Construction


The diagram shows the parts of the classical astrolabe:

The body of the instrument is a shallow, round case. Its raised rim (called the limbus) is graduated to show degrees and hours. Attached to the rim are an eyelet and a ring to allow the instrument to hang vertically. The hollow in the case, the mater, holds one or more flat discs called tympans is made for a specific latitude and shows the observer's horizontal coordinates. Most astrolabes have several interchangeable tympans for different coordinates. The pierced rete rotates above the mater and the tympan. It shows the eccentric path of the sun (the ecliptic) and bears pointers indicating the positions of stars. It represents a map of the heavens. European instruments frequently include a pointer at the top for reading off the time from the limbus. Astrolabes are primarily used for determining the hour.

The back of the astrolabe (the dorsum) usually bears a circle linking the calendar with the ecliptic, and its edge is divided into 360 degrees. In front of this there is a two-armed ruler (the alhidade ) with two sight vanes of pierced metal, which is used for measuring altitude. The instrument is suspended in mid-air by the ring, and its symmetrical construction causes it to hang vertically. The mater, tympans, rete, alidade and pointers are held together by a central bolt which is secured by an inserted pin.