Copernicus  -  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  -  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 Galilei -  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

Heinrich Kepler -  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 -  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 -  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.

Barbara Muehleck -  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.

Susanna Pettinger -  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 -  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.

Astronomia Nova -   · The Astronomia Nova, or the New Astronomy was Kepler's masterpiece. Published in 1609, it was the result of over eight years of work. Kepler spent those years trying to work out the shape of the orbit of Mars. Using Tycho's data about the motion of the planets, Kepler was finally able to determine the shape of the orbit more accurately than anyone who had come before him. This resulted in the formation of his first two laws, which were published in the Astronomia Nova.

Geocentric -   · A geocentric system is one in which the earth is at the center of the universe. For thousands of years, scientists, philosophers, and theologians believed that the universe was geocentric. They were unwilling to believe Copernicus when he challenged that assumption.

Harmonice Mundi -   · The Harmonice Mundi, or Harmony of the World was the culmination of Kepler's life-long study of the structure of the universe. Published in 1618, it described a system in which the spacing between the planets was determined by universal harmonies. The theory was wrong, but the book is nonetheless important, as it marks the first appearance of Kepler's third law.

Heliocentric  -   · A heliocentric system is one in which the sun is at the center of the universe. The system that Copernicus introduced was a heliocentric system. This was not a completely original idea – some of the philosophers of ancient Greece had imagined that the universe might be constructed in this way. However, the dominant view had always been that the universe was geocentric, so Copernicus's claims were a shock to the European system.

Kepler's Three Laws  -   · Kepler is best known today for his contribution of the three planetary laws, which were instrumental in Newton's later development of his theory of universal gravitation. They are as follows: 1. The planets travel around the sun in elliptical orbits with the sun located at one focus. 2. As the planets travel around their orbits, they sweep out the same amount of area per unit of time, no matter where they are on the orbit. 3. The distance a planet's orbit is from the sun, cubed, is directly proportional to the time it takes the planet to travel around the orbit, squared. Mathematically, this can be stated as a 3/p 2 = K where "a" is the distance a planet's orbit is from the sun, "p" is the period, the time it takes for a planet to revolve around the sun once, and "K" is a constant.

Mysterium Cosmographicum -   · Published in 1597, the Mysterium Cosmographicum, or Mysteries of the Cosmos, was Kepler's first major work. It described his theory of the perfect solids, which, although he never fully admitted it, was completely wrong. More importantly, the Mysterium was Kepler's first step to rejoining physics and astronomy, as he grasped for physical explanation for the structure of the universe. He was the first astronomer in centuries to do so. It is in the Mysterium that Kepler first proposes that the sun be moved to the exact, physical center of the universe, and that a force from the sun is responsible for moving the planets around their orbits. The Mysterium was also the major work in fifty years to support the Copernican system.

Perfect solid -   · A perfect solid a three dimensional figure, such as a cube, whose sides are all identical. There are only five perfect solids: the tetrahedron (which has four triangular sides), cube (six square sides), octahedron (eight triangular sides), dodecahedron (twelve pentagonal sides), and icosahedron (twenty triangular sides). Each perfect solid can be inscribed in and circumscribed around a sphere. In the beginning of his career, Kepler believed that the planetary orbits could all be inscribed in one of the perfect solids.