49 Fun Facts About Johannes Kepler

Which word did Kepler coin in 1610 for the moons Galileo had found circling Jupiter?

Satellites, meaning attendants, was Kepler's word for the four bodies escorting Jupiter. Galileo had named them the Medicean Stars to flatter his patrons, but it was Kepler's term that lasted.

In which city did Kepler join Tycho Brahe in 1600 and go on to do his greatest work?

Prague took Kepler in as a religious refugee in 1600, after he refused to change his faith to keep a teaching job. Within two years he had inherited both Tycho's court post and his unrivalled observations.

Which planet's orbit did Kepler analyse under Tycho's direction, leading to his first two laws?

Mars exposed the flaw in circular orbits. Kepler's best circular model still missed Tycho's observations by eight minutes of arc, and rather than blame the data he threw the model out and started again.

What shape did Kepler finally settle on for a planet's path around the Sun?

An ellipse, with the Sun at one focus. Kepler spent years on circles and then on an egg-shaped oval before finding that the true path lay midway between the two.

Kepler's second law says that a line from the Sun to a planet sweeps out what?

Equal areas in equal times, which means a planet has to speed up when it is near the Sun and slow down when far away. Kepler found this rule before he knew the true shape of the orbit.

Kepler's third law links a planet's year to what?

Its distance from the Sun: the square of the year grows as the cube of the distance. Kepler recorded that the idea came on 8 March 1618, was rejected after a botched calculation, and returned for good on 15 May.

Kepler's 1619 book, which contains his third law, tried to explain the design of the heavens through what?

Musical harmony: Kepler believed the planets' changing speeds sounded out notes in a God-given scale. The law for which the book is remembered appears in its last part, as just one more harmony among many.

In which present-day country was Kepler born in 1571?

Germany: he was born in Weil der Stadt, then a tiny free city of the Holy Roman Empire near Stuttgart. Religious upheaval kept him moving, and almost all his famous work was done far from home.

What childhood illness left Kepler with weak vision and crippled hands?

Smallpox in early childhood marked him for life. With eyesight too weak for fine observing, Kepler became the great theorist instead, dependent on other people's measurements.

Which 1577 celestial event did the young Kepler watch from 'a high place' with his mother?

The Great Comet of 1577 drew the eyes of astronomers all over Europe that winter. Three years later Kepler was called outdoors to watch a lunar eclipse and remembered the Moon turning quite red.

What was Kepler's father Heinrich's profession before he abandoned the family?

Mercenary: though a Protestant, Heinrich hired himself out to the Catholic Spanish army fighting Protestant rebels in the Netherlands. He is thought to have died in that war.

At which university did Kepler study, intending to become a minister?

Tübingen was meant to make a pastor of him. Instead his mathematics professor there, Michael Maestlin, one of the earliest converts to a Sun-centred universe, turned him toward astronomy.

Which Protestant church barred Kepler from communion in 1612 for refusing to sign its creed in full?

Lutheran, the church he had once trained to serve. Kepler doubted part of its Formula of Concord and would not condemn Calvinists, whom he called brothers in Christ, so he never signed; the ban was never lifted.

In which Austrian city did Kepler take a job teaching mathematics and astronomy in April 1594, aged 22?

Graz gave him his first job, at a Protestant school. Six years later a Counter-Reformation commission banished him, along with sixty other men, for refusing to change his faith.

Kepler's Mysterium Cosmographicum (1596) explained planetary distances by nesting what inside spheres?

The five Platonic solids, nested one inside another, separate exactly six spheres, one for each planet then known, and Kepler took that for God's blueprint. The idea struck him in the middle of teaching a class.

What had Kepler's bride Barbara Müller already been twice when they married in 1597?

Widowed twice by her early twenties, Barbara was a wealthy mill owner's daughter with a small girl of her own. Her father thought a poor schoolteacher beneath her, and the match nearly fell through while Kepler was away.

Which Holy Roman Emperor made Kepler his imperial mathematician in 1601?

