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Let's go to Mars

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.

A page from an old printed book showing seven hand-drawn concentric circles around a small ringed sun at the centre, each circle labelled in Latin script, with the book's text faintly showing through the paper.
The 1543 diagram itself: nested orbits round the Sun, with Mars given the fourth ring and a two-year circuit — the arrangement that made the backwards loop a trick of perspective. · Image: Nicolaus Copernicus, De revolutionibus orbium coelestium, 1543

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; his circle handed down the story that a copy reached him on the day he died, 24 May. He had held the manuscript for decades, having circulated a short sketch, the Commentariolus, by 1514 at the latest.

For eighteen hundred years the retrograde loop had been something to reproduce: bolt on an epicycle, run the planet 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 and 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 a faster one's window.

In Ptolemy's system the epicycle and deferent ratios had no common scale; in the heliocentric geometry each becomes a real distance measured against Earth's orbit. Copernicus could put the planets in a definite order and scale them against one another. The solar system had a shape, though nobody could yet say how big it was.

The book did not win on accuracy. Copernicus kept perfect circles and uniform motion; his quarrel with Ptolemy was the equant, not the Earth, and removed it with epicycle machinery indistinguishable from what astronomers at Maragha and Damascus had built for the same purpose two to three centuries earlier. His tables were no more accurate than the Alfonsine tables they were meant to replace. Andreas Osiander, overseeing the printing, inserted an unsigned preface, without the author's permission, describing the system as a convenient hypothesis rather than a claim about the world. A print run of about four hundred failed to sell out.

No observer in 1543 could measure the residual that would break the circles. That took another sixty-six years.