities. For example, the Anishinaabe watched the Big Dipper as it moved across the sky. When it was high overhead in the early evening, they knew that spring was close. (a) Figure 9 (a) The Big Dipper (b) Cassiopeia Because the stars are so far away, their motion relative to each other is very slight, so we see the constellations unchanged from year to year. However, the stars in a constellation are all moving through space. The pattern of stars in a constellation changes over a very long time. The change in the Big Dipper over years is shown in Figure 10. (a) (b) Figure 10 This sequence of diagrams shows the movements of the main stars in the Big Dipper: (a) The pattern years ago (b) How the Big Dipper looks today (c) How the Big Dipper will look years from now (b) (c) 372 Unit D Space Exploration

6 The Celestial Sphere In the Middle Ages ( CE), people still believed that the sky was solid, with the stars fixed to an invisible crystal sphere. They believed that the planets and the Sun were fixed in separate orbits closer to Earth, while the stars were held in positions farther from Earth. Since the planets were closer, they appeared to travel farther than the stars in the background. If such a sphere existed, it would rotate around Earth once a day, and it would move all the bodies in the sky with it. For convenience, this is called the celestial sphere. We now understand that it is Earth that rotates, not the sky. If Earth s equator were extended out to the celestial sphere, it would create a celestial equator and divide the night sky into two hemispheres (Figure 11). Within these hemispheres, astronomers use a plotting system similar to latitude and longitude to mark the positions of stars and bodies in the sky. North Celestial Pole ecliptic (Earth s orbital plane) South Celestial Pole celestial equator equator 23.5 Figure 11 The celestial sphere is an imaginary crystal sphere around Earth in which all the objects in the sky are embedded. The celestial poles and the celestial equator are projected onto the sphere from the poles and equator on Earth. The path the Sun takes through the sky is marked against the 12 constellations of the zodiac. This path is called the plane of the ecliptic. Because Earth is tilted 23.5 on its axis, the angle between the ecliptic and the celestial equator is also The two dates when the Sun, on the ecliptic, crosses the celestial equator are around March 21 and September 22, the spring (vernal) and fall (autumnal) equinoxes. On these days, the number of hours that the Sun is above the horizon and below the horizon is equal. The Sun follows this apparent path because of Earth s rotation and revolution around the Sun. The solstices occur when the Sun reaches its highest and lowest positions in the sky, when Earth is tilted toward or away from the Sun (due to the 23.5 axis tilt). The summer solstice occurs when the Sun reaches its highest position in the sky, usually on June 21. The summer solstice marks the first day of summer, and we experience the longest photoperiod of the year. Conversely, the winter solstice occurs when the Sun reaches its lowest position in the sky, and we experience the shortest photoperiod of the year. Because of Earth s tilt, the Sun does not rise very high in the sky. Therefore, it spends less time above the horizon, and we have fewer hours of daylight. The winter solstice usually occurs on December 21 and marks the beginning of winter The Night Sky 373

7 If you would like to learn more about astronomy and the night sky, go to GO Circumpolar Constellations In the northern hemisphere, especially in Canada, some constellations never disappear below the horizon as Earth rotates. A constellation that is always above the horizon is said to be circumpolar (Figure 12). The three main circumpolar constellations are Ursa Minor (which contains the Little Dipper, with Polaris at the end of its long handle), Ursa Major (which contains the Big Dipper), and Cassiopeia. GO spring Big Dipper winter Polaris summer fall W horizon E Figure 12 The Big Dipper and Polaris are always above our horizon. TRY THIS: You Are Earth Skills Focus: conducting, communicating Materials: diagrams of the Big Dipper, the Little Dipper, Cassiopeia, Aquila, Pegasus, Orion, and Leo In this activity, you will turn your classroom into a planetarium. 1. Place diagrams of the Big Dipper, the Little Dipper, and Cassiopeia on the ceiling, in their correct orientation. 2. Place these four constellations on the walls: Aquila (summer) on the east wall Pegasus (autumn) on the north wall Orion (winter) on the west wall Leo (spring) on the south wall 3. Pick one classmate to be the Sun in the middle of the room. Pretend that you are Earth, and stand east of the Sun. When your back is to the Sun, it is night on Earth. 4. Slowly spin around counterclockwise. When you are facing the Sun, it is daylight. 5. Repeat this activity for positions north, west, and south of the Sun to represent the positions of Earth during different seasons. A. Which constellation is visible in which season? B. Which constellation could you not see? Why? 374 Unit D Space Exploration

8 12.1 CHECK YOUR Understanding 1. Describe how the rotation of Earth is responsible for the movement of the stars. 2. Describe Earth s rotation in terms of its axis and length of time to rotate. 3. Describe how the movement of Earth is responsible for day and night. 4. If you were to look down on Earth from above the North Pole, in which direction would Earth appear to rotate? 5. What is the photoperiod? 6. How does the photoperiod differ between the two solstices? 7. What is unique about the photoperiod at the two equinoxes? 8. On any given day, why does the photoperiod vary throughout the world? 9. What is precession? 10. Using Figure 5 on page 370, name the star that might be considered the Pole Star 5000 years from now. 11. Describe Earth s revolution around the Sun in terms of the shape of the orbit and the length of time to complete one revolution. 12. Describe the path of the Sun through the sky using terms you have learned in this section. 13. What is the astronomical significance of the zodiac constellations? 14. The cause of the seasons on Earth is often mistakenly attributed to the elliptical orbit of Earth around the Sun. What is the actual cause of the seasons? 15. What is the astronomical significance of January 3 and July 4? Table Copy Figure 13 into your notebook. On your drawing, indicate (a) the direction of Earth s rotation and revolution (b) the four seasons (c) Earth s tilt Figure 13 March 20/21 Sun North Pole June 21/22 December 21/22 September 22/ Explain why we see different constellations as Earth revolves around the Sun. 18. If you were to visit another planet, you might be able to see the same stars but would you see the same constellations? Explain your answer. 19. Describe the celestial sphere in your own words. 20. Why do the planets appear to move through the constellations instead of with the constellations as Earth rotates? 21. Copy Table 1 into your notebook. Place checkmarks in the boxes to relate the phenomena to one of Earth s two distinct motions. Apparent movement Phenomenon Perihelion Aphelion Seasons of stars Solstices Equinoxes Photoperiod Rotation Revolution 12.1 The Night Sky 375

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