Planetary Motion:The History of an Idea [ 174 ]

Planetary Motion: The History of an Idea That Launched the Scientific Revolution

By Holli Riebeek. Design by Robert Simmon. July 7, 2009

In the black dome of night, the stars seem fixed in their patterns. They rotate through the sky over the seasons so unchangingly that most cultures have used the presence of one or another constellation to tell time. The planets, however, are different, puzzling. They glide slowly and seemingly erratically across the sky. Attempts to explain why the planets move as they do led to modern science’s understanding of gravity and motion.

Many ancient and medieval cultures believed the stars and the planets rotated around a fixed Earth. The complex motions of the planets—which sometimes move backwards across the sky (retrograde motion, shown in the photo)—led Renaissance astronomers to question this geocentric theory. These astronomers discovered the laws of orbital mechanics, transforming natural philosophy into the practice of science. (Photograph ©2007–08 Tunç Tezel.)

Evolution of an Idea

“We revolve around the Sun like any other planet.” —Nicolaus Copernicus

“Of all discoveries and opinions, none may have exerted a greater effect on the human spirit than the doctrine of Copernicus. The world has scarcely become known as round and complete in itself when it was asked to waive the tremendous privilege of being the center of the universe.” —Johann Wolfgang von Goethe

The ancient Greek philosophers, whose ideas shaped the worldview of Western Civilization leading up to the Scientific Revolution in the sixteenth century, had conflicting theories about why the planets moved across the sky. One camp thought that the planets orbited around the Sun, but Aristotle, whose ideas prevailed, believed that the planets and the Sun orbited Earth. He saw no sign that the Earth was in motion: no perpetual wind blew over the surface of the Earth, and a ball thrown straight up into the air doesn’t land behind the thrower, as Aristotle assumed it would if the Earth were moving. For Aristotle, this meant that the Earth had to be stationary, and the planets, the Sun, and the fixed dome of stars rotated around Earth.

A long-exposure photograph reveals the apparent rotation of the stars around the Earth. (Photograph ©1992 Philip Greenspun.)

For nearly 1,000 years, Aristotle’s view of a stationary Earth at the center of a revolving universe dominated natural philosophy, the name that scholars of the time used for studies of the physical world. A geocentric worldview became engrained in Christian theology, making it a doctrine of religion as much as natural philosophy. Despite that, it was a priest who brought back the idea that the Earth moves around the Sun.

In 1515, a Polish priest named Nicolaus Copernicus proposed that the Earth was a planet like Venus or Saturn, and that all planets circled the Sun. Afraid of criticism (some scholars think Copernicus was more concerned about scientific shortcomings of his theories than he was about the Church’s disapproval), he did not publish his theory until 1543, shortly before his death. The theory gathered few followers, and for a time, some of those who did give credence to the idea faced charges of heresy. Italian scientist Giordano Bruno was burned at the stake for teaching, among other heretical ideas, Copernicus’ heliocentric view of the Universe.

In 1543, Nicolaus Copernicus detailed his radical theory of the Universe in which the Earth, along with the other planets, rotated around the Sun. His theory took more than a century to become widely accepted. [Adapted from Nicolaus Copernicus, 1543, De revolutionibus orbium coelestium (“On the Revolutions of the Heavenly Spheres.”)]

But the evidence for a heliocentric solar system gradually mounted. When Galileo pointed his telescope into the night sky in 1610, he saw for the first time in human history that moons orbited Jupiter. If Aristotle were right about all things orbiting Earth, then these moons could not exist. Galileo also observed the phases of Venus, which proved that the planet orbits the Sun. While Galileo did not share Bruno’s fate, the Spanish Inquisition did, under threat of torture, force him to retract his support for a heliocentric solar system, and they placed him under house arrest for life at the age of 69.

Galileo discovered evidence to support Copernicus’ heliocentric theory when he observed four moons in orbit around Jupiter. Beginning on January 7, 1610, he mapped nightly the position of the 4 “Medicean stars” (later renamed the Galilean moons). Over time Galileo deduced that the “stars” were in fact moons in orbit around Jupiter. [Adapted from Galileo Galilei, 1610, Sidereus Nuncius (“The Starry Messenger.”)]

At about the same time, German mathematician Johannes Kepler was publishing a series of laws that describe the orbits of the planets around the Sun. Still in use today, the mathematical equations provided accurate predictions of the planets’ movement under Copernican theory. In 1687, Isaac Newton put the final nail in the coffin for the Aristotelian, geocentric view of the Universe. Building on Kepler’s laws, Newton explained why the planets moved as they did around the Sun and he gave the force that kept them in check a name: gravity.

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The Science: Orbital Mechanics

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