My favorite professor during undergraduate was Kathleen Howell. She was a brilliant orbital mechanicist and a great teacher – but what I particularly liked about her was the way she began each course. She spent the first day of every semester explaining the history of orbital mechanics and the lesson we should all take from it.
The ancient Greek astronomer Hipparchus was among the first to formulate a theory of the geometry of planetary motion in the 3rd century B.C.E. We know very little about Hipparchus as most of his papers were destroyed, but we do know that he proposed a geocentric universe which was endorsed by Aristotle. Aristotle believed that all planets and the sun circled the earth in perfect circles. This made sense – after all, from our vantage point, the sun, the moon and all the stars revolve around us and surely, they must travel in circles. However, observations did not match the theory – something wasn’t quite right.
In C.E. 150, the Greek scientist Ptolemy published The Almagest in which he outlined his theory of planetary motion which has come to be known as the Ptolemaic system. His work was based on that of both Hipparchus and Aristotle, but introduced a new idea to account for deviations from observations – the epicycle. Ptolemy expanded Aristotle’s theory by explaining that planets traveled on small circles (epicycles) which were superimposed on their larger orbital circles. In addition, these bodies orbited a common point called an ‘equant’ which could lay outside the sphere of the earth. The result would be imperfect planetary motion. Despite years of efforts to calculate the number and characteristics of these epicycles, the method never adequately explained observations.
Over the course of the next thirteen hundred years, the idea of a geocentric universe slowly fell out of favor as more evidence to support a heliocentric universe emerged. In the early sixteenth century, a Prussian astronomer named Nicolaus Copernicus formulated and wrote his theory of a heliocentric universe, eventually allowing it to be published in 1543, the year that he died. Copernicus was the first to develop significant support for a theory which placed the sun at the center of the universe with planets again laying on epicycles – circles upon circles. Galileo’s endorsement of the Copernican model added even more weight to the idea that the earth was not in fact the center of the universe! The battle lines were drawn – the Ptolemaic system which placed the earth at the center of circular orbits vs. the Copernican system with the sun at the center of…you guess it – circular orbits. Neither one explained observations no matter how many epicycles you used, yet the fight raged on.
It was around this time that a Danish nobleman named Tycho Brahe decided to answer the question once and for all – was the earth the center of circular planetary motion, or was the sun? He spent his life making painstakingly accurate measurements of celestial positions, managing only to show that no known combination of circles could account for observations. When Brahe died in 1601, his German assistant, Johannes Kepler inherited the data and continued the necessary observations. It took eight years of pouring over the data for Kepler to finally put it all together.
Over seventeen hundred years had been spent arguing two systems, neither of which explained observable data. One group challenged the idea of the sun being the center of the universe while the other challenged the earth, but nobody thought to challenge the fundamental assumption behind both theories – that orbits were circular. In 1609, Kepler published Astronomia Nova in which his first two laws of planetary motion were detailed. The sun was the center of the solar system, he argued, but planets did not travel in circles - they traveled in ellipses. In 1631, using his three newly formulated laws or orbital motion (his third law was published in 1619), Kepler became the first scientist to accurately predict a transit of Venus across the sun (an event which happens only four times every 243 years).
Although he was not able to explain why planets travel in ellipses (that would take Isaac Newton’s laws of motion formulated some 78 years later), Kepler was able to break a 1700 year gap in progress by doing the one thing nobody else thought to do – consider the possibility that the initial assumption is flawed.
What strikes me about this story is not how long a battle raged over a flawed assumption but how little we have learned from history. We live in a world in which countless groups of people spend their lives debating, damning and killing each other over ancient ideals they are certain to be true, given one crucial assumption. How many of them have ever bothered to check it?