Moving the sun to the center

A planet that walks backward

Aristotle’s nested spheres explained the heavens in principle. Astronomers who had to predict where a planet would be on a given night needed more. The story continues through the most systematic science of the ancient world, astronomy, and it begins with Claudius Ptolemy.

Ptolemy lived about four and a half centuries after Aristotle. Astronomy in his day meant watching the sky with the naked eye and measuring angles with simple sighting instruments, like a graduated rod. The precision was rough. Even so, the observations had already outgrown Aristotle’s simple spheres, and the worst problem was the planets’ backward loops.

Watch Mars from night to night and it drifts slowly eastward against the stars. Then, every two years or so, it stops, moves backward for about two months, stops again, and resumes its eastward course. Astronomers call this retrograde motion. Mercury does it about three times a year. Stranger still, Mars is at its brightest in the middle of its backward loop, and Mercury and Venus never stray far from the sun. For people who believed the heavens moved in perfect, steady circles, this was deeply unsettling. Why would a perfect body turn back on itself?

Circles on circles

The Greek answer, as you saw in Course 1, was to combine motions. A ship makes the idea clear. A sailor at the top of the mast drops a spyglass. To the sailor, it falls straight down to the foot of the mast. To someone standing on shore, it traces a curve, because the ship carries it forward as it falls. One motion added to another produces a new shape.

Ptolemy gave each planet a small circle, the epicycle, whose center moved around a larger circle, the deferent. Each circle turned perfectly and steadily. Together they could make the planet swing outward, loop backward, and move on. When the planet is on the inside of its loop, it is also closest to Earth, and therefore brightest. To match the data more closely, Ptolemy set the Earth slightly off the center of the deferent. He also measured each planet’s steady motion from a separate point, called the equant, rather than from the center of its circle. Epicycles could even carry further epicycles.

The result was complicated, but it worked. It was arguably the first model of the heavens in history that could both explain what people saw and predict it: where a planet would appear, months or years ahead. Its predictions agreed with the rough naked-eye observations of the time. With small repairs, Ptolemy’s system served astronomers from the second century to the Renaissance.

Why the question reopened

Science never changes in isolation. By the 1400s and 1500s, the Greek classics were circulating in Europe again, and the printing press spread them fast. Discontent with the church and with the political order was growing. The idea of a world where everything and everyone has a natural place sat badly with a new urge to free the individual. And the age of exploration made fresh demands on astronomy. Ships crossing oceans navigated by the stars, and rough predictions could send them far off course. Social causes and scientific causes were so intertwined that it is hard to say which drove which.

Copernicus

In 1543, the year he died, the Polish astronomer Nicolaus Copernicus published On the Revolutions of the Heavenly Spheres. It set out a sun-centered system in full mathematical detail. Notice what did not drive him. Copernicus had no new observations showing that the earth moves. Nobody could feel it move, and critics hammered him on exactly that point for decades.

What bothered Copernicus was Ptolemy’s untidiness. The equant broke the rule of steady circular motion, and the earth sat awkwardly off center. He wanted to restore the ancient ideal of perfect, uniform circles. He was also drawn to a revived form of Plato’s philosophy that treated the sun as the image of the highest good, the source of light and life. In a famous passage, he wrote:

In the middle of all sits the Sun. For who would place this lamp of a most beautiful temple in any other or better place, from which it can light up everything at once?

Nicolaus Copernicus, On the Revolutions, book I, chapter 10

But Copernicus kept the perfect circles, so he still needed epicycles to match the data. His system was not much simpler than Ptolemy’s, and it didn’t predict the planets much better. What it did offer was a more natural account of the backward loops. The earth, on an inner and faster track, simply overtakes the outer planets, and as it passes them they seem to slide backward against the distant stars.