Rudolf II gave him the post within two days of Tycho Brahe's death. Paying him was another matter: five months passed before the first salary arrived, and Kepler was still chasing arrears when he died.

What kind of advice did the emperor regularly seek from Kepler as his imperial mathematician?

Astrological: the emperor wanted horoscopes and readings of the heavens in times of political trouble. Kepler supplied them while dismissing most of the craft as evil-smelling dung, in which a patient hen might still find a grain of gold.

What kind of object was the bright new star Kepler observed in October 1604?

A supernova, the most recent stellar explosion seen in our own galaxy. Because it never shifted against the background stars, Kepler argued it lay far beyond the planets, in heavens that were supposed never to change.

Kepler's manuscript Somnium, describing a trip to the Moon, is sometimes called the first work of what?

Science fiction, though the aim was serious. By describing how the sky would look to someone standing on the Moon, Kepler was arguing that a moving Earth is only a matter of where you stand.

What accusation was made against Kepler's mother Katharina in Leonberg in 1615?

Witchcraft: a neighbour claimed that a drink from Katharina had made her ill. Kepler ran his mother's defence himself, and she refused to confess even when threatened with torture; she was freed in 1621.

Kepler worked out the minute of his own conception and concluded he had been born how?

Prematurely: by his own reckoning Kepler was a seven-month baby, conceived on his parents' wedding night. He described his childhood self as weak and sickly.

What did Kepler spend two years choosing from a shortlist of eleven?

A second wife: in one long letter Kepler weighed eleven candidates in turn, then went back to the fifth, Susanna Reuttinger, an orphan his friends thought beneath him. The marriage was far happier than his first.

Which comet did Kepler track across the sky in 1607?

Halley's Comet: Kepler tracked it in 1607 and decided that comets fly in straight lines. Decades later Edmond Halley used those very records to show it was one comet, returning about every 76 years.

What purchase in 1613 led Kepler to write a book on measuring the volumes of containers?

Wine for his household arrived in casks, and Kepler watched the merchant gauge each one by poking a rod in diagonally, whatever its shape. Unconvinced, he set about working out the true volumes himself.

Kepler's 1615 book on the volumes of curved containers was an early step towards which branch of mathematics?

Calculus: Kepler found each volume by treating the solid as an infinite number of thin slices and adding them up, long before the method was made rigorous. It was also the first book ever printed in Linz.

Whose work on logarithms did Kepler adopt to speed up the calculations for his planetary tables?

John Napier's logarithms delighted Kepler but came with no account of how they were built. So he derived his own tables from first principles and published them in 1624.

What destroyed the press that was printing Kepler's planetary tables in 1626?

Fire during the Peasant War siege: rebels blockaded Linz that summer, and the printer's house and press went up in flames. Kepler's manuscript survived, and he carried it up the Danube to be printed in Ulm instead.

Which imperial general, newly Duke of Sagan, became Kepler's last patron in 1628?

Wallenstein set Kepler up with a printing press at Sagan in Silesia. Kepler felt isolated there by an unfamiliar dialect, and in 1630 he set off again to collect money he was owed.

In which city did Kepler die on 15 November 1630, while travelling to collect money owed him?

His grave in the Protestant cemetery of St Peter was destroyed a few years later in the Thirty Years' War.

Kepler's 1604 book, begun to explain a puzzling eclipse measurement, is regarded as the foundation of which field?

Modern optics grew out of a puzzle: at the solar eclipse of July 1600, Kepler's pinhole camera made the Moon look too small. Tracing the light with actual threads, he arrived at the idea of the light ray.

What did Kepler discover about the image formed on the retina?

It is projected inverted and reversed. The upside-down picture did not trouble Kepler, who left to others the puzzle of why we still see the world the right way up.

How did the telescope Kepler designed in 1611 differ from the ones already in use?

It used two convex lenses, where earlier telescopes paired a convex lens with a concave one. That gave higher magnification, and the design became the basis of the astronomical telescopes that followed.

How did Kepler respond in 1610 when Galileo announced four moons circling Jupiter?

