Page of 7 KY CONCPT arth rotates on a tilted axis and orbits the Sun. BFOR, you learned Stars seem to rise, cross the sky, and set because arth turns The Sun is very large and far from arth arth orbits the Sun NOW, you will learn Why arth has day and night How the changing angles of sunlight produce seasons VOCABULARY axis of rotation p. 44 revolution p. 45 season p. 46 equinox p. 46 solstice p. 46 XPLOR Time Zones What time is it in Iceland right now? PROCDUR Find your location and Iceland on the map. Identify the time zone of each. Count the number of hours between your location and Iceland. Add or subtract that number of hours from the time on your clock. MATRIAL time zone map WHAT DO YOU THINK? By how much is Iceland s time earlier or later than yours? Why are clocks set to different times? arth s rotation causes day and night. When astronauts explored the Moon, they felt the Moon s gravity pulling them down. Their usual down arth was up in the Moon s sky. As you read this book, it is easy to tell which way is down. But is down in the same direction for a person on the other side of arth? If you both pointed down, you would be pointing toward each other. arth s gravity pulls objects toward the center of arth. No matter where you stand on arth, the direction of down will be toward arth s center. There is no bottom or top. Up is out toward space, and down is toward the center of the planet. As arth turns, so do you. You keep the same position with respect to what is below your feet, but the view above your head changes. In what direction does gravity pull objects near arth? Chapter : arth, Moon, and Sun 43
Page of 7 The globe and the flat map show the progress of daylight across arth in two ways. This location is experiencing sunrise. noon The directions north, south, east, and west are based on the way the planet rotates, or turns. arth rotates around an imaginary line running through its center called an axis of rotation. The ends of the axis are the north and south poles. Any location on the surface moves from west to east as arth turns. If you extend your right thumb and pretend its tip is the North Pole, then your fingers curve the way arth rotates. night moves westward midnight At any one time, about half of arth is in sunlight and half is dark. However, arth turns on its axis in 4 hours, so locations move through the light and darkness in that time. When a location is in sunlight, it is daytime there. When a location is in the middle of the sunlit side, it is noon. When a location is in darkness, it is night there, and when the location is in the middle of the unlit side, it is midnight. If it is noon at one location, what time is it at a location directly on the other side of arth? Rotation What causes day and night? In this model the lamp represents the Sun, and your head represents arth. The North Pole is at the top of your head. You will need to imagine locations on your head as if your head were a globe. PROCDUR 3 4 Face the lamp and hold your hands to your face as shown in the photograph. Your hands mark the horizon. For a person located at your nose, the Sun would be high in the sky. It would be noon. Face away from the lamp. Determine what time it would be at your nose. Turn to your left until you see the lamp along your left hand. Continue turning to the left, through noon, until you just stop seeing the lamp. SKILL FOCUS Making models MATRIALS lamp TIM 5 minutes WHAT DO YOU THINK? What times was it at your nose in steps, 3, and 4? When you face the lamp, what time is it at your right ear? CHALLNG How can a cloud be bright even when it is dark on the ground? 44 Unit: Space Science
Page 3 of 7 arth s tilted axis and orbit cause seasons. Just as gravity causes objects near arth to be pulled toward arth s center, it also causes arth and other objects near the Sun to be pulled toward the Sun s center. Fortunately, arth does not move straight into the Sun. arth moves sideways, at nearly a right angle to the Sun s direction. Without the Sun s gravitational pull, arth would keep moving in a straight line out into deep space. However, the Sun s pull changes arth s path from a straight line to a round orbit about 300 million kilometers (00,000,000 mi) across. Just as a day is the time it takes arth to rotate once on its axis, a year is the time it takes arth to orbit the Sun once. In astronomy, a revolution is the motion of one object around another. The word revolution can also mean the time it takes an object to go around once. arth s rotation and orbit do not quite line up. If they did, arth s equator would be in the same plane as arth s orbit, like a tiny hoop and a huge