PARTS OF THE ASTROLABE

The limb: The ring around edge of disk marked in degrees into twenty-four hours.

The mater: The hollow section of the disk.

The plate (also called a climate or tympanum): Inserts into mater and shows local latitude by circles of altitude (distance celestial object appears to be above horizon) and azimuth (angular distance along horizon to location of celestial object, usually measured from north towards the east along horizon).

The rete: A disk fitted over the plate that has pointers representing a number of fixed stars and a circle showing the ecliptic (projection of the sunís annual path in the sky divided into 30 degree sections representing the signs of the zodiac).

The rule: A hand that sits on top of rete often used to find the declination of celestial objects (not all astrolabes had this).

The pin: This holds the rule and rete in position and allows them to rotate over the plate. It also represents the north or south celestial poles.

The ring: This sits at the top of the astrolabe so that the instrument can be suspended by a cord to take measurements of celestial objects.

The throne: The top of the instrument where the ring is connected.

The back: The back usually contains various scales that have a number of purposes: measuring angles; determining the sunís longitude for any date; solving trigonometry problems (a shadow square); determining Isalmic prayer times (cotangent scale), finding direction to Mecca (the qibla); finding sines and cosines; converting between unequal (seasonal) and equal (12 hours for night and 12 hours for day); and identifying specific astrological information.

The alidade: The sighting apparatus at the back of astrolabes for measuring the altitude of celestial objects.