Earth in Space Lesson 2.pptx

Earth in Space Lesson 2 Holt The Seasons Misconception: The seasons change because Earth’s distance from the sun changes. Fact: Seasons are the result of Earth’s tilted axis. Evidence: Earth’s distance from the sun does change, but that’s not why Earth has seasons. If that were the cause, people i...

Earth in Space Lesson 2 Holt The Seasons Misconception: The seasons change because Earth’s distance from the sun changes. Fact: Seasons are the result of Earth’s tilted axis. Evidence: Earth’s distance from the sun does change, but that’s not why Earth has seasons. If that were the cause, people in the Northern and Southern hemispheres would have the same seasons at the same time. Instead, seasons in the Northern and Southern hemispheres are reversed. As Earth moves around the sun, sometimes the Northern Hemisphere is tilted toward the sun. At other times the Southern Hemisphere is tilted toward the sun. January 21 Where are you and what are you doing today? Before you read the rest of this lesson, answer the questions below. 1. Why are summers generally warmer than winters? 2. Where on Earth is the tilt of Earth least likely to affect seasons? Why? How Does Earth Move? Until a few hundred years ago, most people thought that Earth stood still and the sun, moon, and stars moved around it. But today, scientists know that Earth itself moves and that objects seem to move across the sky because of Earth’s motion. Earth moves in space in two major ways: rotation and revolution. Rotation The imaginary line that passes through Earth’s center and the North and South poles is Earth’s axis. The spinning of Earth on its axis is called rotation. Earth’s rotation causes day and night, as you can see on the next slide. As Earth rotates eastward, the sun appears to move west across the sky. As Earth continues to turn to the east, the sun appears to set in the west. Sunlight can’t reach the side of Earth facing away from the sun, so it is night there. It takes Earth about 24 hours to rotate once. As you know, each 24-hour cycle of day and night is called a day. Rotation Interpret Diagrams Draw an arrow to show the direction in which Earth rotates. Then draw a line separating day from night. 1. Sequence In which city will the sun set next? 2. CHALLENGE Where on Earth is it noon? Midnight? Revolution In addition to rotating, Earth travels around the sun. Revolution is the movement of one object around another. One revolution of Earth around the sun is called a year. Earth’s path, or orbit, is a slightly elongated circle, or ellipse. Earth’s orbit brings the planet closest to the sun in January. Revolution and Rotation Apply Concepts Draw a diagram showing Earth’s motions. Include both rotation and revolution in your diagram. Sun Calendars People of many cultures have divided time based on the motions of Earth and the moon. They have used the motions to establish calendars. A calendar is a system of organizing time that defines the beginning, length, and divisions of a year. Calendars The most common calendar today is divided into years, months, and days. One year equals the time it takes Earth to turn once on its axis. People also divide the year into months based on the moon’s cycle. The time from one full moon to another is about 29 days, though modern months do not match the moon’s cycle exactly. The History of the Calendar Egyptians The ancient Egyptians created one of the first calendars. Based on star motions, they calculated that the year was about 365 days long. They divided the year into 12 months of 30 days each, with an extra 5 days at the end. The History of the Calendar Roman The Romans borrowed the Egyptian calendar. But Earth’s orbit actually takes about 365 ¼ days. The Romans adjusted the Egyptian calendar by adding one day every four years. You know this fourth year as “leap year,” when February is given 29 days instead of its usual 28. Using leap years helps to ensure that annual events, such as the beginning of summer, occur on the same date each year. The History of the Calendar Gregorian The Roman calendar was off by a little more than 11 minutes a year. Over the centuries, these minutes added up. By the 1500s, the beginning of spring was about ten days too early. To straighten things out, Pope Gregory XIII dropped ten days from the year 1582. He also made some other minor changes to the Roman system to form the calendar that we use today. Assess Your Understanding 1a. Identify What are the two major motions of Earth as it travels through space? b. Explain Which motion causes day and night? What Causes Seasons? Many places that are far from Earth’s equator and its poles have four distinct seasons: winter, spring, summer, and autumn. But there are differences in temperature from place to place. For instance, it is generally warmer near the equator than near the poles. Why? How Sunlight Hits Earth The next slide shows how sunlight strikes Earth’s surface. Notice that, near the equator, sunlight hits Earth’s surface from almost overhead. Near the poles, sunlight arrives at a steep angle. As a result, it is spread out over a greater area. That’s why it is warmer near the equator than near the poles. Near the equator, sunlight does not spread very far. The sun’s energy is concentrated in a smaller area. Near the poles, the same amount of sunlight spreads over a greater area. Sunlight on Earth The diagram shows how Earth’s tilted axis affects the strength of sunlight in different places Infer Draw a circle around the area where sunlight is most direct. Mark an X on the places that sunlight reaches but where it is less direct. Earth’s Tilted Axis If Earth’s axis were straight up and down relative to its orbit, temperatures in an area would remain fairly constant yearround. There would be no seasons. Earth has seasons because its axis is tilted as it revolves around the sun. Earth’s Tilted Axis On the last image the Earth’s axis is always tilted at an angle of 23.5° from the vertical. The North Pole always points in the same direction. As Earth revolves around the sun, the north end of its axis is tilted away from the sun for part of the year and toward the sun for part of the year. Summer and winter are caused by Earth’s tilt as it revolves around the sun. June In June, the north end of Earth’s axis is tilted toward the sun. In the Northern Hemisphere, the noon sun is high in the sky and there are more hours of daylight than darkness. The sun’s rays are concentrated. It is summer in the Northern Hemisphere. At the same time south of the equator, the sun’s energy is spread over a larger area. The sun is low in the sky and days are shorter than nights. It is winter in the Southern Hemisphere. June March December In December, people in the Southern Hemisphere receive the most direct sunlight, so it is summer. At the same time, the sun’s rays in the Northern Hemisphere are more slanted and there are fewer hours of daylight. So it is winter in the Northern Hemisphere. December March Seasons The diagram shows how Earth moves during the year. It is not drawn to scale. Make Generalizations Describe the weather and sunlight in the Northern and Southern hemispheres in March and September. September Solstices The sun appears farthest north of the equator once each year and farthest south once each year. Each of these days is know as a solstice (SOHL stis). The day when the sun appears farthest north is the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere. This solstice occurs around June 21 each year. It is the longest day of the year in the Northern Hemisphere and the shortest day in the Southern Hemisphere. The sun rises to the northeast and sets to the northwest. Solstices Similarly, around December 21, the sun appears farthest south. This is the winter solstice in the Northern Hemisphere and the summer solstice in the Southern Hemisphere. The sun rises to the southeast and sets to the southwest. Equinoxes Halfway between the solstices, neither hemisphere is tilted toward the sun. The noon sun is directly overhead at the equator, rises due east, and sets due west. Each of these days is known as an equinox, which means “equal night.” During an equinox, day and night are each about 12 hours long everywhere. The vernal (spring) equinox occurs around March 21 and marks the beginning of spring in the Northern Hemisphere. The fall, or autumnal, equinox occurs around September 22. It marks the beginning of fall in the Northern Hemisphere. Assess Your Understanding Define The noon sun is directly overhead at the equator during (a solstice / an equinox). Relate Cause and Effect What causes the seasons? Finish the Sentence Today I learned