# Rationale

There is general agreement among historians of science that Johannes Kepler (1571-1630), through the discovery of his three laws of planetary motion, is the father of modern astronomy and a highly significant figure in the development of modern science (Koyré 1973; Westfall 1977; Hall 1985; Holton 1988). Kepler's freeing of astronomy from the mindset of uniform circular motion and Galileo's contributions to the understanding of terrestrial physics combined to help to break down the notion that celestial physics was to be distinguished from terrestrial and opened the way for Newton's formulation of mechanics and the universal theory of gravitation.

In the words of one of the historians invited to give a keynote talk at this Special Session, 'Kepler's *Astronomia nova* is indeed a new astronomy. It combines coherently in a revolutionary way many layers of different kinds of knowledge which together offer a most powerful system of inquiry whose scientific fruits we still enjoy today.' This Special Session will celebrate the 400th anniversary of the publication of Kepler's *Astronomia nova*, a seminal work containing the mathematical derivation of the two planetary laws that control the motion of each individual planet. The First Law stated Kepler's discovery that the path of the planet is an ellipse with the Sun at one of the foci; the Second Law explained that the planet moved round its orbit in such a way that the time taken was proportional to the area swept out. Meanwhile, it was Kepler's great desire to synthesize the planetary system, and in *Harmonice mundi* he finally succeeded, by determining the mathematical formula that gives the relationship between the (mean) distances of the planets and their orbital periods about the Sun. At this Special Session we shall additionally take the opportunity to recall the immense contributions that Kepler made to optics, and the development of the calculus and the theory of logarithms.

Research over the past few decades into the life and work of Kepler has led to a major reappraisal of the many contributions to science made by this key figure in astronomical history. Traditionally, Kepler's works have been viewed as rambling and difficult to follow, the precious few grains of wheat being hidden in a mountain of chaff. In fact, these very same works - especially *Astronomia nova* (1609) and *Harmonice mundi* (1619) - have been found by historians and philosophers of science to contain much that is of the greatest value in tracing the birth of modern astronomy. With its many descriptions of unsuccessful models and final successes, what Kepler has in fact given posterity in his *Astronomia nova* is an account of the workings of one of the greatest ever scientific minds. Kepler is also the founding father of modern optics. He was the first to give a correct description of the optics of the human eye and the pinhole camera, and he gave the first correct explanation of how the refracting telescope works.

At this Special Session, historians and Kepler specialists of note will explain and discuss these new findings. Talks and posters on any aspect of Kepler's life and work, and on the Keplerian scientific heritage are encouraged. On this 400th anniversary of the publication of *Astronomia nova* it is surely fitting that the representative body of the world's professional astronomers should proudly celebrate this milestone in astronomical history.

**References**

Hall, A. R. 1985,

*The Revolution in Science 1500-1750*(London: Longman)

Holton, G. 1988,

*Thematic Origins of Scientific Thought: Kepler to Einstein*(Cambridge, Mass: Harvard Univ. Press)

Koyré, A. 1973,

*The Astronomical Revolution*(London: Methuen)

*The Construction of Modern Science: Mechanisms and Mechanics*(Cambridge: CUP)