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Inventory no. 52869 - Epact entry

Epact number: 52721


circa 1400; origin unknown
Brass; 162 mm in diameter

Main text

The most significant feature of this astrolabe is its use of a universal projection which was developed by az-Zarqellu in Toledo in the 11th century. The projection was used on instruments made in the Islamic tradition, and known in medieval Christian Europe as 'saphæa arzachelis'. It is described in various manuscripts, but was little used until it was revived in the 16th century by Gemma Frisius as his 'astrolabum catholicum'. It was then used on many Flemish astrolabes in particular.

On the inside of the mater of this instrument is one of the very few examples of the use of this projection on a non-Islamic astrolabe prior to its re-introduction by Gemma.

Source museum: Museum of the History of Science, Oxford
Museum number: Inventory no. 52,869

Detailed text

The mater has the rim soldered to the back plate [no - it is rivetted - SAJ 24/1/08]. The limb has a scale of hours 0 to 12 twice, divided to 1 hour, subdivided to 20 minutes and to 4 minutes, numbered by 1 hour. The inside of the mater has a 'saphæa' universal projection, with the outer circle marked for declination (90 to 0 to 90 to 0 to 90, divided to 6, subdivided to 1, numbered by 6), the equator divided for right ascension (0 to 180 to 360, divided to 6, numbered by 6) and the ecliptic line with the names of the signs, each divided to 6 degrees by dots, not numbered. Arcs for celestial longitude link the boundaries of the signs with the ecliptic poles. Right ascension and declination arcs are drawn every 6 degrees.

The rete has pointers for 23 named stars. The east-west band is counterchanged 10 times. The ecliptic band has the named of the signs, each with a 30-degree scale divided to 10, subdivided to 2, not numbered. The outer band, for the tropic of Capricorn, is cut with 120 teeth, each marked with a dot, and there is an incomplete or worn scale marked with the months of the year.

There are two plates for latitudes 42, 45 and 48, 51 degrees. On each there is an outer degree scale 0 to 360, divided to 5, subdivided to 1, numbered by 10. There are unnumbered almucantars and azimuths, circles for the equator and tropics, a crepuscular line, and east-west and meridian lines. There are divisions for unequal hours, numbered 1 to 12.

The back has an outer degree scale 90 to 0 to 90 to 0 to 90, divided to 5, subdivided to 1, numbered by 5, and a zodiacal scale that shares the degree divisions in sections for each names zodiacal sign, 0 to 30, numbered by 5. Inside is an eccentric calendar scale with the months named in Latin. each with a scale of days divided to 5, subdivided to 1, numbered as appropriate by 5. The first point of Aries is at 11|1/4| March. The central space has a double shadow square and unequal hour diagram.

Jim Bennett

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