Non-fiction: A Treatise on the Astrolabe

Overview
A Treatise on the Astrolabe is a concise, practical manual written in Middle English by Geoffrey Chaucer in 1391 for his son Lewis. Framed as a tutor's letter, it explains the astrolabe as a single, portable instrument for solving many routine problems of medieval astronomy and timekeeping. Plain in tone and concrete in detail, the text aims to make technical operations intelligible to a non-Latinate reader by translating and adapting specialist material into the vernacular.

Content and structure
The treatise proceeds in a clear stepwise fashion: introductory remarks and vocabulary, an anatomical description of the instrument, instructions for constructing and inscribing its parts, and worked procedures for observational and computational tasks. Chaucer names and describes the astrolabe's main components, the mater or "mother," the limb and degree scale, the rete with star pointers, the plates (tympans) cut for particular latitudes, and the alidade used for sighting. He gives detailed directions for making the instrument, arranging the scales, and engraving the plates for different latitudes, so that a single text can be used across regions.

Practical instruction and examples
Chaucer emphasizes hands-on procedures. He shows how to measure the altitude of the sun and stars, how to read or tell the hour by day and night, how to identify star positions and rising and setting times, and how to determine the sun's position in the zodiac. Many explanations are illustrated by worked examples and simple arithmetic, with attention to the small adjustments and mnemonic aids that make calculations feasible in the field. The treatise teaches not only how to manipulate the instrument but also how to translate observational results into useful calendrical, navigational, and horological information.

Sources, method, and language
The material reflects established medieval astronomy and the technical tradition of astrolabe manuals circulating in Latin and Arabic across Europe. Chaucer adapts that specialist knowledge into English, choosing clear phrasing and stepwise explanation rather than abstruse scholastic argument. The vernacular presentation is striking for its era: most scientific writing then appeared in Latin. Chaucer's method mixes literal description with pragmatic shortcuts and examples, aiming to equip a practitioner with both the instrument and the know-how to use it reliably.

Significance and legacy
The treatise is notable as one of the earliest sustained pieces of scientific prose in English and as a rare example of explicit technical instruction addressed to a named lay reader. It reveals Chaucer's curiosity about applied science and his competence with mathematical instruments, and it helped establish a model for later English technical writing. Modern readers value the text both as a source on medieval instrument design and practice and as evidence of how learned knowledge was translated into practical skills for everyday use.