Planetary Motion

Jacob Kirsten
The primary author is the individual who drafted the first version of this section; a section that could have been modified since it was originally published.

Today, when someone says "the Earth orbits the Sun," we don't blink twice because it is widely accepted as fact. That wasn't always the case in our history. At one point, people were burned at the stake for suggesting that the something doesn't revolve around Earth.

Stars have had consistent patterns throughout the seasons, but, earlier in history, people have thought the planets had an inconsistent pattern across the sky. The understanding of their movements led to the modern understanding of gravity and motion.

There were conflicting theories about the reasons planets moved across the sky. Because he had seen no sign that the Earth was even in motion, Aristotle believed that planets and the Sun orbited Earth. This theory dominated human's scientific understanding for almost 1,000 years. This worldview also was a part of Christian theology.

In 1515, Copernicus proposed that Earth orbited the Sun, like any other planet in our solar system. He published his De Revolutionibus in 1543 after being concerned of criticism from the scientific community. Over time, the theory gradually got more and more followers supported by the mathematical genius of Newton and Kepler. It wasn't until 1610 that Galileo noticed that moons were orbiting Jupiter contradicting Aristotle's theory that everything orbit's Earth.

Around the same time as Galileo, Kepler published laws that described the orbits of the planets around the sun, a Geocentric view of the Universe because the orbit of Mars was difficult to place. Copernicus' system suggested that the planets orbited the sun in a circular motion, but Kepler finally realized that planets orbited the Sun in an elliptical pattern and that Mars was the most elliptical.

Newton built on Kepler's laws explaining that gravity was the reason planets orbited the sun. Kepler had inspired Newton to create laws of his own to explain why the planets moved as they do: The Laws of Motion. As Newton explained, "If you press a stone with your finger, the finger is also pressed by the stone."

On page 95 of Unflattening, Nelson Goodman is quoted, "The Sun moves or is still can both be true statements of the same world." This suggests that because people have different viewpoints of the world, it doesn't make either one of them wrong; much like Aristotle and Copernicus. Both of them had different viewpoints of the universe, but at the time they were using the knowledge they had in examining our solar system. Schrödinger's cat is a good example of this situation, where until you open the box, you must assume that the cat is both alive, and dead.

For Further Reading

    Riebeek, Holli. "Planetary Motion: The History of an Idea That Launched the Scientific Revolution : Feature Articles." Earth Observatory, 7 July 2009. Web. 21 June 2015.

    "Johannes Kepler: The Laws of Planetary Motion." University of Tennessee. n.d. Web. 21 June 2015.

    Sommerville, J.P. "Nikolaus Copernicus, Tycho Brahe, and Johannes Kepler." University of Wisconsin-Madison. n.d. Web. 21 June 2015.





Title page from the second edition of Nicolaus Copernicus' De revolutionibus orbium coelestium