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Geocentric Model
Many ancient people believed that the sun and the other planets orbited around the Earth. The Roman astronomer Claudius Ptolemy created the most influential geocentric model in the second century. He theorized that each planet moved around a circle he called an epicycle. Each epicycle moved on a giant circle around Earth called a deferent. The center of the deferent was a point halfway between Earth and another point called the equant. The equant was defined as the point where the epicycle's movement seemed uniform. Most medieval Christians accepted this model as proof of God's concern for mankind. The Roman Catholic Church saw any challenge to it as a challenge to the faith.
Nicholas Copernicus
The Polish astronomer Nicholas Copernicus developed his heliocentric model of the universe in the early 16th century. He placed the sun in the center of the solar system and eliminated the concept of the equant. However, his model was only a limited revision of Ptolemy and still explained planetary motion with epicycles. Copernicus's theory was not published until he was near death in 1543 but it laid the groundwork for important developments such as Johannes Kepler's laws of planetary motion and Isaac Newton's theory of gravitation.
Tycho Brahe
The Danish astronomer Tycho Brahe made many important observations before the invention of the telescope. He proved that, contrary to common belief, supernovae and comets exist in space beyond the moon, made important observations of Mars that contributed to Kepler's laws of planetary motion and developed the rules of evidence that still guide modern researchers. He also proposed a compromise between the heliocentric model and the geocentric model. His concept placed Earth at the center but allowed the planets to move around the sun as it moved around the Earth. This explained the observation that the planets move around the sun but preserved the belief that Earth was the center of the cosmos.
Giordano Bruno
Theologian and philosopher Giordano Bruno was an early proponent of Copernicus's theory of a heliocentric universe. However, he went beyond it, developing a concept of an infinite universe without a physical center. He argued his position from both a theological and scientific viewpoint. He believed that the Bible gave instructions in ethics not astronomical data and argued that if God's love is infinite, then his creation is also infinite. Bruno supported his belief with Brahe's observations of comets and his own belief that universe was composed of particles called atoms and a mysterious substance called ether. Bruno was burned at the stake as heretic in 1600, although the exact charge against him has been lost.