A Treatise the Astrolabe by Geoffrey Chaucer is the work of an avid amateur astronomer who happened also to be England’s greatest medieval poet. A user of the astrolabe can plot the movement of the stars, tell time, and calculate numerous other results. Chaucer translated and revised a standard Latin treatment of the astrolabe. His treatise, which is generally regarded as one of the first technical manuals in English and a model of how technical manuals should be written. Not since 1872 has a free-standing edition of A Treatise the Astrolabe been published. Thanks to the expertise of its editor, Sigmund Eisner, who supplies sixty-eight illustrations, this Variorum edition provides a more detailed exposition than previously available. Eisner’s extensive labors result in the first complete record of textual variants found in the thirty-two surviving manuscripts of the work and in all the major printed text published between 1532 and 1987. This landmark edition also presents a thorough digest of all published commentary on Chaucer’s treatise. Amplified by sixty-eight illustrations, this variorum edition of Chaucer’s A Treatise on the Astrolabe provides a more detailed exposition of the treatise than has ever before been available.