A standard quadrant from Ottoman Turkey, signed by Ibrâhîm and dated 1284 Hijra [= 1867/68]

Dozens of wooden quadrants survive from the Ottoman period. One side bears astrolabic markings for a specific latitude, here 39°, and the other side a sexagesimal grid for all manner of calculations to base 60 (as was standard in ancient Greek and medieval practice).

The astrolabic markings comprise one-half of the markings on a standard astrolabe plate, the rete being replaced by a string and movable bead attached at the astronomical centre of the markings. The bead is set in the appropriate position to display the solar declination, using the ecliptic scale (a semi-crescent divided for the zodiacal signs). This can then be rotated so that the bead falls on the appropriate altitude circle or any one of a series of curves for the prayer-times. The latter are for the twilight prayers (labelled f for fajr, daybreak, and sh for shafaq, nightfall), the beginning and end of the afternoon prayer ('asr awwal and thânî), as well as the time for fasting in Ramadan (imsâk) and the time for the early morning prayer at the two festivals (labelled 'îdayn). Inside these main markings there is a set of scales for various astronomical functions and a universal horary quadrant for the six seasonal hours.

On the back the sexagesimal grid is fitted with two axial semicircles for facilitating the determination of sines and cosines, as well as two non-linear curves for finding the altitude of the sun at the beginning and end of the 'asr prayer. The inscriptions in the lozenge in the front and the ribbon on the back relate to the use of the markings for specific operations in Ramadan.

At the apex of the markings for the universal horary quadrant on the front is the maker's name, Ibrâhîm. If he is known by any other quadrants, this will become apparent when the long-awaited repertory of Muslim instrument makers and their works by the late Alain Brieux and Francis Maddison is published. At one of the two corners of the trigonometric markings on the back is the date 1284 Hijra [= 1866/67].

Bibliography: On this kind of quadrant see Morley, "Arabic Quadrant", and King, In Synchrony with the Heavens, II, pp. 77-78.