Revolutions of Earth

Earth, and the other seven planets, revolve around the Sun in an orbit that is slightly elliptical.

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Explaining Motion in the Universe

Earth Orbits a Star

Certainly no one today doubts that Earth orbits a star, the Sun. Photos taken from space, observations made by astronauts, and the fact that there has been so much successful space exploration that depends on understanding the structure of the solar system all confirm it. But in the early 17th century saying that Earth orbited the Sun rather than the reverse could get you tried for heresy, as it did Galileo. Let’s explore the evolution of the idea that Earth orbits the Sun.

The Geocentric Universe

To an observer, Earth appears to be the center of the universe. That is what the ancient Greeks believed. This view is called the geocentric model, or "Earth-centered" model, of the universe.

One problem with the geocentric model is that some planets seem to move backwards (in retrograde) instead of in their usual forward motion around Earth. A demonstration animation of retrograde motion of Mars as it appears to Earth can be found here: http://projects.astro.illinois.edu/data/Retrograde/index.html.

The Heliocentric Universe

At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric model, or "sun-centered" model.

Although Copernicus’ model was simpler  it still did not perfectly describe the motion of the planets. Johannes Kepler solved the problem a short time later when he determined that the planets moved around the Sun in ellipses (ovals), not circles (Figure below). Kepler’s model matched observations perfectly.

Animation of Kepler’s Laws of Planetary Motion: http://projects.astro.illinois.edu/data/KeplersLaws/index.html

Kepler’s model showed the planets moving around the sun in ellipses.

In their elliptical orbits, each planet is sometimes farther away from the Sun than at other times. This movement is called revolution. At the same time, Earth spins on its axis. Earth’s axis is an imaginary line passing through the planet’s center that goes through both the North Pole and the South Pole. This spinning movement is called Earth’s rotation.

Earth’s Revolution

Copernicus, Galileo, and Kepler were all right: Earth and the other planets travel in an elliptical orbit around the Sun. The gravitational pull of the Sun keeps the planets in orbit. This ellipse is barely elliptical; it's very close to being a circle.

Earth and the other planets in the solar system make elliptical orbits around the Sun.

For Earth to make one complete revolution around the Sun takes 365.24 days. This amount of time is the definition of one year.

Summary

• In the geocentric model of the universe, Earth is at the center.
• In the heliocentric model of the universe, the Sun is at the center. The heliocentric model is the modern view of the solar system, but not the entire universe.
• Earth and the other planets of the solar system revolve around the Sun.

Interactive Practice

Use this resource to answer the questions that follow.

1. What does geocentric mean?

2. What does heliocentric mean?

3. When did the heliocentric theory gain popularity?

4. Why was did geocentric theory remain popular for so long?

5. Why did the heliocentric theory eventually take precedence?

Review Questions

1. How does the heliocentric model differ from the geocentric model?

2. Why do you think people had a hard time switching from one worldview to the other?

3. Describe Earth’s orbit around the Sun.

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