KEPLER, Johannes.

First collected edition – a comprehensive version of Johannes Kepler’s theory of comets. It gathers three astronomical works, revised, translated or enlarged from the original, based on his research for the comets of 1607 (i.e., later Halley’s, very bright and with a double tail) and 1618 (which he was the first to see through a telescope). The German astronomer Kepler (1571-1630) was assistant of Tycho Brahe at Prague and the mathematician to three Holy Roman Emperors. He famously adapted the Copernican theory by suggesting planets had orbits that were elliptical, nor circular, with the Sun, and he explained the speed by which planets move around the ellipsis. In ‘De cometis’, Kepler sought, following Brahe, to overcome the Aristotelian theory by which comets were not considered ‘heavenly bodies’ but phenomena caused by changes in the weather (Cantamessa).

Kepler and Galileo were united in continuing the work of Copernicus, Galileo by astronomical observation and Kepler by development of Copernican ideas. In 1610 their connection was close as Kepler helped Galileo in his struggle for ‘Sidereus Nuncius’. Eight years later they famously differed on the origin of comets – the case of the three comets discussed here. Galileo defended their earthly origin, Kepler maintained their origin as cosmic and of course was right. In fact, he thought comets as ‘spherical transparent objects refracting the sun’s rays’ (Heidarzadeh, p.65). The collection begins with ‘Astronomicus’, on theorems of the movement and trajectory of comets, and a discussion on their aspect (including tails) and height. The folding diagram detailing the trajectory of the Halley comet shows how he sought to map its route through the heavens, making its trajectory a straight line. Part II, ‘Physicus’, focuses on the physiology of comets, i.e., their nature and formation, and the composition of their tails. Part III, ‘Astrologicus’, discusses the interpretation or meaning of the 1607 comet, originally published in German, with an added section on the comet of 1618. By applying the rules of judicial astrology, which he criticised without rejecting completely, Kepler examined the influence of comets from the present and the past, connecting, for instance, the 1607 comet to the fatal illness of Empress Anne. A most important work.