351-18(2): Nikolaus Copernicus, Tycho Brahe, & Johannes Kepler


A key figure in the origins of the seventeenth-century scientific revolution was Nikolaus Copernicus (1473-1543), a Polish clergyman, who wrote De revolutionibus orbium coelestium (On the revolution of the the celestial orbs) arguing that the earth revolves around the sun.
The model of the universe of the ancient astronomer, Ptolemy (c. 87-150 AD) had long been accepted.

Ptolemy's universe showed the Earth at the center, with the moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn, in succeeding celestial spheres.
Ptolemy (like Aristotle) believed that the heavenly bodies moved in perfect circles; unlike earthly bodies which were flawed and moved in straight lines.

To account for discrepancies in the perceived motion of the plants, Ptolemy postulated epicycles - a smaller circle around the point moving steadily around the celestial sphere.


Copernicus' heliocentric system placed the Sun immobile at the center of the other planets - in the [correct] order Mercury, Venus, Earth, Mars, Jupiter, Saturn.

Copernicus argued not only that the Earth moved, but also that it moved in two different ways - diurnal motion on its axis, and annual motion around the Sun.


Like Ptolemy, Copernicus thought that the planets moved in circles, and he too postulated epicycles. This meant that his predictions of planetary motion were actually less accurate than Ptolemy's.
In addition, Copernicus' system flew in the face of Scripture which stated unequivocally that "the world also is stablished, that it cannot be moved” (Psalms 93:1).
"Then spake Joshua to the Lord in the day when the Lord delivered up the Amorites before the children of Israel, and he said in the sight of Israel, Sun, stand thou still upon Gibeon" (Joshua 10:12). If the Sun had already been standing still, Joshua  should have ordered the cessation of the Earth's diurnal motion in order to get more time for slaughtering Amorites.
Copernicus' book put forward his theory as an hypothesis, not dogmatically (though the passage that stated it was an hypothesis was in fact inserted by Andreas Osiander, who saw the book through the press at a time when Copernicus was very ill) but the decades after his death were inauspicious for heterodox theses. Roman Catholics were stressing obedience to the Church's authority and Protestants were insisting on basing all knowledge on Scripture.


"There is talk of a new astrologer who wants to prove that the earth moves and goes round instead of the sky, sun and moon, just as if somebody moving in a carriage or ship might hold that he was sitting still and at rest while the earth and trees walked and moved. But that is how things are nowadays:  when a man wishes to be clever he must needs invent something special, and the way he does it must needs be the best! That fool wants to turn the whole art of astronomy upside-down. However, as Holy Scripture tells us, so did Joshua bid the sun to stand still and not the earth"

Martin Luther, Table Talk, on Copernicus.


Copernicus was not much of a writer, De Revolutionibus was almost unreadable. Only about 1,000 copies were printed in 1543 and many went unsold.

Tycho Brahe

Tycho Brahe (1546-1601) was a member of a wealthy Danish noble family. He lost a chunk of his nose in a duel, and had to wear a band (made of a gold and silver alloy) to cover the gap. He worked initially near Copenhagen for the Danish royal house, but after a dispute with Christian IV he took employment with the Holy Roman Emperor, Rudolf II.


Brahe tried to reconcile the Ptolemaic and Copernican systems. He suggested that the Sun and Moon revolve around the Earth (the center of the universe), while the other planets revolve around the Sun.

Tycho Brahe brought new standards to astronomical observation, making great efforts to ensure that his instruments were of the highest quality and that they remained accurate. Until Brahe, astronomers generally relied on astronomical tables (often derived from ancient observations and often inaccurate).
In 1572 he observed a new star, (as did many others - this supernova was was visible to the naked eye for eighteen months, and was initially so bright that it could be seen in daylight). In 1577 tracked a comet, and showed that it was at least six times as distant as the moon. These observations undermined the Aristotelian axiom that the heavens (beyond the moon) were perfect and unchanging.
Since the comet moved across the orbit of the planets, Brahe's observation also discredited the existence of heavenly "spheres".
Tycho's Brahe's observations were not published for some time, but a manuscript copy of some of them was of immense help to Johannes Kepler.

Johannes Kepler

Johannes Kepler (1571-1630) was born in Swabia, Germany. Johannes' father was a soldier and a tavern-keeper, who deserted his family. Johannes was a short-sighted child, who suffered from many illnesses.
In 1594, he became professor of mathematics and astronomy at Graz (Styria). Part of his duty there was to produce an annual set of astrological predictions.
In 1596, Kepler published his Mysterium cosmographicum which argued that the structure of the universe was based on simple geometric forms - circle, triangle, square &c.

Kepler sent a copy of the Mysterium to Tycho de Brahe, who recognized Kepler's abilities and from 1600 employed him as his assistant. Brahe gave Kepler the job of observing and explaining the motion of the planet Mars.
On Brahe's death in 1601, Kepler took over his job as court astronomer to Emperor Rudolf II.
Mars has a very elliptical orbit, so Kepler's observations would never exactly accord with the hypotheses he developed on the basis of circular motion. After six years of exhausting labor, he finally constructed a revolutionary theory that agreed with the evidence - and discovered that the planets move not in a perfect circle but elliptically.


Kepler had framed the elliptical thesis by 1605, but lack of money and other problems meant that his Nova Astronomia was not published until 1609.

In 1619, Kepler published his Harmonice Mundi which contained his third law linking the velocities of the planets to their distance from the sun.

Kepler's three planetary laws

1. The planets travel around the sun in elliptical orbits, with the sun at one focus. [Hyperlink to animation}
2. A planet's orbit is such that a line from the planet to the sun covers equal areas in equal times.
3. The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their mean distances from the sun (i.e. their semi-major axes).

Kepler's planetary laws were the first "natural laws" (in the scientific sense) ever promulgated - i.e. the first mathematically-precise, verifiable, universal statements about physical phenomena.
Kepler also worked on optics and designed a better telescope, but it was his discovery of elliptical motion that revolutionized thought.
Just as Brahe recognized Kepler's abilities, so the latter instantly acknowledged the path-breaking work of Galileo Galilei.


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