Fixed Stars After an Explanation of the Method for Determining Star

chapter sixteen

After an explanation of the method for determining star positions in Almagest VII.4, Ptolemy presented a catalogue of the fixed stars in Alm. VII.5 and VIII.1. The catalogue gives the name or descrip- tion, the ecliptic coordinates (longitude and latitude), and the mag- nitude of stars grouped in 48 constellations. In his catalogue Ptolemy listed 1,025 stars, of which 3 stars ascribed to the constellation of Leo (a group called Plókamos, meaning “lock” or “tuft of hair” [of Berenice]) were not included in the count of 1,022 stars (see Kunitzsch 2002, p. 22). Moreover, 3 stars are listed twice (not those in Leo), so that the number of entries in his catalogue is 1,028. Although subject to some controversy, the epoch of Ptolemy’s star catalogue is 137 AD (Toomer 1984, p. 340, n. 91). This catalogue was mostly known to western astronomers through the translation into Latin of the Arabic versions of the Almagest made by Gerard of Cremona around 1175. A modern edition of this Latin translation can be found in Kunitzsch 1990. Another line of transmis- sion to the West came through the star catalogue for 964 AD compiled by al-Sūfị̄ which, in turn, depended on the star catalogue in Ptolemy’s Almagest (see Comes 1990). The total precession in al-Sūfī’ṣ text is 12;42° (Samsó and Comes 1988, p. 69), and this is the amount to be added to the star’s longitude given by Ptolemy to obtain that of al-Sūfī.̣ This work was adapted into Castilian in 1256 by Judah ben Moses ha- Cohen with the help of Guillén Arremón Daspa, both in the service of King Alfonso X of Castile, and it was later revised (1276) by the king himself, in collaboration with other astronomers in his scriptorium. The title of this text is Libro de las estrellas de la ochaua espera and it is also known as Libro de las XLVIII figuras de la VIII spera (Rico Sinobas 1863–1867, vol. 1, pp. 5–145). The total precession in this text is 17;8°. Ptolemy’s star catalogue gave rise, directly or indirectly, to a wealth of other star tables, mostly much shorter. The number of stars and their names vary widely, and in many cases quantities other than ecliptic longitude and latitude were tabulated, e.g., meridian altitude (altitudo) 186 chapter sixteen or coordinates related to the equator, such as mediation (medium celi; i.e., the longitude of the point on the ecliptic that has the same right ascension as the star when it culminates), as well as the names of the planets associated astrologically with each star (natura). These star tables can be grouped into three categories:

(i) short lists of about 20−40 stars, usually listing the stars to be used on astrolabes or other instruments; (ii) long lists of about a few hundred stars, much more uncommon and too long to be used on an astronomical instrument; (iii) catalogues of about 1,000 stars that depend on the Almagest.

(i) As an example of short list we have chosen that appearing in Oxford, Bodleian Library, MS Can. Misc. 27, f. 129v (see Table 16A). At the right margin of the table, a note reads: Ista tabula fuit composita anno 1225. For each of the 34 stars we are given the longitude on the ecliptic, without any indication of its sign, specified here between square brackets, and mediation, m, which is the co-culminating degree of the ecliptic. Mediation (m) and right ascension (α) are related by means of the following equation: tan m = tan α / cos ε, where ε is the obliquity of the ecliptic. Now, the right ascension is obtained from the given ecliptic coordinates of the star, λ, and latitude, β, by means of the modern equations tan α = tan λ · cos (θ + ε) / cos θ and tan θ = tan β / sin λ, where θ is an auxiliary angle. At the far right of Table 16A we have added the modern designation of each star. Comparison with the coordinates in Ptolemy’s star catalogue shows an increment of 14;55° in longitude. Thus, this list should be related to Kunitzsch’s type XIII (35 stars with a mixture of increments of 14;55° and 15;7°) and XIV (32 stars with increments of 14;55°), both associated with the Toledan Tables (Kunitzsch 1966, pp. 87−97; see also F. S. Pedersen 2002, pp. 1494−1501). The list displayed in Table 16A does not quite match either of them: on the one hand, it only gives longitude and mediation (both found in type XIII lists, among other