Unit 3 Lesson 1 Earth’s Days, Years, and Seasons Copyright © Houghton Mifflin Harcourt Publishing Company

Spinning in Circles What determines the length of a day? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons Spinning in Circles What determines the length of a day? Each planet spins on its axis. The spinning of a body, such as a planet, on its axis is called rotation. The time it takes a planet to complete one full rotation on its axis is called a day. Copyright © Houghton Mifflin Harcourt Publishing Company 2

What determines the length of a day? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What determines the length of a day? Earth rotates in a counterclockwise motion around its axis when viewed from above the North Pole. As a location on Earth’s equator rotates from west to east, the sun appears to rise in the east, cross the sky, and set in the west. Only one-half of Earth faces the sun at any given time. Copyright © Houghton Mifflin Harcourt Publishing Company 3

What determines the length of a day? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What determines the length of a day? People on the half of Earth facing the sun experience daylight, and this period is called daytime. People on the half of Earth facing away from the sun experience darkness, and this period is called nighttime. Earth completes one rotation on its axis in 24 hours, or one day. Copyright © Houghton Mifflin Harcourt Publishing Company 4

What determines the length of a day? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What determines the length of a day? How does Earth’s rotation make the sun appear to move across the sky? Copyright © Houghton Mifflin Harcourt Publishing Company 5

What determines the length of a year? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What determines the length of a year? As Earth rotates on its axis, it also revolves around the sun. The motion of a body that travels around another body in space is called revolution. Earth completes a full revolution around the sun in 365 ¼ days, or about one year. We have divided the year into 12 months, each lasting 28 to 31 days. Copyright © Houghton Mifflin Harcourt Publishing Company 6

What determines the length of a year? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What determines the length of a year? As Earth rotates on its axis, it revolves around the sun. Copyright © Houghton Mifflin Harcourt Publishing Company 7

Tilt-a-Whirl What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons Tilt-a-Whirl What conditions are affected by the tilt of Earth’s axis? Earth’s axis is tilted at 23.5°. During each revolution, the North Pole may be tilted toward the sun or away from the sun. When the North Pole is tilted toward the sun, the Northern Hemisphere has longer periods of daylight than the Southern Hemisphere has. When the North Pole is tilted away from the sun, the opposite is true. Copyright © Houghton Mifflin Harcourt Publishing Company 8

What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What conditions are affected by the tilt of Earth’s axis? At each position of Earth’s orbit, which hemisphere is experiencing longer hours of daylight? Copyright © Houghton Mifflin Harcourt Publishing Company 9

What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What conditions are affected by the tilt of Earth’s axis? The angle at which the sun’s rays strike each part of Earth’s surface changes as Earth moves in its orbit. When the North Pole is tilted toward the sun, the sun’s rays strike the Northern Hemisphere more directly, and this region is warmer. When the North Pole is tilted away from the sun, the sun’s rays strike the Northern Hemisphere less directly, and this region is cooler. Copyright © Houghton Mifflin Harcourt Publishing Company 10

What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What conditions are affected by the tilt of Earth’s axis? The spherical shape of Earth also affects how the sun warms up an area. At a point near the equator, the sun’s rays hit Earth’s surface more directly, so temperatures are higher there. At a point near one of the poles, the sun’s rays hit Earth’s surface less directly, so temperatures are lower there. Copyright © Houghton Mifflin Harcourt Publishing Company 11

What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What conditions are affected by the tilt of Earth’s axis? Compare the area over which the sun’s rays are spread at the equator and at the poles. Copyright © Houghton Mifflin Harcourt Publishing Company 12

What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What conditions are affected by the tilt of Earth’s axis? The number of daylight hours on any given day of the year varies with location. Areas around Earth’s equator receive about 12 hours of light a day. Copyright © Houghton Mifflin Harcourt Publishing Company 13

What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What conditions are affected by the tilt of Earth’s axis? Areas on Earth’s surface that are tilted toward the sun have longer than 12-hour days, as they travel a longer path through the lit part of Earth. Areas on Earth’s surface that are tilted away from the sun have shorter than 12-hour days, as they travel a shorter path through the lit part of Earth. Copyright © Houghton Mifflin Harcourt Publishing Company 14

What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What conditions are affected by the tilt of Earth’s axis? Which place has the longest days? Which has the shortest? Copyright © Houghton Mifflin Harcourt Publishing Company 15

What conditions are affected by the tilt of Earth’s axis? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What conditions are affected by the tilt of Earth’s axis? During summer in the Northern Hemisphere, areas north of the Arctic Circle receive 24 hours of daylight. At the same time, areas south of the Antarctic Circle receive 24 hours of darkness. In winter in the Northern Hemisphere, the polar areas experience the reverse conditions. Copyright © Houghton Mifflin Harcourt Publishing Company 16

Seasons Change … What causes seasons? Unit 3 Lesson 1 Earth’s Days, Years, and Seasons Seasons Change … What causes seasons? Most locations on Earth experience seasons. Each season is characterized by a pattern of temperature and other weather trends. We experience seasons due to changes in the intensity of sunlight and the number of daylight hours as Earth revolves around the sun. Copyright © Houghton Mifflin Harcourt Publishing Company 17

Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What causes seasons? At an equinox, 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 sunlight reaches more of one hemisphere than the other. At a solstice, the area of sunlight is at a maximum in one hemisphere and at a minimum in the other hemisphere. Copyright © Houghton Mifflin Harcourt Publishing Company 18

Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What causes seasons? During the September equinox, sunlight shines equally on both poles. During the March equinox, half of each hemisphere is lit, and the sunlight is centered on the equator. Copyright © Houghton Mifflin Harcourt Publishing Company 19

Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What causes seasons? At the December solstice, the North Pole leans away from the sun and is in complete darkness, and the South Pole is in complete sunlight. At the June solstice, the North Pole leans toward the sun and is in complete sunlight, and the South Pole is in complete darkness. Copyright © Houghton Mifflin Harcourt Publishing Company 20

Unit 3 Lesson 1 Earth’s Days, Years, and Seasons What causes seasons? At which point in Earth’s orbit does the Northern Hemisphere experience each season? Copyright © Houghton Mifflin Harcourt Publishing Company 21