On Kepler and Howell
My favorite undergraduate professor, Dr. Kathleen Howell, spent the first day of each 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. The geocentric universe he proposed was endorsed by Aristotle who believed that all planets and the sun circled the earth in perfect circles. However, the theory did not explain planetary observations. Some four hundred years later in C.E. 150, the Greek scientist Ptolemy outlined his theory of planetary motion which has come to be known as the Ptolemaic system. Ptolemy expanded Aristotle’s theory by explaining that planets traveled on small circles (epicycles) which were superimposed on their larger orbital circles. Despite years of efforts to calculate the number and characteristics of these epicycles, this theory also failed to explain 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 first began 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 our planet earth orbited the sun in a circular orbit. Yet neither the Ptolemaic system nor the Copernican system explained observations no matter how many epicycles were used.
Around the same time, a Danish nobleman named Tycho Brahe was making painstakingly accurate measurements of celestial positions in an effort to determine empirically which system was correct – the Ptolemaic or the Copernican. A lifetime of observation managed 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 solve the mystery.
Seventeen hundred years had been spent trying to make theory match observation by changing the orbit’s center, yet 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, 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 was flawed.
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. The geocentric universe he proposed was endorsed by Aristotle who believed that all planets and the sun circled the earth in perfect circles. However, the theory did not explain planetary observations. Some four hundred years later in C.E. 150, the Greek scientist Ptolemy outlined his theory of planetary motion which has come to be known as the Ptolemaic system. Ptolemy expanded Aristotle’s theory by explaining that planets traveled on small circles (epicycles) which were superimposed on their larger orbital circles. Despite years of efforts to calculate the number and characteristics of these epicycles, this theory also failed to explain 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 first began 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 our planet earth orbited the sun in a circular orbit. Yet neither the Ptolemaic system nor the Copernican system explained observations no matter how many epicycles were used.
Around the same time, a Danish nobleman named Tycho Brahe was making painstakingly accurate measurements of celestial positions in an effort to determine empirically which system was correct – the Ptolemaic or the Copernican. A lifetime of observation managed 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 solve the mystery.
Seventeen hundred years had been spent trying to make theory match observation by changing the orbit’s center, yet 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, 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 was flawed.
posted by Nicholas at 10:18 PM
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