Theory of planetary motion

For 2,000 years, astronomers placed the earth at the center of the universe and assumed that all heavenly bodies moved in perfect circles around it. Copernicus discovered that the sun lay at the center of the solar system, but still assumed that all planets traveled in perfect circles.

Copernicus put forth the idea of the heliocentric universe in his book De Revolutionibus Orbium Coelestium. Howevr, he was not the forts person to arote about it. The idea can be traced back almost two thousand years earlier to the Greek mathematician and philosopher Aristarchus of Samos (310-230 BC).

Johannes Kepler was born in Southern Germany in 1571, 28 years after the release of Copernicus’s discovery.

Kepler used Copernicus’s model as the foundation for his laws of planetary motion.  Kepler accepted Copernicus’s heliocentric idea, but he also found parts of De Revolutionibus troubling. Kepler believed that earth and other know planets revolved around the sun, called the heliocentric model.

Elliptical orbits became Kepler’s first law, Kepler then added his Second Law each planet’s speed altered as a function of its distance from the sun. As a planet flew closer, it flew faster.

Kepler published his discoveries in 1609 and then spent the next 18 years calculating detailed tables of planetary motion and position for all six known planets.
Theory of planetary motion

Copernicus and his discovery

Copernicus and his discovery
In 1499 Copernicus graduated from the University of Bologna, Italy; was ordained priest in the Catholic Church; and returned to Poland to work for his uncle, Bishop Waczenrode, at the Frauenburg Cathedral.

Copernicus was given the top rooms in a Cathedral tower so he could continue his astronomy measurements.

At that time people still believed a model of the universe created by the Greek scientist, Ptolemy more than 1,500 years earlier.

According to Ptolemy, the earth was the center of the universe and never moved. The sun and planets revolved around the earth in great circles while the distant stars perched way out on the great spherical shell of space.

But careful measurement of the movement of planets didn’t fit with Ptolemy’s model.

So astronomers modified Ptolemy’s universe of circles by adding more circles within circles or epi-circles. The model now claimed that each planet travel along a small circle (epi-circle) that rolled along that planets big orbital circle around the earth.

Century after century, the errors in even this model grew more and more evident. More epi-circles were added to the model so that planets moved along epi-circles within epi-circles.

Copernicus hoped to use “modern” (sixth-century) technology to improve on Ptolemy’s measurements and hopefully, eliminate some of the epi-circles.

For almost 20 years Copernicus painstakingly measured the position of the planets each night. But his tables of findings still made no sense in Ptolemy’s model.

Over the years, Copernicus began to wonder what the movement of the planets would look like from another moving planet. When his calculations based on this idea more accurately predicted the planets’ actual movement, he begin to wonder what the notion of the planets would look like of the earth moved. Immediately, the logic of this notion became apparent.

Each planet appeared at different distances from earth at different times throughout a year. Copernicus realized that this meant Earth could not lie at the center of the other planets’ circular paths.

From 20 years of observations he knew that only the sun did not vary in apparent size over the course of a year.

This meant that the distance form earth to the sun had always remain the same. If the earth was not at the center, then the sun had to be. He quickly calculated that of he placed the sun at the universe’s center and had the earth orbit around it, he could not completely eliminate all epi-circles and have the known planets travel in simple circles the sun.

For fear of retribution from the Church, Copernicus dared not released his findings during his lifetime. They were made public in 1543, and even then they were consistently scorned and ridiculed by the Church, astronomers and universities alike.

Finally, 60 years later, first Johannes Kepler and then Galileo Galilei proved that Copernicus was right.
Copernicus and his discovery