hoop lying on the same tabletop. Instead, arth rotates at about a 3 angle, or tilt, from this lined-up position. reading tip Use the second vowel in each word to help you remember that an object rotates on its own axis, but revolves around another object. orbit 3 3 Not to scale arth s axis points in a constant direction as arth orbits the Sun. arth is tilted 3 from its orbit. Use your thumb to represent the North Pole. Keep it steady as you move your hand in a counterclockwise circle on a tabletop. As arth moves, its axis always points in the same direction in space. You could model arth s orbit by moving your right fist in a circle on a desktop. You would need to point your thumb toward your left shoulder and keep it pointing that way while moving your hand around the desktop. arth s orbit is not quite a perfect circle. In January, arth is about 5 million kilometers closer to the Sun than it is in July. You may be surprised to learn that this distance makes only a tiny difference in temperatures on arth. However, the combination of arth s motion around the Sun with the tilt of arth s axis does cause important changes of temperature. Turn the page to find out how. January 53,000,000 km 48,000,000 km July Not to scale arth s orbit is almost a circle. arth s distance from the Sun varies by only about 5,000,000 km about 3% during a year. Chapter : arth, Moon, and Sun 45
Page 4 of 7 VOCABULARY Remember to put each new term into a frame game diagram. reading tip The positions and lighting can be hard to imagine, so you might use a model as well as the diagram on the next page to help you understand. Seasonal Patterns Most locations on arth experience seasons, patterns of temperature changes and other weather trends over the course of a year. Near the equator, the temperatures are almost the same year-round. Near the poles, there are very large changes in temperatures from winter to summer. The temperature changes occur because the amount of sunlight at each location changes during the year. The changes in the amount of sunlight are due to the tilt of arth s axis. Look at the diagram on page 47 to see how the constant direction of arth s tilted axis affects the pattern of sunlight on arth at different times of the year. As arth travels around the Sun, the area of sunlight in each hemisphere changes. At an equinox (-kwuh-nahks), sunlight shines equally on the northern and southern hemispheres. Half of each hemisphere is lit, and half is in darkness. As arth moves along its orbit, the light shifts more into one hemisphere than the other. At a solstice (SAHL-stihs), the area of sunlight is at a maximum in one hemisphere and a minimum in the other hemisphere. quinoxes and solstices happen on or around the st days of certain months of the year. 3 4 September quinox When arth is in this position, sunlight shines equally on the two hemispheres. You can see in the diagram that the North Pole is at the border between light and dark. The September equinox marks the beginning of autumn in the Northern Hemisphere and of spring in the Southern Hemisphere. December Solstice Three months later, arth has traveled a quarter of the way around the Sun, but its axis still points in the same direction into space. The North Pole seems to lean away from the direction of the Sun. The solstice occurs when the pole leans as far away from the Sun as it will during the year. You can see that the North Pole is in complete darkness. At the same time, the opposite is true in the Southern Hemisphere. The South Pole seems to lean toward the Sun and is in sunlight. It is the Southern Hemisphere s summer solstice and the Northern Hemisphere s winter solstice. March quinox After another quarter of its orbit, arth reaches another equinox. Half of each hemisphere is lit, and the sunlight is centered on the equator. You can see that the poles are again at the border between day and night. June Solstice This position is opposite the December solstice. arth s axis still points in the same direction, but now the North Pole seems to lean toward the Sun and is in sunlight. The June solstice marks the beginning of summer in the Northern Hemisphere. In contrast, it is the winter solstice in the Southern Hemisphere. In what month does winter begin in the Southern Hemisphere? 46 Unit: Space Science
