If there was one scientific fact that was perfectly obvious to everyone in the 15th century, it was that the Earth was the center of the universe. After all, hadn't God fashioned it and separated it from the sky on the first day of the Creation? And wasn't it home to every living creature? And didn't the sun and the moon and the stars perform their dance across its sky each day and night while, as Ptolemy had written almost 1,000 years earlier, the Earth stood absolutely still and watched the show? Case closed.
EARTH IN THE CENTER OF EVERYTHING
In 1473, Nicolaus Copernicus - or rather, Mikolaj Kopernik - was born in Poland into a family of wealthy merchants. His father died when Mikolaj was 10, and his maternal uncle, a Catholic bishop, became the boy's guardian and oversaw his education. At 18 Mikolaj enrolled in the University of Krakow, where he studied astronomy, mathematics, philosophy, and the sciences. Four years later, he had completed his course of study but not obtained a degree (as I've been learning while writing these posts, wealthy young European gentlemen of that era were not usually expected to obtain degrees; it was considered a rather middle-class thing to do, suggesting as it did that one might need to someday make a living). Mikolaj was then sent by his uncle to the University of Bologna to study canon law, with the goal of becoming a Church official. Mikolaj - who by now had Latinized his name to Nicolaus Copernicus - spent three years in Bologna pretty much ignoring the study of canon law and devoting himself instead to his true passions: mathematics and astronomy. He also learned Greek, because many of the foremost astronomical treatises still being used, included Ptolemy's, had been written by ancient Greeks (yes, really) and were not available in translation. If Nicolaus had been trying to subvert his uncle's plans for him, however, he failed. His uncle pulled enough strings to have him appointed a canon in the Polish Church, although Nicolaus was still living in Italy! Nicolaus's feelings about having been done this "favor" are not recorded.
In 1501, Copernicus returned to his uncle's home in Poland, where of course he was expected to jump right into his canonical duties, but instead he managed to wheedle two more years in Italy out of his uncle, this time ostensibly to study medicine. At the University of Padua, he focused on his studies of astronomy - no surprise there - but amazingly, also managed to successfully complete courses of study in medicine and law, all simultaneously. When Copernicus returned to Poland at the age of 30, it was as a physician, a canon, a lawyer, and an astronomer. His travels abroad were over; he lived for another 40 years, and never left Poland again. Ah, but in his mind, he waltzed among the stars.
From 1503 until 1510, Copernicus lived in the castle of his uncle the bishop, serving as his physician and personal secretary and accompanying him to parliamentary meetings throughout Poland. At about the time of his uncle's death in 1512, Copernicus moved to the town of Frombork, where he settled permanently (he never married or had children). He lived the life of an important and respected citizen, taking on many political and administrative duties over the years as well as serving as physician to the series of bishops who succeeded to his uncle's post.
But all this time, as busy as he was with other professional and civic obligations, Copernicus was living a second, semi-secret life as a mathematician and astronomer. Other astronomers already knew that there were some serious problems with Ptolemy's geocentric (Earth-centered) view of the universe; for example, it did not account for the fact that Mars would occasionally be seen to travel backward across the sky, a phenomenon known as "retrograde motion." If all the planets revolved around the Earth, as Ptolemy said they did, this wouldn't happen. Rather than applying this information to assess whether Ptolemy had been right or wrong, however, astronomers instead came up with tortured explanations of how he could have been right despite retrograde motion. But even while his uncle was still alive, Copernicus had begun to quietly question whether Ptolemy had been infallible after all.
In 1514, Copernicus circulated to his friends his own handwritten book setting forth his theories. The cornerstone of his thoughts was that the sun, not the Earth, lay near (but not at) the center of the planets, and that Earth, like the other planets, revolved around it. Earth also rotated on its axis, causing the rising and setting of the sun, the movement of the stars, and the changing of the seasons.
The only way to observe the sky at that time was with the naked eye. Telescopes did not exist until the late 1500's, and no one trained one on the sky until Galileo did in the early 1600's. But by 1532, having combined his own observations of the sky with the field of applied mathematics, Copernicus had completed the first draft of what would prove to be his life's work: a book entitled The Revolutions of the Celestial Spheres. He showed it only to his circle of close friends, but it was so groundbreaking that soon word of it began to spread. Everyone was curious, including the Pope, but Copernicus showed no inclination to satisfy their curiosity. His book remained unpublished.
In 16th-century Catholic countries, saying or doing anything that the Church might interpret as heresy tended to greatly reduce one's life expectancy, and Copernicus (a prudent man) had every reason to be wary about how his book might be received by religious authorities. But things changed for him in 1539 when Georg Joachim Rheticus, a German mathematician, came to Frombork to spend two years learning everything he could from Copernicus. And what Rheticus learned convinced him that Copernicus was a genius and that his book must be published.
Eventually, Copernicus agreed to release a sort of trial balloon: he allowed Rheticus to publish a short summary of Copernicus's theories. There was no reaction from the Church. Copernicus had made the excellent tactical move of dedicating his book to the Pope, but that's probably not what saved him; it's much more likely that the book created no stir because it was so esoteric that almost no one understood it. And so, Copernicus finally entrusted his precious manuscript to Rheticus, to have it published in Germany. The Revolutions of the Celestial Spheres rolled off the presses in 1643, shortly before its author's death; the legend is that the proofs were brought to Copernicus as he lay on his deathbed, after which he closed his eyes and was able to die in peace.
Copernicus's theories were not perfect; like Ptolemy, he made mistakes, which others who came after him were able to correct through the use of improved technology (like telescopes). But Copernicus was one of the giants on whose shoulders Galileo and Kepler later stood when they reached past him even deeper into the heavens.
p.s. The reason for retrograde motion is that Earth is revolving around the sun on one circuit, and Mars is revolving on another, and occasionally Earth overtakes and passes Mars.