John of Saxony (astronomer)

John of Saxony or Johannes de Saxonia or John Danko or Dancowe of Saxony was a medieval astronomer. Although his exact birthplace is unknown it is believed he was born in Germany, most likely Magdeburg. His scholarly work is believed to date from the end of the 13th century into the mid 14th century. He spent most of his active career, from about 1327 to 1355, at the University of Paris.

John of Saxony is quoted in various medieval manuscripts and referenced as the author of various astronomical or astrological treatises, even though his authorship in some cases is questionable.

A computus dating back to 1297 is attributed to John of Saxony even though the author is listed as Iohannes Alemanus. The fact that the geographical longitudes of Paris and Magdeburg, considered John's birthplace, among other facts is why some historians consider him the author.

Viewed with less suspicion is his commentary on Liber Introductorius ad Magisterium Idiciorum Astrorum written by al-Qabisi (Alcabitius) an Arab scholar from the late 10th century. John of Saxony's commentary on this treatise is preserved in many manuscripts, several incunabula and old prints, the latest of which dates from the middle of the 16th century.

Among his other contributions were a commentary on Alcabitius's Introduction to the art of astrology, which went through eleven printed editions, and an Almanach for the years 1336 to 1380 that he computed for the meridian of Paris using the Alfonsine tables.

John of Saxony was of the opinion that astronomers of that time wrote their works in such a confusing manner that those who weren't trained in the field couldn't comprehend nor use appropriately. Under the tutelage of his master, John of Ligneres, he sought to write tables that accounted for any situation and could be put to practical use. As he puts it "Therefore to the praise of glorious God, the honor of my master, and the profit of scholars who wish to learn the workings of astronomical tables, I, John of Saxony, with God's aid intend to give examples of all the operations which are commonly performed with tables so that there may be no one henceforth who will shrink from the use and employment of tables of the stars because of the difficulty working with them."

The more widely recognized work by John of Saxony is his Canons on the Alfonsine Tables. The Alfonsine Tables are the mathematical calculations from Ptolemy's Almagest with limited modifications. These tables for calculating planetary positions, prepared under the auspices of Alfonso X, were originally in Castillian. They were also used to derive ecliptic longitudes or planets for any chosen time and observer's position, lunar phases, lunar and solar eclipses, as well as calendar dates. Like the 11th century Toledan Tables before them, the Alfonsine Tables were based on the geocentric model of the planetary system as described in Ptolemy's Almagest. Around 1320, the Alfonsine Tables appeared in Paris where they were studied at the University of Paris by astronomers John of Ligneres, John of Saxony, and later Johannes de Muris.

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