3.5: Earth's Revolutions

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What kind of revolution are we talking about?

Copernicus caused a revolution. He said that Earth revolved around the Sun. With his telescope, Galileo found a lot of evidence for this. He could see moons orbiting Jupiter. If moons can orbit Jupiter, surely Earth can orbit the Sun. Yes? In the two images above, you can see Jupiter at two different times, showing moons in different places.

Earth's Revolution

Earth orbits a star. That star is our Sun. One revolution around the Sun takes 365.24 days. That is equal to one year. Earth stays in orbit around the Sun because of the Sun's gravity (Figure below).

Earth's orbit is not a circle. It is a bit elliptical. So as we travel around the Sun, sometimes we are a little farther away from the Sun. Sometimes we are closer to the Sun.

Students sometimes think the slightly oval shape of our orbit causes Earth's seasons. That's not true! The seasons are due to the tilt of Earth’s axis, as discussed in the previous concept.

Planets in the solar system make elliptical orbits around the Sun

Earth and the other planets in the solar system make elliptical orbits around the Sun. The ellipses in this image are highly exaggerated.

The distance between the Earth and the Sun is about 93 million miles, or 150 million kilometers. Earth revolves around the Sun at an average speed of about 27 kilometers (17 miles) per second. Mercury and Venus are closer to the Sun, so they take shorter times to make one orbit. Mercury takes only about 88 Earth days to make one trip around the Sun. All of the other planets take longer amounts of time. The exact amount depends on the planet's distance from the Sun. Saturn takes more than 29 Earth years to make one revolution around the Sun. How old would you be if you were on Jupiter?

Summary

Earth's orbit around the Sun is somewhat elliptical.
Earth's seasons are not caused by the shape of its orbit.
Earth and the other planets of the solar system revolve around the Sun.

Review

How long does it take for Earth to make one revolution around the Sun?
Is Earth farther from the Sun in the winter and closer in the summer? Explain.
Describe Earth’s orbit around the Sun. Describe the orbits of the other planets.

Explore More

Use this resource to answer the questions that follow.

What did Aristotle and Ptolemy say about the relationship of Earth to other celestial bodies?
In what ways did planets not fit their ideas?
Why was the geocentric model a hypothesis that needed to be thrown out?
What did Copernicus say about the solar system? What was the heliocentric model?
Why was Copernicus’ model wrong?
What was Kepler’s innovation?
What contributions did Galileo’s telescope make to accepting the Copernicus/Kepler model of the solar system?
What did The Church do with the heliocentric model?
What is wrong with the heliocentric model?
Why do we know that the geocentric model is wrong today?