He backed him in print, months before he had the use of a telescope to check for himself. Galileo wrote to thank him for being the first, and practically the only, person to have complete faith in his claims.

Kepler's New Year's gift to a friend at court in 1611 was a pamphlet about the shape of what?

Snowflakes: Kepler asked why every one of them has six corners. The light-hearted letter, full of puns, turned out to be a pioneering study of the regular arrangements from which crystals are built.

The Kepler conjecture, proved only after nearly four centuries, concerns the most efficient way to do what?

Pack spheres: Kepler guessed in 1611 that the pyramid used for stacking cannonballs cannot be beaten. Thomas Hales proved it in 1998, but referees were only '99% certain' until computers checked every step in 2014.

Who made the first ever observation of a transit of Mercury in 1631, on the date Kepler predicted?

Pierre Gassendi caught it from Paris on 7 November 1631, a year after Kepler's death. Kepler had been so unsure of Mercury's orbit that he told observers to keep watch the day before and the day after as well.

Which of Kepler's books, published in three parts, became the standard textbook in its field for a generation?

Epitome of Copernican Astronomy was set out as questions and answers, the way textbooks of the day were. Despite its title, it is more an introduction to Kepler's own astronomy than to that of Copernicus.

Who derived Kepler's laws from a force-based theory of universal gravitation in the 1687 Principia?

The mathematical challenge became known as "solving the Kepler problem".

Before gravity was understood, what force did Kepler think the Sun used to drive the planets around?

Magnetism: once William Gilbert had shown that the Earth is a magnet, Kepler pictured a spinning Sun sweeping the planets round. His aim, he wrote, was to show the heavens work like a clock, not like a divine living being.

What cause of death did the 2010 exhumation of Tycho Brahe rule out, clearing Kepler of an old suspicion?

Mercury poisoning had been suspected since tests on Tycho's beard hair in the 1990s, and a 2004 book cast Kepler as the killer. The team that reopened the tomb found too little mercury to have harmed him.

Which composer's 1957 opera tells the life of Kepler?

Paul Hindemith wrote the libretto himself and drew a symphony of the same name out of the opera's music while he was still composing it. Philip Glass followed in 2009 with a Kepler opera of his own.

What was NASA's Kepler space telescope built to find?

Planets around other stars: the telescope watched for the tiny dip in a star's brightness as a planet crossed in front of it. It had found more than 2,600 by the time it ran out of fuel and was retired in 2018.

Which monarch bought Kepler's surviving manuscripts in 1773 for a national academy of sciences?

Catherine the Great bought eighteen volumes of his papers for the academy in St Petersburg, after they had been pawned and then forgotten for decades. That is how the bulk of Kepler's surviving manuscripts came to rest in Russia.

Which biblical sign did Kepler link to a rare triple meeting of Jupiter and Saturn?

The Star of Bethlehem: having watched a new star appear soon after the two planets met in his own day, Kepler reasoned that their meetings before the Nativity had produced one too. He also dated Christ's birth several years 'before Christ'.

In which language did Kepler write his major scientific books?

Latin was the language of all his schooling and of scholars across Europe, so his books could be read anywhere. When he wanted ordinary readers, as with his first tract on the new star of 1604, he wrote in German.

What did Kepler say causes the tides, an idea Galileo dismissed as occult nonsense?

The Moon's pull: Kepler wrote that if the Earth stopped holding its waters, the seas would rise and flow to the Moon. Galileo, who put the tides down to the Earth's motion, scorned the idea as an occult fancy.

In the epitaph he wrote for himself, Kepler says he once measured the heavens and now measures what?

The shadows of the earth: 'I used to measure the heavens, now I shall measure the shadows of the earth.' His gravestone was swept away in the Thirty Years' War, but the lines he had composed were written down and survived.

Which 1609 book announced Kepler's first two laws?

Astronomia Nova, the 'New Astronomy', sat finished for years while Tycho Brahe's heirs disputed Kepler's right to use the observations. He later sold the whole edition to the printer to recover some of his unpaid salary.