The quadrant, with astrolabic marking for a specific latitude on one side and a trigonometric grid on the other side, was the most popular astronomical instrument in the central regions of the Islamic world (Egypt, Syria and Turkey) from the 15th century onwards. The trigonometric grids of orthogonal lines for calculations with trigonometric functions to base 60, standard in the Middle Ages, were first devised in the 9th and 10th centuries. The clever idea of putting one-half of the markings of an astrolabe plate on a quadrant and replacing the star-map with a thread and a movable bob dates from 12th century Egypt (for an introduction to Islamic instrumentation in the Islamic world see King D. A., Islamic Astronomical Instruments, A-B, London Variorum, 1987, and on the quadrant in particular see the overview "Rub", in The Encyclopaedia of Islam, New Edition, Leiden, 1986).
Half a dozen examples of such instruments with both sets of markings from the 14th century survive, all made of brass; otherwise most surviving examples are in wood and date from the 18th or 19th centuries (see Christie's South Kensington-Fine Scientific Instruments, 13.12.1996, London, 1996, pp.40-41, lot 599 and Ackerman S. and King D. A., "A Remarkable Syrian Quadrant", forthcoming, on a quadrant from Damascus at the end of the 19th century). No quadrant from Syria or Egypt are known from between the 14th and 18th century, except the one under consideration here.
The astronomical markings on this quadrant are mainly standard for such instruments, and are competently executed. The inscriptions are in a legible, functional nashki script, with numbers in standard Arabic alphanumerical (adjab) notation. There are two sights on the radial axis serving as meridian. The astrolabic quadrant has altitude circles for each 2° and azimuth circles for each 5°. There are additional curves for determining the duration of twilight (see the article "Shafak" in Enc. Islam, op.cit), labelled fajr, "morning twilight" and shafaq, "evening twilight" (these time-intervals for angles of solar depression of 19° and 17°, respectively, are measured from the east-west radius, so that the curves are the horizon). The outer scale is divided and labelled for each 5° in both directions , and continued for between 15° and 20° inside the main markings below the east-west radius. Inside the outer scale is a solar declination scale labelled qaws al-mayl, "arc of declination", with the obliquity of the ecliptic specified (see below). Also, altitudes for each 6° are indicated on the meridian radius. Inside these astrolabic markings is a universal horary quadrant, with the hours for the seasonal hours labelled sâ'ât zamâniyya, "seasonal". The hours to midday are labelled 1-6 in the Eastern Arabic forms of the Hindu numerals.
On the trigonometric quadrant there is the standard sexagesimal (base 60) grid, with both radial scales and the outer scale marked and labelled for each 5 units in the both directions. The additional marking are also standard. These consist of: a quarter-circle radius 24 units for determining the solar declination from the solar longitude; two axial semi-circles for the finding sines and cosines with facility; and two straight lines for determing the altitudes of the sun at the beginning and end of the afternoon prayer ('asr, on the time of Muslim prayer see the article "Mîkât" in Enc. Islam, op.cit.). The last-mentioned markings, being drawn as straight lines, are approximations to the actual curves defining these functions graphically, which are not rectilinear.
The following additional markings and inscriptions are of particular historical interest. First, at the outer end of the meridian radius there is an inscription: 'amal Shams al-Dîn ibn ? 'Izz al-Dîn al Halabî, (bi-madînat?) Halab sanat 967 "made by Shams al-Dîn ibn 'Izz al-Dîn ? al-Halabî (in the city of ?) Aleppo in the year 967AH/1559-60AD.
