Zarok's astronomical clock

Zarok's astronomical clock
The astrolabe in Zarok's planetarium.
Type	Astronomical instrument

An astrolabe was an astronomical instrument dating to ancient times. It served as a star chart, and its various functions also made it an analog calculation device capable of working out several kinds of problems in astronomy. It was able to measure the altitude above the horizon of a celestial body, day or night; it could be used to identify stars or planets, to determine local latitude given local time (and vice versa), to survey, or to triangulate.

History

14th century

There was an astrolabe on the floor of the planetarium in Zarok the Sorcerer's castle.

Construction

Mater

The mater was the main body of the astrolabe. The edge of the mater was called the limb, on which the degree scale and scale of hours were engraved. The hollowed-out part of the mater was called the womb and contained a latitude plate.

Latitude plate

To an observer on the earth it appeared that the planet was at the centre of an immense sphere with the stars and other heavenly bodies located on its inside surface. Called the celestial sphere, it appeared to rotate around the earth.

The celestial sphere was mapped on the plate of an astrolabe using a mathematical technique called stereographic projection. This technique allowed the 3-dimensional sphere to be represented on the 2-dimensional flat plate. Each latitude needed its own projection, and so most astrolabes came with a variety of plates for particular latitudes, usually stacked one on top of the other, within the astrolabe. It is unknown whether Zarok's astrolabe had multiple plates.

Ecliptic ring

The ecliptic ring was the annual path of the sun through the sky, as seen from the earth. A belt extending around 6 degrees north and south of the ecliptic is called the zodiac. Within this belt the apparent motions of the sun and planets take place.

Rule

The rule was a bar which rotated across the front of the astrolabe. It was used to locate positions on the plate, and to relate them to the scale of hours marked on the limb.