Christoph Clavius · 1611
Read the translation (PDF) · Editorial footnotes (PDF)
Tomus III holds Clavius’s astronomy. The first and larger part is his commentary on Sacrobosco’s De sphaera — the most influential Renaissance astronomy textbook — reprinting Sacrobosco’s brief text and surrounding it with a vast commentary on spherical astronomy.
Clavius opens with an argument for the usefulness of astronomy, then follows Sacrobosco chapter by chapter, each short passage answered by a commentary many times its length. The division of the sphere of the world comes first, then the circles of the sphere, the number and order of the elements, and the count and motions of the celestial spheres.
The commentary is at its most interesting where it stops expounding and starts arguing from evidence. Clavius sets out how it was determined that the heavens move from east to west; by what reasoning it was concluded that there is also a motion from west to east; how the lower heavens were observed to move about different poles; and on account of which phenomena astronomers attributed the motion of trepidation to the fixed stars. From there the work proceeds to risings and settings, the climes, the motions of the Sun and planets, eclipses, and the computus.
The second part, the Astrolabium, is a comprehensive treatise on the construction and use of the astrolabe, with its projection geometry and tables. Both parts are richly illustrated with woodcut diagrams.
Sacrobosco’s Sphere was the book through which most of educated Europe first met the structure of the cosmos, and for more than a century Clavius’s edition was how the Sphere itself was read. That makes this volume the best single record of what mainstream astronomy taught, and how it justified itself, in the decades on either side of 1600 — the settled position against which Tycho, Kepler and Galileo were arguing.
The translated source is the Mainz printing of 1611.