Kepler’s derivation of the bisection of the earth’s orbit in Astronomia Nova

Chapter 22 of Astronomia Nova is focused on the calculation of the Earth’s eccentricity. This is carried out by observing the effect of the Earth’s motion on the apparent position of Mars. Kepler’s method to derive the exact eccentricity, however, requires as data a set of longitudes of Mars while that planet and the Earth are in a very particular and restricted number of possible configurations. This paper explains how Kepler understood and tackled the Earth problem in theoretical terms, and also how he drew information from Tycho’s observational registers in a methodical way in order to obtain the necessary data to calculate the desired parameter, i.e. the eccentricity of the Earth’s orbit. In doing so, I will analyze not only Astronomia Nova’s relevant passages, but also Kepler’s preliminary annotations, as published in the Gesammelte Werke.