2. Before, you learned Stars seem to rise, cross the sky, and set because Earth turns The Sun is very large and far from Earth Earth orbits the Sun Now, you will learn Why Earth has day and night How the changing angles of sunlight produce seasons VOCABULARY axis of rotation, revolution, season, equinox, solstice Four Square Diagram = Include: definition, example(s), picture, description, pronounce, origin, sentence. Axis of Rotation Define Sentence

5. Earth's rotation causes day and night. When astronauts explored the Moon, they felt the Moon's gravity pulling them down. Their usual "down"—Earth—was up in the Moon's sky. As you read this, it is easy to tell which way is down. But is down in the same direction for a person on the other side of Earth? If you both pointed down, you would be pointing toward each other. Earth's gravity pulls objects toward the center of Earth. No matter where you stand on Earth, the direction of down will be toward Earth's center. There is no bottom or top. Up is out toward space, and down is toward the center of the planet. As Earth 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 Earth?

7. The directions north, south, east, and west are based on the way the planet rotates, or turns. Earth 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 Earth turns. If you extend your right thumb and pretend its tip is the North Pole, then your fingers curve the way Earth rotates. At any one time, about half of Earth is in sunlight and half is dark. However, Earth turns on its axis in 24 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 Earth?

9. Earth's tilted axis and orbit cause seasons. Just as gravity causes objects near Earth to be pulled toward Earth's center, it also causes Earth and other objects near the Sun to be pulled toward the Sun's center. Fortunately, Earth does not move straight into the Sun. Earth moves sideways, at nearly a right angle to the Sun's direction. Without the Sun's gravitational pull, Earth would keep moving in a straight line out into deep space. However, the Sun's pull changes Earth's path from a straight line to a round orbit about 300 million kilometers (200,000,000 mi) across. Just as a day is the time it takes Earth to rotate once on its axis, a year is the time it takes Earth 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. Earth's rotation and orbit do not quite line up. If they did, Earth's equator would be in the same plane as Earth's orbit, like a tiny hoop and a huge hoop lying on the same tabletop. Instead, Earth rotates at about a 23° angle, or tilt, from this lined-up position. Use the second vowel in each word to help you remember that an object rotates on its own axis, but revolves around another object.

10. As Earth moves, its axis always points in the same direction in space. You could model Earth'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. Earth's orbit is not quite a perfect circle. In January, Earth 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 Earth. However, the combination of Earth's motion around the Sun with the tilt of Earth's axis does cause important changes of temperature. Earth’s equatorial speed is… 1,669.8 km/h.

11. Seasonal Patterns Most locations on Earth 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 Earth's axis. Look at the diagrams to see how the constant direction of Earth's tilted axis affects the pattern of sunlight on Earth at different times of the year. As Earth travels around the Sun, the area of sunlight in each hemisphere changes. At an equinox (EE-kwuh-NAHKS), sunlight shines equally on the northern and southern hemispheres. Half of each hemisphere is lit, and half is in darkness. As Earth 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. Equinoxes and solstices happen on or around the 21st days of certain months of the year.

13. September Equinox When Earth 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, Earth 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.

14. March Equinox After another quarter of its orbit, Earth 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. Earth'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?

16. How are temperatures throughout the year affected by the angles of sunlight?

17. Lengths of Days Seasonal temperatures depend on the amount of daylight, too. In Chicago, for example, the summer Sun heats the ground for about 15 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 20 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 12 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 12 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. Equinox means "equal night"—daylight and nighttime are equal in length.

18. KEY CONCEPTS 1. What causes day and night? 2. What happens to Earth's axis of rotation as Earth 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? CHALLENGE 6. Infer If Earth'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?