The astrolabe
To obtain the time, the user first measured the
altitude of the sun. Then, having noted the sun’s
position for the day in the zodiacal circle, he
rotated the rete until the sun’s position coincided
with the circle on the plate corresponding to the
observed altitude. A line drawn through the point
of coincidence and the center of the instrument,
given by the edge of the alidade, to a marginal
circle of hours showed the time. All the stars’
position can then be referenced to the local
celestial coordinates engraved on the typan
(plates) that stays below the rete.
It is believed to have been a Greek
instrument, invented by Hipparchus
of Biythynia (150 B.C.), that played
an important part in the history of
civilization. Its earliest known
description is by John Philoponus
from Alexandria dating from the 6th
century. Alexandrian knowledge
seem to migrate through the Christian
abbeys of Syria after the Arab
conquest of Egypt. With the
Abbasids, Islamic science became
acquainted with Greek sources as
well as with Indian and Persian and
through them those of ancient
Babylonians and Egyptians.