Besides his print of Chaucer's Astrolabe, Mr. Brae has reprinted some curious and interesting critical notes of his own, and has added some essays on Chaucer's 'prime,' on 'the Carrenare,' and 'shippes opposteres.' To all that he has done I am much indebted.
§ 27. Works on the Subject. The works upon, and descriptions of, the astrolabe, are numerous. I have had neither time nor inclination to make researches into the subject; for which reason I here note the names of a few books which may be examined by the curious reader.
In his Universal Lexicon, Zedler explains that astrolabes are of two kinds, 'universal' and 'particular.' He speaks of the astrolabes (1) of Gemma Frisius; see Petri Apiani Cosmographia, per Gemmam Phrysium restituta; (2) of Johan de Rojas, a Spaniard, A.D. 1550; (3) of De la Hire the elder, professor of mathematics at Paris, A.D. 1702; (4) of Johannes Stoflerinus (or Stöffler), A.D. 1510. The last of these varied from the others in adopting a different and more convenient system of projection, viz. that upon the plane of the equator, or one parallel to it, the eye being in the antarctic pole, and the arctic pole being made the centre of the instrument. This projection is the same as that which was used by Ptolemy, and it is adopted in the diagrams which accompany Chaucer's treatise in some of the MSS. It should be observed here that the term 'astrolabe' alone is vague; it was originally a general name for any circular instrument used for observation of the stars; but in the sixteenth and seventeenth centuries it was restricted to the particular kind called the 'Astrolabe Planisphere,' or astrolabe on a flat surface, in which sense alone the word is used throughout this volume. See the English Cyclopaedia, Arts and Sciences, s.v. Astrolabe.
The simplest work is that by Stöffler or Stoflerinus, as he calls himself; see also Gemma Frisius, Metius, Clavius Bambergensis, the Cursus Mathematicus of Dechales, vol. iv. p. 161, Delambre's History of Astronomy, and other works. The plates in Metius are most exquisitely engraved, and on a large scale, and give a better representation of the instrument than any others that I have seen.
One of the MSS., viz. MS. E., refers to an astrolabe belonging to Merton College, Oxford[[57]]. There is a very nice one, made of brass, and by a Dutch engraver, in the library of King's College, Cambridge. It has several discs or plates, or, as Chaucer calls them, 'tables[[58]].' Of this instrument the same library contains a written description, with some account of the problems it will solve, and an investigation of its probable date, by H. Godfray, Esq., of St. John's College.
There is a book entitled 'A verie briefe and most plaine description of Mr. Blagrave his Astrolabe,' &c., by Mr. Blundevill; London, printed by William Stansby. But it turns out to be of little practical assistance, because Blagrave's astrolabe was on a different principle.
§ 28. Description of the Astrolabe Planisphere. There is not, however, much need of reference to books to understand what the astrolabe used by Chaucer was like. The instrument may be readily understood from a brief description, and from the Plates in this volume.
The most important part of the 'astrolabe planisphere' consisted of a somewhat heavy circular plate of metal from four to seven inches in diameter, which could be suspended from the thumb by a ring (i. 1), working with such freedom as would allow the instrument to assume a perfectly perpendicular position (i. 2). One side of the plate was perfectly flat, and was called the back. This is represented in [Fig. 1]. On it was described a number of concentric rings, marked with various divisions, which may be readily understood from the figure. Beginning at the outermost ring, the first two represent the ninety degrees into which each quadrant of a circle can be divided (i. 7). The next two represent the signs of the zodiac, each subdivided into thirty degrees (i. 8). The next two represent the days of the year, and are rather difficult to mark, as the circle has, for this purpose, to be divided into 365¼ equal parts (i. 9). The next three circles shew the names of the months, the number of days in each, and the small divisions which represent each day, which coincide exactly with those representing the days of the year (i. 10). The two innermost rings shew the saints' days, with their Sunday-letters. Thus, above the 21st of December is written 'Thome,' i.e. St. Thomas's day, its Sunday-letter being E; the rest can easily be traced by the tables in a Prayer-book (i. 11). These may be thus briefly recapitulated:—
1 and 2. Circles of degrees of the quadrant and circle.
3 and 4. Circles of the zodiacal signs, with their degrees.