Page 5 of 7 Seasons arth s orbit and steady, tilted axis produce seasons. September quinox Half of the sunlight is in each hemisphere. The strongest sunlight is on the equator. 4 June Solstice More than half of the Northern Hemisphere is in sunlight. The strongest sunlight is north of the equator, so the Northern Hemisphere grows warmer. December Solstice Less than half of the Northern Hemisphere is in sunlight. The strongest sunlight is south of the equator, so the Southern Hemisphere grows warmer. Not to scale 3 March quinox Half of the sunlight is in each hemisphere. The strongest sunlight is on the equator. View from the Sun If you could stand on the Sun and look at arth, you would see different parts of arth at different times of year. fall winter spring summer spring summer fall winter September quinox December Solstice 3 March quinox 4 June Solstice The equinoxes and solstices mark the beginnings of seasons in the two hemispheres. Warmer seasons occur when more of a hemisphere is in sunlight. Look at the poles to help you see how each hemisphere is lit. When is the South Pole completely in sunlight? Chapter : arth, Moon, and Sun 47
Page 6 of 7 RSOURC CNTR CLASSZON.COM Learn more about seasons. Angles of Sunlight You have seen that seasons change as sunlight shifts between hemispheres during the year. On the ground, you notice the effects of seasons because the angle of sunlight and the length of daylight change over the year. The effects are greatest at locations far from the equator. You may have noticed that sunshine seems barely warm just before sunset, when the Sun is low in the sky. At noon the sunshine seems much hotter. The angle of light affects the temperature. When the Sun is high in the sky, sunlight strikes the ground at close to a right angle. The energy of sunlight is concentrated. Shadows are short. You may get a sunburn quickly when the Sun is at a high angle. When the Sun is low in the sky, sunlight strikes the ground at a slant. The light is spread over a greater area, so it is less concentrated and produces long shadows. Slanted light warms the ground less. Near the equator, the noonday Sun is almost overhead every day, so the ground is warmed strongly year-round. In the middle latitudes, the noon Sun is high in the sky only during part of the year. In winter the noon Sun is low and warms the ground less strongly. How are temperatures throughout the year affected by the angles of sunlight? Sun Height and Shadows Winter Solstice, P.M. Spring quinox, P.M. Summer Solstice, P.M. Winter shadows are long because sunlight is spread out. The Sun appears low in the sky even at noon. location on arth Spring and fall shadows are of medium length, and the noon Sun appears higher in the sky. Summer shadows are short because the light is concentrated in a small area. The noon Sun appears high in the sky. 48 Unit: Space Science
Page 7 of 7 midnight 6 A.M. noon 6 P.M. Lengths of Days Seasonal temperatures depend on the amount of daylight, too. In Chicago, for example, the summer Sun heats the ground for about 5 hours a day, but in winter there may be only 9 hours of sunlight each day. The farther you get from the equator, the more extreme the changes in day length become. As you near one of the poles, summer daylight may last for 0 hours or more. Very close to the poles, the Sun does not set at all for six months at a time. It can be seen shining near the horizon at midnight. Tourists often travel far north just to experience the midnight Sun. At locations near a pole, the Sun sets on an equinox and then does not rise again for six months. Astronomers go to the South Pole in March to take advantage of the long winter night, which allows them to study objects in the sky without the interruption of daylight. Very near the equator, the periods of daylight and darkness are almost equal year-round each about hours long. Visitors who are used to hot weather during long summer days might be surprised when a hot, sunny day ends suddenly at 6 P.M. At locations away from the equator, daylight lasts hours only around the time of an equinox. Near the pole in the summer, the Sun stays above the horizon, so there is no night. This series of photographs was taken over the course of a day. reading tip quinox means equal night daylight and nighttime are equal in length. KY CONCPTS. What causes day and night?. What happens to arth s axis of rotation as arth orbits the Sun? 3. How do the areas of sunlight in the two hemispheres change over the year? CRITICAL THINKING 4. Apply If you wanted to enjoy longer periods of daylight in the summertime, would you head closer to the equator or farther from it? Why? 5. Compare and Contrast How do the average temperatures and the seasonal changes at the equator differ from those at the poles? CHALLNG 6. Infer If arth s axis were tilted so much that the North Pole sometimes pointed straight at the Sun, how would the hours of daylight be affected at your location? Chapter : arth, Moon, and Sun 49