Monument to Copernicus, Adler Planetarium, Chicago USA

Not all revolutionaries are high-profile attention-seekers. Nicolaus Copernicus (1473-1543), a Polish canon who revolutionized our view of the Earth’s place in the Universe, was one of the quiet kind.

He spent over 30 years working on his theory that the Sun, and not the Earth, was at the center of the Universe. Yet the final book describing the details was published only on his death bed, in 1543.

Arthur Koestler, in his book on the history of astronomy called The Sleepwalkers, called Copernicus the "Timid Canon." What made Copernicus so circumspect?

Greek influence
At the start of the 16th century, when Copernicus became interested in astronomy, science had advanced little since the time of the ancient Greeks. Astronomers still accepted the view of second-century Greek scientist Ptolemy who said that the Sun, planets and stars all orbited the Earth on invisible spheres.

Such a system is known as geocentric, meaning Earth-centered, and it was a natural enough assumption. After all, the Earth really does seem to stand still while all else revolves around it. But this model wasn’t very good at predicting the observed motions of the planets in the sky.

Copernicus thought he could do better by removing the Earth from the center and relegating it to the status of a planet orbiting the Sun like Mercury, Venus, Mars, Jupiter and Saturn (Uranus, Neptune and Pluto were not known at that time).

From Earth-centered to Sun-centered
There were several strong arguments in favor of this Sun-centered, or heliocentric, arrangement. One was that it provided a natural link between the distance of a planet from the Sun and the time it takes to complete one orbit – the innermost planets would get around the quickest and the outermost ones the slowest.

Secondly, Copernicus’s theory explained why Mercury and Venus are never seen far from the Sun – they are closer to the Sun than we are, and have smaller orbits.

And thirdly, it explained why the outer planets appear to double back on themselves at times, an event known as a retrograde loop. This is now simply explained as an illusion caused as the Earth caught up with and overtook the slower-moving outer planet.

A giddy ride
Copernicus first outlined his revolutionary new view of the Universe in a handwritten manuscript circulated around 1510. But he realized that he had a long way to go to convince others of the truth of his cosmology. Most of his contemporaries were not prepared to accept the dizzying idea of the Earth speeding around the Sun once a year while twirling on its axis every day.

Worse, Copernicus’s model of the heavens contained a basic flaw inherited from his predecessors: he remained wedded to the Greek notion that the planets moved on invisible spheres. Copernicus’s theory improved the predictions of planetary motions, but he still could not produce the accuracy for which he strove. So his reluctance to publish was more to do with dissatisfaction with his own results than fear of ridicule – or even of possible accusations of heresy, since the concept of a central stationary Earth was then part of religious as well as scientific orthodoxy.

His final theory, with all its imperfections, was published in a book titled De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres). Copernicus suffered a stroke at the end of 1542 while the book was going through press. Legend has it that a finished copy was placed in his hands the day he died.

The Copernican revolution
Initial reaction to the book was muted. While many astronomers appreciated the improvements Copernicus made in predicting the movements of the planets, few could bring themselves to believe in the reality of the heliocentric model on which they were based.

That all changed early in the 1600s when the Italian scientist Galileo Galilei threw his weight behind the heliocentric model. He supported it with evidence from his experiments on the motion of bodies and his observations with the newly invented telescope.

The clinching argument, though, came from a German mathematician, Johannes Kepler. Kepler finally did away with the cumbersome spherical motions that had blemished Copernicus’s theory. In 1609 Kepler published the first two of his three laws of planetary motion which showed that planets orbit the Sun on ellipses, not on combinations of spheres or circles as the Greeks had maintained.

We now know that although the Sun is the center of the Solar System, it is not the center of the Universe after all. It is just one star in a Galaxy of billions of others. All the same, Copernicus had quietly started a revolution by forcing scientists to look at the Universe around them in a completely new way.