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Who was Johannes Kepler? Kepler was a mathematics teacher at a seminary school in Graz, where he became an associate of Prince Hans Ulrich von Eggenberg. Later he became an assistant to the astronomer Tycho Brahe in Prague, and eventually the imperial mathematician to Emperor Rudolf II and his two successors Matthias and Ferdinand II. He also taught mathematics in Linz and was an adviser to General Wallenstein. Additionally, he did fundamental work in the field of optics, invented an improved version of the refracting (or Keplerian) telescope, and was mentioned in the telescopic discoveries of his contemporary Galileo Galilei. Last but most certainly not least, he was a corresponding member of the Accademia Dei Lincei in Rome. All of these titles, ranks, and positions that Kepler held came as no easy feat, but Kepler, being the mathematical genius of his time, achieved them all.   What was he famous for?  Johannes Kepler is best known for his three laws of planetary motion:   1) Every planet’s orbit is an ellipse with the Sun at a focus;  2) A line joining the Sun and a planet sweeps out equal areas in equal times, and  3) The square of a planet’s orbital period is proportional to the cube of the semi-major axis of its orbit.

These laws will be elaborated on in detail later on in this article. Kepler’s laws have an important place in the history of astronomy, cosmology, and science in general. They marked a key step in the revolution which moved the center of the universe from the Earth (geocentric cosmology) to the Sun (heliocentric), and they laid the foundation for the unification of heaven and earth, by Newton, a century later (before Newton, the laws which governed celestial phenomena were widely believed to be disconnected with those controlling things which happened on Earth; Newton showed with his universal law of gravitation that the same law rules both heaven and earth).

A-List of Kepler's Firsts

First to correctly explain planetary motion, thereby, becoming the founder of celestial mechanics and the first "natural laws" in the modern sense; being universal, verifiable, precise. In his book Astronomia Pars Optica, for which he earned the title of founder of modern optics, he was the: First to investigate the formation of pictures with a pinhole camera; First to explain the process of vision by refraction within the eye; First to formulate eyeglass designing for nearsightedness and farsightedness; First to explain the use of both eyes for depth perception. In his book Dioptrice (a term coined by Kepler and still used today) he was the: First to describe real, virtual, upright, and inverted images and magnification; First to explain the principles of how a telescope works; First to discover and describe the properties of total internal reflection.

The astronomical works of Johannes Kepler

The ideas that Kepler would pursue for the rest of his life were already present in his first work, Mysterium cosmographical (1596; “Cosmographic Mystery”). Kepler had become a professor of mathematics at the Protestant seminary in Graz, Austria, in 1594, while also serving as the district mathematician and calendar maker. In 1595, while teaching a class, Kepler experienced a moment of illuminatory eureka. It struck him suddenly that the spacing among the six Copernican planets might be explained by circumscribing and inscribing each orbit with one of the five regular polyhedra. This came to Kepler after knowing Euclid’s proof that there can be five and only five such mathematical objects made up of congruent faces. Kepler was confident that if the ratios of the mean orbital distances agreed with the ratios obtained from circumscribing and inscribing the polyhedrons, he had discovered the architecture of the universe. 

Finally, Kepler published the first textbook of Copernican astronomy, Epitome Astronomiae Copernicanae (1618–21; Epitome of Copernican Astronomy). The title mimicked Maestlin’s traditional-style textbook, but the content could not have been more different. The Epitome began with the elements of astronomy but then gathered together all the arguments for Copernicus’s theory and added to Kepler’s harmonics and new rules of planetary motion. This work would prove to be the most important theoretical resource for the Copernicans in the 17th century. Galileo and Descartes were probably influenced by it. It was capped by the appearance of Tabulae Rudolphinae (1627; “Rudolphine Tables”). The Epitome and the Rudolphine Tables cast heliocentric astronomy and astrology into a form where detailed and extensive counterarguments would force opponents to engage with its claims or silently ignore them to their disadvantage. Eventually, Newton would simply take over Kepler’s laws while ignoring all references to their original theological and philosophical framework.

The last decade of Kepler’s life was filled with personal anguish. His mother fell victim to a charge of witchcraft that resulted in a protracted battle with her accusers, lasting from 1615 until her exoneration in 1621; she died a few months later. Kepler used all means at his disposal to save his mother’s life and honour, but the travels, legal briefs, and manoeuvres that this support required seriously disrupted his work.  I used to measure the heavens, Now I shall measure the shadows of the earth. Although my soul was from heaven, the shadow of my body lies here. Sky-bound was the mind, earthbound the body rests.

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