Copernicus explains Mars's backwards loop
De revolutionibus turned Mars's retrograde loop into a trick of perspective — Earth overtaking it on the inside. The predictions were no better than Ptolemy's.
Johannes Petreius's press in Nuremberg finished printing De revolutionibus orbium coelestium in the spring of 1543. Copernicus was seventy, in Frombork, and paralysed after a stroke; the story handed down by his circle is that a copy reached him on the day he died, 24 May. He had been sitting on the manuscript for decades, having circulated a short sketch of the idea, the Commentariolus, by 1514 at the latest.
The payoff for Mars is the cleanest thing in the book. For eighteen hundred years, the retrograde loop had been something to reproduce — bolt on an epicycle, let the planet ride backwards for a few weeks each opposition, adjust until the tables match. Copernicus made the loop an artefact of the observer. Earth goes round the Sun in 365 days; Mars takes 687. Every 780 days Earth catches Mars and passes it on the inside track, and for roughly ten weeks Mars appears to slide backwards against the stars, the way a slower train seems to run backwards from the window of a faster one. The loop is not a motion of Mars. It is a motion of us.
There was a second prize nobody had expected. In Ptolemy's system the epicycle and deferent ratios were free-floating numbers with no physical meaning; in the heliocentric geometry every one of them becomes a real distance, measured against Earth's orbit. Copernicus could therefore put the planets in a definite order and scale them against one another. For the first time the solar system had a shape, even though nobody could yet say how big any of it was in miles.
The book did not win on accuracy. Copernicus kept perfect circles and uniform motion; his real quarrel with Ptolemy was the equant, not the Earth, and he removed it with epicycle machinery indistinguishable from what astronomers at Maragha and Damascus had built for the same purpose two centuries earlier. His tables were no more accurate in practice than the Alfonsine tables they were meant to replace. Andreas Osiander, overseeing the printing, inserted an unsigned preface describing the whole system as a convenient hypothesis rather than a claim about the world — without the author's permission. A print run of about four hundred failed to sell out.
Mars finished the argument, and not in this book. The planet Copernicus had explained so cleanly would not fit the perfect circles he had kept, and the residual that eventually broke them had not yet been measured — nobody in 1543 could observe well enough to see it. Putting the Sun at the centre explained why the loop happens. The shape of the orbit took another sixty-six years, and the most stubborn planet in the sky.