Kepler's Textbook
Kepler's textbook Epitome of Copernican Astronomy (Epitome astronomiae Copernicanae), was one of hismost ambitious and drawn out project, second perhaps only to his Rudolphine Tables. Books I through III appeared in 1618, Book IV in 1620, and Books V-VII in 1621. The complete work was reprinted in 1635, and for a long time remained one of the few comprehensive textbook on the topic.
In the passage reproduced above (from Book IV), Kepler applies his Third Law of Planetary motion to infer the sizes of the orbits of the Galilean moons of Jupiter.