Chapter 19: Earth, Moon, and Sun Section 1: Earth in Space Key terms: astronomy, axis, rotation, revolution, orbit, solstice, equinox Key concepts: How does Earth move in space? What causes the cycle of seasons on Earth?

How Earth moves Earth moves through space in two major ways: rotation and revolution Rotation: spinning of Earth on its axis. It causes day and night. It rotates once per 24 hours. Axis: imaginary line that passes through Earth’s center and North and South poles

Revolution: movement of one object around another Revolution: movement of one object around another. One revolution of the earth around the sun takes 24 hours. Earth follows a path, or orbit, which is an ellipse.

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Seasons of the earth and how sunlight hits the earth How sunlight hits: most directly at the equator. Near the poles, it happens at a steep angle. That is why it is warmer at the equator. Earth’s tilted axis: Earth has seasons because its axis is tilted as it revolves around the sun. It is approx. at 23.5 degrees from the vertical. As it revolves around the sun, the north end of the earth is tilted away from the sun for part of the year and toward the sun for part of the year.

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Important to note: The change in seasons is not caused by changes in Earth’s distance from the sun.

Earth in months: In June: the north end of Earth’s axis is tilted toward the sun. In the N. hemisphere, the noon sun is high in the sky and there are more hours of daylight. S. hemisphere has the sun’s energy spread over a larger area, creating winter. In December: S. hemisphere gets more sun, so it is summer.

Solstices When the sun is farthest north or south of the equator. When the sun is farthest north is the summer solstice in the N. hemisphere, winter solstice in S. hemisphere

Equinoxes Halfway between the solstices, neither hemisphere is tilted toward or away from the sun. The noon sun is directly overhead at the equator. Equal hours of day and night everywhere on Earth.