Inventory no. 52528 - Epact entry
Epact number: 79288
Early 16th century; origin unknown
Brass; 115 mm in diameter
Some unusual features of this astrolabe have led to doubts being cast on its authenticity. Some of its features seem medieval, but the engraved letters are in a later style, which itself is not standard throughout. The rete is made from a rather thick brass plate and has been incorrectly divided.
However, it can be difficult to distinguish between a fake instrument, one that is the work of a non-commercial and eccentric maker, and one that has later additions. Although puzzles remain with this instrument, it is probably not straightforwardly a counterfeit.
Source museum: Museum of the History of Science, Oxford
Museum number: Inventory no. 52,528
The limb is soldered to the back plate of the mater, inside blank, throne, shackle and suspension ring. The limb has a degree scale 90 to 0 to 90 to 0 to 90, divided to 10, subdivided to 5 and to 1, numbered by 10, and hours 1 to 12 twice, numbered by 1.
Rete for 13 named stars, the ecliptic circle with the names of the signs, each with a scale of 30 degrees divided to 10, subdivided to 2, numbered by 10. The ecliptic circle is also divided on the verso. The diametric bar is counterchanged three times. The pointers are numbered 1, 2 or 3 as an indication of stellar brightness.
There are three plates, for latitudes 37 and 40°; 43 and 46°; 49 and 52°. Each side is engraved with azimuths for every 10 degrees, and almucantars for every 3 degrees, numbered by 6 to 42 (latitude 49 to 36, latitude 52 to 30), lines beneath the horizon for unequal hours numbered 1 to 11, meridian and east-west lines, tropics and equator.
The back has a degree scale on the limb, 90 to 0 to 90 to 0 to 90, divided to 10, subdivided to 5 and to 1, numbered by 10. The same degree divisions serve a zodiacal scale inside, with names and symbols of the signs, each with a 30-degree section, numbered by 10. Within this is a calendar scale with month names, each with a scale of days divided to 10, subdivided to 5 and to 1, numbered by 10 as appropriate. The first point of Aries is at 10|1/2| March. The lower half of the central space has a double shadow square with scales 0 to 12 to 0 to 12 to 0, divided to 3, subdivided to 1, numbered by 3, marked 'VER' / 'RECTA' / 'SA'. Above this to the right is a diagram of unequal hours, while to the left of this is a linear scale on the radius of the same circle, 0 to 60, divided to 10, subdivided to 5 and to 1, numbered by 10, used for finding the sines of angles.
Straight alidade with double pinhole sights; threaded bolt and nut.
See F. R Maddison, A Supplement to a Catalogue of Scientific Instruments in the Collection of J. A. Billmeir (Oxford, 1957), p. 35.