Epact Scientific Instruments of Medieval and Renaissance Europe

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.