A Treatise on the Astrolabe

It describes both the form and the proper use of the instrument, and stands out as a prose technical work from a writer better known for poetry, written in English rather than the more typical Latin.

Robinson believes that it indicates that had Chaucer written more freely composed prose it would have been superior to his translations of Boethius and Renaut de Louhans.

[2] Chaucer’s exact source is undetermined but most of his ‘conclusions’ go back, directly or indirectly, to Compositio et Operatio Astrolabii, a Latin translation of Messahala's Arabic treatise of the 8th century.

A collotype facsimile of the second part of the Latin text of Messahala[5] (the portion which is parallel to Chaucer's) is found in Skeat’s Treatise On The Astrolabe.

[1][7] Paul Kunitzsch argued that the treatise on the astrolabe long attributed to Messahala was in fact written by Ibn al-Saffar.

Chaucer explains this departure from the norm thus: Chaucer proceeds to labour the point somewhat: He continues to explain that it easier for a child to understand things in his own language than struggle with unfamiliar grammar, a commonplace idea today but radical in the fourteenth century.

Chaucer's appeal is an early version of the phrase "the King's English": Skeat identifies 22 manuscripts of varying quality.

Skeat observes that the errors are just those described in "Chaucers Wordes unto Adam, His Owne Scriveyn": A has indeed been rubbed and scraped then corrected by another hand.

Indeed, he draws attention to Chaucer's comment at the end of conclusion 4: The whole of this section describes the form of an astrolabe.

The astrolabe is based on a large plate ("The moder" or "mother") which is arranged to hang vertically from a thumb ring.

With it one could determine the date, time (when the sky was clear), the position of stars, the passage of the zodiac, latitude on the earth's surface, tides and basic surveying.