Giordano Bruno (1548–1600) was an Italian priest who adapted and expanded upon the astronomy of Copernicus. (You can find out more about Copernicus and the Scientific Revolution a little later.) In brief, the theory is
Heliocentricism says that the sun is the center of the solar system and the Earth and other planets revolve around it. This theory is now accepted as fact, but a few hundred years ago, you could have been burned at the stake for teaching it.
Though he was not part of the Protestant Reformation, Bruno was highly critical of Catholic dogma, a most dangerous view to be voiced in those days. In addition to the belief that the sun was the center of the solar system and the Earth and the other planets revolved around it, Bruno posited that there were an infinite number of solar systems supporting an infinite number of planets, many of which supported life, both like and unlike life on Earth.
A radical notion? It is the commonly accepted belief today. Both scientists and the public at large readily accept that we are very likely not alone in the vast universe.
Bruno's views on the cosmos are being validated by modern science. He was way ahead of his time, not a household name like the more famous Galileo, and there have been many attempts over the centuries to discredit him and dismiss him as simply a Renaissance magician. While it is true he had a great interest in magic, there was a scientific method to this magus. Bruno believed that there were more things in Heaven and Earth, as the poet said. There were unseen elements and untapped energies governing the cosmos that, if comprehended and perhaps even harnessed, could have world-transforming results. Unfortunately, there were established political and religions institutions who were quite cozy in their exalted and controlling status quo and who did not take kindly to the notion of a transformed world. Giordano Bruno was imprisoned by the Catholic Church, tortured, and eventually burned at the stake for his beliefs. Such was sadly and often the case for great thinkers who wanted to push the envelope of human knowledge and achievement.
Bruno's statement that someone who fancies himself a philosopher must be ready to doubt everything ranks up there along with Socrates's “Know thyself” and “An unexamined life is not worth living” as a cardinal rule for a philosopher to live by. It was later made better known by Descartes many decades later.