Classical astrolabes were usually made of brass and were commonly about six or eight inches (15 to 20 cm) in diameter, although much larger and smaller ones were made. No doubt many astrolabes were made of cardboard and paper, but few have survived.
Most astrolabe problems were solved using the front of the instrument. The front of an astrolabe has two types of parts: fixed and rotating. The fixed parts represent time scales and the stereographic projection of the sky as seen from a specific latitude. The rotating parts simulate the daily rotation of the sky.
The main body of a typical astrolabe would consist of a brass disk about ¼ inch (6 mm) thick and about six inches (15 cm) in diameter that is hollowed out in the center to hold sets of thin brass plates. The ring around the edge of the disk (the limb) was marked in degrees and, on many European astrolabes, into 24 hours with noon at the top and midnight at the bottom for telling time. Astrolabes from the Islamic world usually did not have the hours shown. Inserted into the hollow section of the disk (the mater, Latin for "mother"; rhymes with "later") is a plate (also called a climate or tympanum) for the local latitude engraved with circles of altitude and azimuth for a certain latitude. Old instruments included several plates engraved on both sides so the instrument could be used at several latitudes. It was possible to take the instrument apart and insert the plate closest to the user's latitude in the mater.
Over the plate is fitted a disk (the rete, Latin for net), also made of brass, that was mostly cut away (pierced) so you could see the plate under it. Pointers represented a number of fixed stars. A circle showing the projection of the Sun's annual path in the sky (the ecliptic) was included on the rete. The ecliptic circle was always divided into 30 degree sections representing the signs of the zodiac. The rete is assumed to rotate in one sidereal day to simulate the daily rotation of the stars in the sky.
On top of the rete was a clock-type hand called the rule. Not all astrolabes had a rule depending on the intended use of the instrument. The rule and the rete were held in position by a pin through the center of the instrument and could rotate over the plate. The pin was often held in place with a wedge shaped like the head of a horse (screws had not yet been invented).
The entire instrument was suspended by a cord connected to a ring located at the top of the astrolabe for taking measurements of the Sun or a star. The top of the instrument where the ring was connected is called the throne (kursi on Islamic instruments). On European instruments the attachment mechanism was called the fixed armilla and was equipped with a swivel called the armilla (Latin for ring).
The back of the instrument was engraved with a wide variety of scales depending on where and when the astrolabe was made. All astrolabes included scales for measuring angles and scales for determining the Sun's longitude for any date. Additional scales were included at the maker's option. Almost all European astrolabes, and many Islamic ones, had a scale for solving simple trigonometry problems called the shadow square. A cotangent scale was added to many Islamic astrolabes for determining prayer times. Islamic instruments might also include a scale for finding the direction to Mecca (the qibla), mathematical scales of sines and cosines or astrological information. European instruments often had a scale for converting between unequal (planetary) hours and equal hours.
The back of every astrolabe included an alidade for measuring the altitude of celestial objects.
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This page constructed by James E. Morrison, Janus.