Copernicus was a Polish astronomer and clergyman who, in 1543, introduced a new heliocentric system of the universe. In Copernicus's system, the planets revolved on a complex system of epicycles, but they all revolve around the sun. This was a revolutionary idea in the sixteenth century. Everyone was firmly convinced that the earth was motionless at the center of the universe. To imagine that it moved around the sun seemed ridiculous. It took several decades for the Copernican system to become fully accepted by astronomers and the public. Kepler was the first major astronomer to publicly acknowledge his support of it.
Tycho de Brahe was a Danish nobleman who made a name for himself in the late sixteenth century as Europe's best observational astronomer. He kept a closely guarded collection of astronomical observations, the most accurate astronomical data available at the time. Eager to use Tycho's figures to develop his own system, Kepler traveled to Prague to work in Tycho's lab. In addition to being a brilliant astronomer, Tycho was also an arrogant and temperamental man. Tycho and Kepler had a love-hate relationship; they respected one another, but each was also jealous of the other's achievements and potential. Several times, Kepler fled the lab, only to return full of apologies. When Tycho died, he expressed a hope that Kepler would use his data to develop the Tychonic system of the universe, in which the planets orbited the sun, which orbited the earth. Instead, Kepler applied Tycho's observations to the Copernican system, which led him to discover his first two laws.
Galileo was an Italian astronomer who discovered the moons of Jupiter. Galileo was the first major astronomer to use a telescope to observe the heavens. When these observations yielded findings that the scientific community was reluctant to believe, Kepler lent him public support Galileo later became a symbol of science's break from religion during the scientific revolution. He was put on trial by the Catholic Church and convicted of heresy for his support of the Copernican system
Kepler's father, Heinrich, was an itinerant criminal who repeatedly abandoned his family. At one point he owned a tavern, at another, he was nearly hanged for an alleged crime. One of Kepler's younger brothers was forced to run away from home when Heinrich threatened to sell him. Heinrich left for good in 1588 – he was not missed.
Katherine Kepler, Kepler's mother, was born Katherine Guldenmann. She was the daughter of an innkeeper and the niece of a woman who had been burned at the stake as a witch. Kepler later described her as a petty, angry, quarrelsome woman. She came back into Kepler's life in 1615, when her fellow villagers accused her of being a witch. Kepler was quick to come to her defense. After five years of argument and negotiation, Katherine was interrogated under threat of torture. When she continued to deny being a witch, she was finally released. She was driven from her town and died six months later.
Michael Maestlin was Kepler's most influential teacher at the University of Tuebingen. Maestlin was the first to teach Kepler about the Copernican system. In the classroom, Maestlin was a strong supporter of the Copernican system, but on paper, he continued to propound the Ptolemaic system. Kepler turned to Maestlin for help and advice throughout his life, but Maestlin seems to have grown tired of his troublesome student. He often ignored Kepler's letters for years at a time.
Kepler married Barbara Muehleck in 1597. It was a marriage of convenience, not love. Kepler's friends had decided it was time for him to marry and had chosen Barbara as a good mate; Kepler acquiesced. They were married for fourteen years and had four children. Barbara died in 1611 of the Hungarian fever.
Two years after his first wife died, Kepler married the 24-year-old Susanna Pettinger. They had eleven children together and Kepler had nothing negative to say about her in later life – a ringing endorsement considering the way he described most of his family members.
Ptolemy, an astronomer from the second century A.D., formulated a system of the universe that lasted for over one thousand years after his death. His system placed the earth at the center of the universe, with the planets and the stars revolving around it. Ptolemy insisted that the planets in his system moved with uniform circular motion. Because this is not actually how the planets move, he was forced to introduce the following mathematical devices. The deferent is the main circle around which each planet orbits the earth. An epicycle is a smaller circle around which the planet orbits the deferent. Finally, the equant is an imaginary point in the exact center of the planetary orbits. Ptolemy's system was so complex that, by the time of Copernicus, it contained somewhere between forty and eighty epicycles.