Mr. Bradshaw gave me the hint; I afterwards found this remark by Selden, in his Preface to Drayton's Polyolbion: 'his [Chaucer's] Treatise of the Astrolabe, which I dare swear was chiefly learned out of Messahalah.'
Macha-allah or Messahala, an Arabian astronomer, by religion a Jew, flourished towards the end of the eighth century. Latin translations of four of his works (not including the Treatise on the Astrolabe) have been printed, and were published at Nuremberg in 1549. A list of his works is given in Casiri (Bibl. Arab.-hisp. tom. 1er. pag. 434), and in the Biographie Universelle.
This splendid MS., of the thirteenth century, is dated 1276, and illustrated with beautifully executed coloured diagrams. It is a storehouse of information about the Astrolabe, and I have often consulted it.
It is printed in full in my edition of Chaucer's Astrolabe, published for the Early Eng. Text Society in 1872, at pp. 88-104.
In my edition of the 'Astrolabe' for the Early Eng. Text Society (1872), I have inserted a large number of examples of strange blunders in the printed editions.