Kepler was born on 27 December 1571 in the small town of Weil der Stadt, now part of the Stuttgart Region in the German state of Baden-Wurttemberg, and moved to nearby Leonberg with his parents in 1576.
In 1589, after passing through the state-run Protestant education system Kepler enrolled at the University of Tubingen (est.1477), where he was taught geocentric and heliocentric astronomy by Michael Maestlin (1550-1631). Kepler seems to have accepted almost instantly that the Copernican system was physically true. Towards the end of his studies he was recommended for a position as a teacher of mathematics and astronomy at the Protestant school at Graz – later the University of Graz (est.1585) – and he accepted it in April 1594.
Kepler’s first major astronomical work, Mysterium Cosmographicum (‘Cosmographic Mystery’), was published at Tubingen in 1596. Basing his proposals on the Copernican system he suggested that the distance relationships between the six planets known at that time could be understood in terms of the five Platonic solids centred on the Sun and just fitting between the concentric shells and this all enclosed within a sphere representing the orbit of Saturn.
But the comparison with values deduced from observation was not exact. Hoping to obtain better observations he sent a copy of Mysterium to Tycho Brahe. Tycho, then working in Prague, had in fact already written to Maestlin in search of a mathematical assistant and so Kepler went to work for Tycho in Prague in 1600. Two days after Tycho’s death in 1601, Kepler was appointed his successor as imperial mathematician with the responsibility to complete his unfinished work.
Kepler believed the Sun was the source of all motive force in the Solar System. He assumed that this motive force weakens with distance, causing faster or slower motion as planets move closer or farther from it. Based on measurements of the aphelion and perihelion of Earth and Mars he discovered that a planet’s rate of motion is inversely proportional to its distance from the Sun. By late 1602 Kepler had reformulated this relationship to say that an imaginary line joining a planet to the Sun sweeps out equal areas in equal times – his second law of planetary motion.
He then began to calculate the entire orbit of Mars. In early 1605 he at last came to the idea of an ellipse. He immediately concluded that all planets move in ellipses, with the Sun at one focus – his first law of planetary motion. By the end of the year he had completed the manuscript for Astronomia nova, though it would not be published until 1609 because of a legal dispute over the use of Tycho’s observations.
Even today it can seem bizarre that a planetary orbit is an ellipse with the Sun at one focus and with no physical counterpart at the other ’empty’ focus. We now know that a system of massive bodies actually orbits around its barycentre, the centre of mass of the system. As the Sun is very large in comparison with the other masses, the barycentre of the Solar System lies just outside the Sun’s surface, so that the Sun itself performs a complicated orbit about that point. The barycentre of the Earth-Moon system is ≈1600 km (≈1000 miles) below the surface of Earth.
Emperor Rudolf, whose health was failing, was compelled to abdicate in favour of his brother Matthias (62; HRE: 1612-19) who, like Rudolph, was a Catholic but, unlike Rudolph, did not believe in the tolerance of Protestants. Kepler had to leave Prague and he moved to Linz.
Kepler’s Harmonices mundi (‘Harmony of the World’) was published in 1619. In this he attempted to explain the natural world in terms of music, the central set of harmonies being the ‘music of the spheres’. Among many other harmonies, Kepler articulated what came to be known as his third law of planetary motion. He had tried many combinations until he discovered that for any two planets the ratio of the squares of their periods will be the same as the ratio of the cubes of the mean radii of their orbits.
In 1601 Kepler had been commissioned to work with Tycho on the Rudolphine Tables. Calculating tables, the normal business for an astronomer, always involved heavy arithmetic. Kepler was accordingly delighted when in 1616 he came across the work on logarithms (1614) by John Napier (1550-1617). Kepler calculated tables of eight-figure logarithms, which were published with the Rudolphine Tables (finally) in 1627. The tables proved to be accurate over decades and as the years passed the continued accuracy of the tables was seen as an argument for the correctness of Kepler’s Laws and thus for the correctness of heliocentric astronomy.
Kepler left Linz in 1626 and was working for Albrecht von Wallenstein (1583-1634), one of the few successful military leaders in the Thirty Years’ War (1618-1648). Kepler died on 15 November 1630 in Regensburg after a short illness. He had been on his way to collect some money owing to him in connection with the Rudolphine Tables. The accuracy of the Rudolphine Tables eventually produced the first observational evidence that Mercury and Venus orbit inside Earth (i.e. they are nearer to the Sun). Kepler had predicted that the two planets would cross the face of the Sun on 7 November and 6 December 1631. Pierre Gassendi (1592- 1655) saw Mercury as a dark speck flitting across as calculated but he missed Venus because it crossed during the time the Sun was below the horizon. In 1639 Reverend Jeremiah Horrocks (1618-41) observed a transit of Venus which he had predicted from his refinement of Kepler’s tables.
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