Chapter 17 Earth’s Cycles

Lesson 1 – How does Earth move? Earth spins like a top as it circles around and around the Sun. Earth is tipped to one side as it moves. This tilt causes the changing seasons.

Why can’t we feel Earth moving? Everyone agrees that Earth doesn’t seem to be moving. You can’t sense its movement because you are moving right along with it. Another reason you don’t sense Earth’s motion is that it moves steadily and smoothly.

What are some clues that Earth is moving? 1. To people on Earth, the Sun and the stars seem to move across the sky. Why? Because the Earth is constantly turning. 2. Another clue is that the seasons change during the year for most places away from the equator. The equator is the imaginary line that divides the north and south halves of the Earth.

Earth’s Rotation Earth turns around its axis. The axis is the imaginary line that goes through its center. The spinning of Earth around its axis is called its rotation. How long does it take for Earth to spin once on its axis? 23 hours and 56 minutes – 1 day

How does Earth’s rotation cause day and night? As Earth spins or rotates on its axis, there are certain times when areas of Earth are not facing the Sun. The result is darkness or – night. When Earth rotates, it turns from west to east. This causes objects such as the Sun and stars to appear to move from east to west in the sky.

Why do shadows change? Because of Earth’s rotation, shadows change their positions and sizes during the day. Think about it. In the morning and the afternoon your shadow is much longer because the Sun is shining on you at an angle. At 12:00 noon your shadow is fairly short because the Sun is directly over your head.

Earth’s Revolution We have just learned that Earth spins on its axis and that is called rotation. However, Earth is not just rotating. It is also moving in a path around the Sun. This is called revolution. It takes Earth an entire year to make one revolution around the Sun. The path that Earth takes around the Sun is called its orbit. Earth’s orbit is an ellipse. Think of an ellipse as a stretched out circle.

What keeps Earth revolving around the Sun? Why doesn’t Earth just float around in outer space? Gravity! Gravity is the force that pulls two objects toward each other. There is gravity between the Earth and the Sun.

Revolution

Seasons

What causes the seasons to change? The tilt of Earth on its axis. During summers in the Northern Hemisphere, the North Pole tilts toward the Sun and gets more direct sunlight, while the South Pole tilts away from the Sun and gets sunlight that is less direct. United Streaming Video – Seasons (25 minutes)

Lesson 2 – What patterns can you see in the sky? You see stars in different parts of the night sky as Earth rotates on its axis and revolves around the Sun. The Moon revolves around Earth. It also rotates on its own axis. All these movements cause changes in the patters that you see in the sky.

The Moon

The Moon The Moon looks as if it is shining with its own light. However, the moon does not produce light. You can see the Moon because it reflects sunlight off its surface. The Moon orbits around Earth in an elliptical orbit. It takes the Moon about 27.3 days to complete one revolution around Earth. Like Earth, the Moon rotates on its axis. As the Moon orbits Earth, the SAME SIDE of the Moon always faces Earth.

Why does the Moon look different at different times of the month? The changing of the Moon refers to the Moon’s phases. Different amounts of the lighted half of the Moon face Earth at different times. Phases of the Moon Video – 3 minutes

Eclipses An eclipse occurs when one object in space gets between the Sun and another object, and casts its shadow on the other object.

Lunar Eclipse A lunar eclipse occurs when the Moon passes through Earth’s shadow. Lunar Eclipse Video (1 minute)

Solar Eclipse A solar eclipse occurs when the moon comes between the Earth and the Sun. Solar Eclipse video (1 minute)

The Sun The Sun is a medium size star that is nearest Earth. United Streaming Video – The Sun (1 minute)

Constellations A constellation is a star pattern in the sky. For thousands of years, people have noticed that the stars appear in shapes and patterns in the sky. Constellations Video – 45 seconds