There is an illegible word, presumably a proper name, that is, the name of the marker's father, above the 'Izz of 'Izz al-Dîn. The words bi-madînat are not secure. In any case the name of Shams al-Dîn is new to the literature. There was a tradition of astronomy in Aleppo from the 14th to the 19th century, which went slowly but surely downhill. It was the early 14th century that the leading instrument-marker of medieval Islam, Ibn al-Sarrâj, was active, and when Najm al-Dîn al-Misrî wrote his remarkable treatise describing over a hundreds kinds of astronomical instruments (see Christie's, London-Islamic Art and Manuscript, 11 April 2000, lot 22, and more especially, Charette F., Astronomical Instrumentation, PhD. thesis, Frankfurt, 2001). From the later centuries we have only a few modest works on timekeeping compiling in Aleppo (King D. A., Astronomical Timekeeping, II-11.5, 11.11, 11.14, Leiden, in press)
Second, the value of the obliquity of the ecliptic given on the solar declination scale is clearly stated as 23°37'. This is a value not attested in the mediaeval Islamic literature of the period (or of any other period), and appears to be the result of some independent observations and calculations, not by the most competent astronomer. The obliquity decreases over the centuries. Ptolemy ca. 125 AD determined it to be 23°51'20''; Muslim astronomers in the 9th century found it to be 23°35' or 33'; and the astronomer Ulugh Beg in 15th century Samarkand measured 23°30'17'' (see the articles "Mayl" and "Mintaka" in Enc. Islam, op.cit.). In Damascus in the late 14th century the value accepted was 23°31', and thereafter either 23°30' following Ulugh Beg or 23°29', used by Ottoman astronomers after the 17th century (King D. A., Astronomical Timekeeping, II-10 and 11, Leiden, in press). In other words, the incorrect value 23°37' on this quadrant is unexpected and cannot be explained in the light of our present knowledge of the late Islamic astronomy.
Third, below the horizon on the astrolabic markings is an inscription: li-Halab wa-li-kulli balad 'arduhu l-h n "from Aleppo and every locality whose latitude is 35°50'".
This value of the latitude of Aleppo dates at least from the 10th century (Kennedy E. S. and Kennedy M. H., Islamic Geographical Coordinates of Localities from Islamic Sources, Frankfurt, 1987, pp.16-17). It appears to have been derived by calculation from the length of maximum daylight for the fourth climate of Antiquities using the Indian value for the obliquity of the ecliptic, 24°. (The seven climates are latitudinal bands whose midpoints are defined by the maximum daylight values of 13 hours, 13 1/2,..., 16, so that the 4th climate corresponds to 14 1/2 hours (see King, "Bringing Astronomical Instruments Back to Earth: The Geographical Data on Mediaeval Astrolabes", in Vanderjagt A. and Nauta L. eds., Between Demonstration and Imagination: Essays in the History of Science and Philosophy Presented to John D. North, Leiden, 1999, pp.3-53). The value for Aleppo became standard in the late medieval period (Kennedy E. S. and Kennedy M.H., Islamic Geographical Coordinates, Frankfurt, 1987, pp.16-17; King D. A., Studies in Astronomical Timekeeping in Medieval Islam, II-11.14, Leiden, in press), although 36°0' was also used (ibid., II-11.11): the former is even more inaccurate than the latter, for the actual value is 36°14'. In the 14th century when Aleppo was an important centre of astronomy, Najm al-Dîn al-Misrî used the better value, 36°0' (also the latitude of the middle of the 4th climate for obliquity 23°35'), and Ibn al-Sarrâj the over-corrected value 36°30'. It should be borne in mind that any really competent mediaeval astronomer who put his mind to it could determine his local latitude (and the obliquity of the ecliptic-see above) to within a couple of minutes.
Fourth, inside the semi-circle corresponding to the 6th hour on the universal horary quadrant there is a prayer niche (mihrâb) indicating for Aleppo the direction of Mecca (qibla, see the article "Kibla" in Enc. Islam, op.cit.). This inclined to the meridian at an angle of 15°. The "correct" value, based on the most accurate available mediaeval coordinates of Aleppo and Mecca, would be closer to 18° (King D. A., World Map for Finding the Direction and Distance to Mecca: Innovation and Tradition in Islamic Science, Leiden, 1999, pp.620-621); this is turn is different from the value which can be computed from the modern coordinates.
In brief, this new instrument is an important addition to our understanding of mediaeval instrumentation, providing several new insights and rising several new questions.