Astronomical Images : Primary and secondary planets: the Earth and the Moon, and Jupiter and its satellites

Johannes Kepler

Astronomical Images

<p style='text-align: justify;'>Johannes Kepler's <i>Epitome astronomiae copernicanae</i> was published in three successive parts from 1617 to 1621. Intended as a new astronomical textbook for students, the <i>Epitome</i> covered much of the material traditionally associated with the <i>Theoricae planetarum</i> and works on spherical astronomy. Despite its title, the <i>Epitome</i> does not draw its information directly from Copernicus' <i>De revolutionibus</i> but is an original synthesis, mainly based on Kepler's own earlier works. It thus presents a comprehensive survey of Kepler's new astronomy. The text is illustrated by an abundance of images, with some figures printed multiple times. This image serves to illustrate the distinction between primary planets, which orbit the Sun, and secondary planets (i.e. satellites), which orbit another planet. Drawing on the recent telescopic discoveries of 1610-13, in which Galileo identified the four moons of Jupiter, it depicts a simplified cosmos with the Sun at the centre (A), and the orbits of the two planets with observed 'secondary planets' ' the Earth (B) and Jupiter (not labelled). The text also mentions the satellites of Saturn, but, as their existence was still contentious, they are not represented in the accompanying illustration.</p>