High School Earth Science/Earth's Motions

Imagine a line passing through the center of Earth that goes through both the North Pole and the South Pole. This imaginary line is called an axis. Earth spins around its axis, just as a top spins around its spindle. This spinning movement is called Earth's rotation. At the same time that the Earth spins on its axis, it also orbits, or revolves around the Sun. This movement is called revolution.

Lesson Objectives

Describe Earth's rotation on its axis.
Describe Earth's revolution around the Sun.

Earth's Rotation

Figure 24.9: A pendulum at the North Pole always swings in the same direction, but because of Earth's rotation its direction will appear to change to observers on Earth.

In 1851, a French scientist named Léon Foucault took an iron sphere and swung it from a wire. He pulled the sphere to one side and then released it, letting it swing back and forth in a straight line. A ball swinging back and forth on a string is called a pendulum. A pendulum set in motion, will not change its motion, so it will not change the direction of the swinging. However, Foucault observed that his pendulum did seem to change direction. He knew that the pendulum itself could not change its motion, so he concluded that the Earth, underneath the pendulum was moving. Figure 24.9 shows how this might look.

It takes 23 hours, 59 minutes and 4 seconds for the Earth to make one complete rotation on its axis, if we watch Earth spin from out in space. Because Earth is moving around the Sun at the same time that it is rotating, Earth has to turn just a little bit more to reach the same place relative to the Sun, so we experience each day on Earth as 24 hours. At the equator, the Earth rotates at a speed of about 1,700 kilometers per hour. Thankfully, we do not notice this movement, because it would certainly make us dizzy.

Earth's Revolution

Earth's revolution around the Sun takes much longer than its rotation on its axis. One complete revolution takes 365.25 days, or one year. The Earth revolves around the Sun because gravity keeps it in a roughly circular orbit around the Sun. The Earth's orbital path is not a perfect circle, but rather an ellipse, which means that it is like a slight oval in shape (Figure 24.10). This creates areas where the Earth is sometimes farther away from the Sun than at other times. We are closer to the Sun at perihelion (147 million kilometers) on about January 3rd and a little further from the Sun (152 million kilometers) at aphelion on July 4th. Students sometimes think our elliptical orbit causes Earth's seasons, but this is not the case. If it were, then the Northern Hemisphere would experience summer in January!

Figure 24.10: Earth and the other planets in the solar system make regular orbits around the Sun; the orbital path is an ellipse and is controlled by gravity.

During one revolution around the Sun, the Earth travels at an average distance of about 150 million kilometers. Mercury and Venus take shorter times to orbit the Sun than the Earth, while all the other planets take progressively longer times depending on their distance from the Sun. Mercury only takes about 88 Earth days to make one trip around the Sun, while Saturn, for example, takes more than 29 Earth years to make one revolution around the Sun.

Earth revolves around the Sun at an average speed of about 27 kilometers (17 miles) per second. Our planet moves slower when it is farther away from the Sun and faster when it is closer to the Sun. The reason the Earth (or any planet) has seasons is that Earth is tilted 23 1/2 degrees on its axis. This means that during the northern hemisphere "summer" the North pole points toward the Sun, receiving direct solar rays, and in the northern hemisphere "winter" the North Pole is tilted away from the Sun (Figure 24.11)and the rays of the Sun are angled rather than direct. Thus, the season we experience depends not on where the Earth is in its revolutionary orbit around the Sun, but rather the inclination of the axis of the Earth. This means "summer" in the northern hemisphere actually occurs when Earth is farthest from the Sun, but inclined toward it, and "winter" occurs when Earth is closest but inclined away.

Figure 24.11: The Earth tilts on its axis.

Lesson Summary

Earth rotates or spins on its axis once each day and revolves around the Sun once every year.
The tilt of Earth's axis produces seasons.

Review Questions

Describe the difference between Earth's rotation and its revolution.
What is the force that keeps the Earth and other planets in their orbital paths?
The planet Jupiter is about 778,570,000 kilometers from the Sun; Earth is about 150,000,000 kilometers from the Sun. Does Jupiter take more or less time to make one revolution around the sun? Explain your answer.
In its elliptical orbit around the Sun, the Earth is closest to the Sun in January. Even though Earth is closest to the Sun in January, people in the Northern hemisphere experience winter weather. Using your understanding of how the Earth is tilted on its axis, why do you think people in the Northern Hemisphere have winter in January?
Where on Earth would Foucault's pendulum appear to not be moving? Why?

Vocabulary

axis: An imaginary line that runs from the North Pole to South Pole, and includes the center Earth.
ellipse: A shape that looks like a slightly squashed circle.
hemisphere: One half of a sphere.
revolution: The Earth's movement around the Sun in an orbital path.
rotation: The motion of the Earth spinning on its axis.

Points to Consider

What type of experiment could you create to prove that the Earth is rotating on its axis?
If you lived at the equator, would you experience any effects due to Earth's tilted axis?
If Earth suddenly increased in mass, what might happen to its orbit around the Sun?

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