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Ahmad B. Ali Marsil, circa 1776.

18TH CENTURY ARABIC QUADRANT FROM THE MAGHREB REGION OF NORTHWEST AFRICA

Signed along one of the sight vanes (in Arabic), "Made by the little slave of his Lord, Ahmad b'Ali Marsil, year one thousand one hundred and ninety" (1190 A.H., equivalent to 1776-77 A.D.). On one side of the brass plate there is an engraved astrolabe quadrant with data for 35 degrees North latitude. This line of latitude runs through the Maghreb Region in Northwest Africa which comprises Morocco, Algeria and Tunisia. One of the radial edges has been cut with a long indentation to provide two `sight' vanes, and a plumb-bob is attached by a string to the apex. There is also a horary quadrant for unequal hours marked at the apex. Extensive astronomical and mathematical plots are engraved on the surface including a stereographic projection of the sky with astrological symbols, various star positions and the elliptic and azimuth lines. These plots enable the quadrant's use as a computer to find rising and setting times and positions for the sun, the constellations and bright stars as well as the time of day or night, and other solutions for problems in practical astronomy. On the reverse is a sine quadrant. This has been covered by a reinforcing board and back plate with a roundel inscribed 'From the work of as-sayyid Ahmad b, Ali Marsil, may God have mercy.' This suggests that the modification was made by the original maker. At the bottom, the quadrant has an index of 0-90 for taking sights. The quadrant sight used gravity to align the plumb bob with the index reading when the celestial body was observed by eye over the top of the instrument along the sighting vanes. The string was then clamped by the observer's finger against the index, and the angle read.
Height 7.5". Width of arc 10.25".
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  • Provenance:
    Sotheby's, London, The Frank Collection of Scientific Instruments, 1986
    The Kelton Collection of Marine Art & Artifacts.

    Since no horizon need be visually sighted (like the later bubble sextant) it could be used for star sights at night and on land where the horizon was not observable. The reading did not have to be compensated for the altitude of the observer -- the calculation of which was very controversial. The cast shadow method was used to measure the sun's altitude. The basic design of the instrument is of ancient origin. A graduated circle was the earliest instrument for measuring the altitudes of heavenly bodies precisely. It was Ptolemy who first suggested the measurement could be made with a quarter circle or quadrant. The quadrant reproduces data from the entire sky. It is conceptually formed by taking the circle of the sky with all required data, folding it on the East-West line, then folding the resulting semi-circle on the North-South line. Thus the four quadrants of the full circle, with their data, are superimposed over one another. In use it is necessary to select the curves and data appropriate to the quadrant addressed. As a maritime instrument, the quadrant could have been in use by Arab navigators in the Indian Ocean centuries before the Portuguese arrived, although at sea, stabilizing the plumb-bob on a rolling deck was a problem in obtaining an accurate sight. *NOTE: 35 degrees North Latitude seems to be the most central latitude bisecting the Islamic world. It passes through Morocco, Algeria, Tunisia, along the Southern Mediterranean passing through Crete and Cyprus, then into Syria, Iraq, Iran, Afghanistan, Pakistan and Northern India before extending on into China and points east. Although the latitude information constrains the use of the quadrant as an astronomical computer to the latitude for which the data is inscribed, it can be used anywhere in the world for measuring the altitude of the Sun, Moon and stars, the primary measurement required to determine one's latitude. The 90 degree scale inscribed at the bottom is for this function.

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