Copernicus: On the Revolutions

Every day the sun rises in the east and sets in the west. It really looks as if the sun goes around us. For a very long time, almost everyone believed the Earth stood still in the middle of everything, with the sun, moon, planets and stars circling it.

Nicolaus Copernicus was a church official and astronomer in Poland about 500 years ago. He had a bold idea: maybe the sun is in the middle, and the Earth is a planet that goes around it, spinning like a top once a day. The sun only looks like it moves, the way trees seem to slide past when you ride in a car.

His idea explained a strange puzzle. Mars usually creeps slowly eastward among the stars. But every two years or so it stops, goes backward for a few weeks, then goes forward again. Why? Copernicus said: because Earth is passing Mars, like a faster runner on an inside track. From the fast runner, the slow one seems to slide backward for a while.

Who, when and why

Nicolaus Copernicus (1473–1543) was born in Toruń, in the Kingdom of Poland. He studied at Kraków, then in Italy at Bologna, Padua and Ferrara, where he took a doctorate in canon (church) law. For most of his life he was a canon of the cathedral at Frombork, managing church lands, practicing medicine and, in his spare hours, observing the sky. Around 1510 he passed around a short handwritten sketch of a sun-centered system, the Commentariolus. His full book, On the Revolutions of the Heavenly Spheres, came out in Nuremberg in 1543, the year he died.

The problem Copernicus faced

Astronomy had followed Claudius Ptolemy's Almagest (about AD 150) for 1,400 years. Ptolemy put the Earth at the center. Each planet rode on a small circle, the epicycle, whose center moved around a large circle, the deferent. To match the planets' changing speeds, Ptolemy also measured the motion as even not around the deferent's center but around another point, the equant. It worked well enough to predict positions, but Copernicus thought the equant broke the rule that heavenly motions should be uniform circles, and that the system did not hang together as one design.

Idea 1: the sun at the center, the Earth in motion

Book I sets out the new order. The Earth turns on its axis once a day, which makes the whole sky seem to wheel around us. The Earth goes around the sun once a year, which makes the sun seem to move through the zodiac. The planets, in order from the sun, are Mercury, Venus, Earth (with the Moon going around it), Mars, Jupiter and Saturn. The stars are so far away that the Earth's yearly motion causes no shift we can see in them.

Idea 2: backward loops explained

In Ptolemy's system, each planet's backward loop (its retrograde motion) needed its own epicycle, timed just so. In Copernicus's system, the loops come free: they are what we see when the faster Earth overtakes an outer planet, or when an inner planet overtakes us. And it explains a coincidence Ptolemy could only accept: the outer planets loop backward exactly when they are opposite the sun in our sky.

Idea 3: a system whose parts fit

The biggest gain was not accuracy but order. In Ptolemy's scheme each planet was a separate model; nothing fixed how far away each was. In Copernicus's, the sizes of the planets' orbits follow from what we see, and the planets fall into order: the farther from the sun, the slower they go.

Getting the history right

• Copernicus did not get rid of epicycles. Old textbooks say his system was much simpler. In fact, to match the observations he still used small circles on circles, about as many as the Ptolemaic models of his day. What his system dropped was the big epicycles used for the backward loops, and the equant. The story that Ptolemy's followers had piled up "80 epicycles on epicycles" is a later myth; the historian Owen Gingerich showed that the tables astronomers actually used came from Ptolemy's simple models.
• It was not more accurate at first. Tables made from Copernicus's book (the Prutenic Tables of 1551) predicted planet positions about as well as the old tables. Real gains came with Tycho Brahe's far better observations and Kepler's ellipses.
• The unsigned preface. The book opened with a short note, "To the Reader", saying the new system was only a way to calculate, not a claim about the real world. Copernicus did not write it; the Lutheran minister Andreas Osiander, who oversaw the printing, added it without signing it. Kepler later made this public.
• Not banned at once. The book was read and taught for seventy years. In 1616, during the controversy around Galileo, the Catholic Church's Congregation of the Index suspended it "until corrected"; a list of corrections followed in 1620. People of different churches and none argued for and against it, on scientific as well as religious grounds: no one could yet detect the Earth's motion directly.

What later science kept, and what it changed

Kept: the sun near the center of the planets' motions, the Earth as a planet that spins daily and orbits yearly, the order and relative distances of the planets, and the explanation of backward loops. Changed: Kepler replaced the circles with ellipses (1609) and threw out the epicycles for good. Newton explained the motions with gravity, and showed that the sun and planets actually circle their common center of mass, which lies near the sun. The stars are not fixed on a sphere: they are suns, at vastly different distances, and the yearly shift of the nearest one (stellar parallax) was finally measured by Friedrich Bessel in 1838.

Two descriptions, one motion. The visual above shows something surprising: for a single planet, a sun-centered circle plus Earth's circle and an Earth-centered deferent plus epicycle can trace exactly the same path. So which is "true"? The case for Copernicus was not that his geometry fit one planet better, but that one arrangement explained them all at once, and fixed their distances. Choosing a theory because it ties many facts together is a big part of how science judges ideas.

Common sense and motion. If the Earth spins, why don't we feel a wind, or why doesn't a dropped stone land behind us? Copernicus had only partial answers. Galileo's work on motion (Two New Sciences, on this ladder) supplied the idea that we share the Earth's motion and so do not notice it.

Reading the dedication. Copernicus writes to Pope Paul III, mentions the cardinal and the bishop who urged him to publish, and says his book has waited "not nine years merely, but already going on four times nine." Why might he have waited so long? Read his own explanation and form your own view.

Big questions. When our senses and a theory disagree, which should we trust? Is a simpler explanation more likely to be true, or only easier to use?