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*De sphaera mundi* or the *Tractatus de sphaera* of Sacrobosco, first compiled in the 13th century by Johannes de Sacro Bosco, was the most influential pre-Copernican work on astronomy published in Europe. Based on Claudius Ptolemy’s *Almagest* and early Islamic contributions to astronomy, this work became the principal medieval university text for astronomy. By 1700 (long after it had been superseded by the work of Nicolaus Copernicus), it had gone through 200 editions. The frontispiece shown above is from the 1482 edition. Note the hand of God turning the sphere of the earth.

On the first page of his *De sphaera mundi*, shown above, Sacrobosco noted that the treatise on the sphere would be divided into four parts:

- first, discussion of a sphere, its properties, and the world as a sphere;
- second, the circles of which this material sphere was composed;
- third, the rise and significance of signs, i.e. astrology; and,
- finally, circles and the motion of the planets and the causes of eclipses.

A complete English translation of *De sphaera mundi* was made by Lynn Thorndike in 1949.

The diagram above was used to illustrate the principles of a solar eclipse when the moon’s shadow passes the earth.

Sacrobosco (circa 1195-1256), as he was popularly called, was known by other names, including John of Holywood. He was a British monk and a professor at the University of Paris. While best known for his book on astronomy, *Tractatus de Sphaera, *he was also known for his *Algorismus, *a book on counting and arithmetic using the Indo-Arabic numbers.

*The three images above appear through the courtesy of the United States Library of Congress.*

Frank J. Swetz (The Pennsylvania State University), "Mathematical Treasure: Sacrobosco’s De sphaera," *Loci* (April 2014)