Steinar Thorvaldsen
The first transits of Mercury and Venus ever observed were important for quite different reasons than were the transit of Venus observed in the eighteenth century. Good data of planetary orbits are necessary for the prediction of planetary transits. Under the assumption of the central position of the Sun, Johannes Kepler published the theory of elliptical orbital motion of the planets in 1609; this new astronomy made it possible to compute noticeably improved ephemerides for the planets. In 1627 Kepler published the Tabulae Rudolphinae, and thanks to these tables he was able to publish a pamphlet announcing the rare phenomenon of Mercury and Venus transiting the Sun. Although the 1631 transit of Mercury was only observed by three astronomers in France and in Switzerland, and the 1639 transit of Venus was only predicted and observed by two self-taught astronomers in the English countryside, their observation would hardly been possible without the revolutionary theories and calculations of Kepler. The Tabulae Rudolphinae count among Kepler's outstanding astronomical works, and during the seventeenth century they gradually found entrance into the astronomical praxis of calculation among mathematical astronomers and calendar makers who rated them more and more as the most trustworthy astronomical foundation.
Manuscript: jad19_1j.pdf