On February 19, 1473, Nicolaus Copernicus is born in Torun, a city in north-central Poland on the Vistula River. The father of modern astronomy, he was the first modern European scientist to propose that Earth and other planets revolve around the sun.

Copernicus was born into a family of well-to-do merchants, and after his father’s death, his uncle–soon to be a bishop–took the boy under his wing. He was given the best education of the day and bred for a career in canon (church) law. At the University of Krakow, he studied liberal arts, including astronomy and astrology, and then, like many Poles of his social class, was sent to Italy to study medicine and law.

While studying at the University of Bologna, he lived for a time in the home of Domenico Maria de Novara, the principal astronomer at the university. Astronomy and astrology were at the time closely related and equally regarded, and Novara had the responsibility of issuing astrological prognostications for Bologna. Copernicus sometimes assisted him in his observations, and Novara exposed him to criticism of both astrology and aspects of the Ptolemaic system, which placed Earth at the center of the universe.

Copernicus later studied at the University of Padua and in 1503 received a doctorate in canon law from the University of Ferrara. He returned to Poland, where he became a church administrator and doctor. In his free time, he dedicated himself to scholarly pursuits, which sometimes included astronomical work. By 1514, his reputation as an astronomer was such that he was consulted by church leaders attempting to reform the Julian calendar.

The cosmology of early 16th-century Europe held that Earth sat stationary and motionless at the center of several rotating, concentric spheres that bore the celestial bodies: the sun, the moon, the known planets, and the stars. From ancient times, philosophers adhered to the belief that the heavens were arranged in circles (which by definition are perfectly round), causing confusion among astronomers who recorded the often eccentric motion of the planets, which sometimes appeared to halt in their orbit of Earth and move retrograde across the sky.

In the second century A.D., the Alexandrian geographer and astronomer Ptolemy sought to resolve this problem by arguing that the sun, planets, and moon move in small circles around much larger circles that revolve around Earth. These small circles he called epicycles, and by incorporating numerous epicycles rotating at varying speeds he made his celestial system correspond with most astronomical observations on record.

The Ptolemaic system remained Europe’s accepted cosmology for more than 1,000 years, but by Copernicus’ day accumulated astronomical evidence had thrown some of his theories into confusion. Astronomers disagreed on the order of the planets from Earth, and it was this problem that Copernicus addressed at the beginning of the 16th century.

Sometime between 1508 and 1514, he wrote a short astronomical treatise commonly called the Commentariolus, or “Little Commentary,” which laid the basis for his heliocentric (sun-centered) system. The work was not published in his lifetime. In the treatise, he correctly postulated the order of the known planets, including Earth, from the sun, and estimated their orbital periods relatively accurately.

For Copernicus, his heliocentric theory was by no means a watershed, for it created as many problems as it solved. For instance, heavy objects were always assumed to fall to the ground because Earth was the center of the universe. Why would they do so in a sun-centered system? He retained the ancient belief that circles governed the heavens, but his evidence showed that even in a sun-centered universe the planets and stars did not revolve around the sun in circular orbits. Because of these problems and others, Copernicus delayed publication of his major astronomical work, De revolutionibus orbium coelestium libri vi, or “Six Books Concerning the Revolutions of the Heavenly Orbs,” nearly all his life. Completed around 1530, it was not published until 1543–the year of his death.

In the work, Copernicus’ groundbreaking argument that Earth and the planets revolve around the sun led him to make a number of other major astronomical discoveries. While revolving around the sun, Earth, he argued, spins on its axis daily. Earth takes one year to orbit the sun and during this time wobbles gradually on its axis, which accounts for the precession of the equinoxes. Major flaws in the work include his concept of the sun as the center of the whole universe, not just the solar system, and his failure to grasp the reality of elliptical orbits, which forced him to incorporate numerous epicycles into his system, as did Ptolemy. With no concept of gravity, Earth and the planets still revolved around the sun on giant transparent spheres.

In his dedication to De revolutionibus–an extremely dense scientific work–Copernicus noted that “mathematics is written for mathematicians.” If the work were more accessible, many would have objected to its non-biblical and hence heretical concept of the universe. For decades, De revolutionibus remained unknown to all but the most sophisticated astronomers, and most of these men, while admiring some of Copernicus’ arguments, rejected his heliocentric basis. It was not until the early 17th century that Galileo and Johannes Kepler developed and popularized the Copernican theory, which for Galileo resulted in a trial and conviction for heresy. Following Isaac Newton’s work in celestial mechanics in the late 17th century, acceptance of the Copernican theory spread rapidly in non-Catholic countries, and by the late 18th century it was almost universally accepted.