Earth 1

The Sun at the centre

Painted portrait of Nicolaus Copernicus, dark shoulder-length hair, red coat

Nicolaus Copernicus

1473 – 1543 · Toruń → Frombork

A church canon, doctor and administrator who moved the Earth: it spins once a day and goes round the Sun once a year, like every other planet.

Portrait: unknown artist, c. 1580, Toruń Town Hall. Public domain, via Wikimedia Commons

Why planets loop backwards

Every so often Mars slows, stops, drifts backwards against the stars for about two months, then carries on. Ptolemy explained it by putting each planet on a small circle riding a bigger one. Copernicus saw a simpler reason: we are on a faster, inner lane and we are overtaking it. Flip the view below.

1.524 AU

orbit

780 d

laps us every

forwards

moving

Same motions, two viewpoints. From the Sun every orbit is a circle. From the Earth the outer planet loops backwards each time we overtake it: that is retrograde motion, and Ptolemy needed epicycles to draw it.

  • S: the synodic period: years between two overtakes, when the loop happens
  • P: the planet's own year, in Earth years (Mars: 1.88)
  • 1: the Earth's year, one lap per year

For Mars that gives 2.1 years, about 780 days between loops.

“In medio vero omnium residet Sol. (At rest in the middle of everything is the Sun.)”
Copernicus, De revolutionibus, Book I, chapter 10, 1543

He measured the solar system

Put the Sun at the centre and the sizes of the orbits fall out of geometry. For Venus: at its greatest angle from the Sun, the line from Earth just grazes its orbit, so a right angle sits at Venus.

  • a: the planet's distance from the Sun, with Earth's set to 1
  • ε max: its greatest angle away from the Sun seen from Earth: 46° for Venus, so a = 0.72

Distances from the Sun in Earth–Sun units: Copernicus's value, and today's.

Copernicus on money

He also ran the finances of the Warmia chapter, and in 1526 finished a treatise on coinage. Debased coins, he argued, drive out good ones, which people hoard or melt; and the more coin in circulation, the less each buys. The first idea is now called Gresham's law (sometimes Copernicus–Gresham); the second is an early form of the quantity theory of money, the argument still running under quantitative easing.

What he got wrong

He kept perfect circles, so he still needed small extra circles to match the sky, and his tables were not much more accurate than Ptolemy's. Kepler's ellipses (1609) fixed that, Galileo's telescope gave the evidence, and Newton gave the reason.

Timeline

  1. 19 Feb 1473

    Born in Toruń, Royal Prussia, to a merchant family.

  2. 1491 – 95

    University of Kraków: mathematics and astronomy. His uncle, later Prince-Bishop of Warmia, pays his way.

  3. 1496 – 1503

    Italy: canon law at Bologna, where he observes with the astronomer Domenico Maria Novara; medicine at Padua; doctorate in canon law at Ferrara, 1503.

  4. 9 Mar 1497

    Watches the Moon pass in front of the star Aldebaran from Bologna.

  5. c. 1510 – 14

    The Commentariolus, a short handwritten outline of a Sun-centred system, circulates among friends.

  6. 1517 – 26

    Writes on coinage for the Prussian estates; the 1526 Monetae cudendae ratio argues that debased coins drive good ones out of use.

  7. 1539

    Georg Joachim Rheticus, a young Lutheran mathematician, arrives at Frombork and becomes his only student.

  8. 1540

    Rheticus publishes the Narratio prima, the first printed account of the theory.

  9. 1543

    De revolutionibus orbium coelestium printed in Nuremberg, with an unsigned preface by Andreas Osiander calling it a mere calculating device.

  10. 24 May 1543

    Dies at Frombork.

  11. 5 Mar 1616

    The Catholic Church suspends De revolutionibus 'until corrected'.

  12. 22 May 2010

    Reburied in Frombork Cathedral after his remains were identified by DNA matched to hairs in one of his books.

Next: Galileo, who turned a telescope on the sky and proved him right.

Sources