Inventory no. 12765 - Epact entry
Epact number: 56165
circa 1500; Italian
Brass; 166 mm in diameter
This is one of the earliest surviving armillary spheres and the assigned date is consistent with its simple form. Only later do such instruments include more complicated motions and planets. Here there is simply the celestial sphere, incorporating the motion of the sun, and a central earth.
However, there is at least one more ambitious feature. Although the surviving instrument is incomplete, it has a very unusual adjustment for demonstrating Italian hours. In this method of reckoning time, hours are counted from 0 at sunset to 24 the following sunset. Thus the relationship between the scale of hours and the celestial sphere has to be adjusted according to the position of the sun in the zodiac. The scale on the equator is in hours from 1 to 24, and it can be moved, being arranged to slide outside the zodiacal band. Scales on the base give the position of the sun in the zodiac according to the date.
Source museum: Museum of the History of Science, Oxford
Museum number: Inventory no. 12,765
Circular brass base on a low, decorated plinth and an inset glazed magnetic compass marked with the cardinal points. The base has an outer zodiacal scale, with the names of the signs in Latin, each with a 30-degree scale divided to 5, subdivided to 1, numbered by 5. Inside this is an eccentric calendar scale, similarly divided, with the names of the months in Latin, numbered by 10 as appropriate. The first point of Aries is at 11 March. An index arm is pivoted at the centre; on it is scratched faintly 'Volvela Solis'.
A central pillar rises to the intersection of four quadrant supports for the horizon ring, which is also supported by an upright strut at one side, which may be an addition. The ring has a degree scale, divided to 5, subdivided to 1, not numbered. The names of four winds are faintly scratched on the horizon ring. Two of the quadrants, forming a semicircle, have a continuous slot for the meridian ring, which is accommodated also by slots in the horizon ring.
The meridian ring has an unnumbered degree scale similar to that on the horizon ring. There are pivots for the poles of a celestial sphere comprising the equator, tropics, arctic and antarctic circles, and a zodiacal band. The equator, which sits outside the zodiac and can be rotated in the equatorial plane, has an hour scale 0 to 24, divided to 20 minutes, subdivided to 4 minutes, numbered by 1 hour. The zodiacal band has the names of the signs, with a 30-degree scale for each, divided to 5, subdivided to 1, numbered by 10, and is marked with several named stars. There is a slot running along the centre of the band, but nothing is now extant to slide in it. At the centre of a rod between the poles is a wooden sphere representing the earth.
- Armillary spheres
- Inventory no. 12765 - Former Display Label
- Picture Caption in Francis Maddison, Medieval Scientific Instruments