Circa 1776


Signed in Arabic along one of the sight vanes "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.). One side of the brass plate with 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 current-day 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 of the sun, the constellations and bright stars, as well as the time of day or night, and other solutions for problems in practical astronomy. Reverse with a sine quadrant, which 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 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".
Condition: Age appropriate surface wear, wood with a split.

The absence of a condition report does not imply an object is free of defects. All items may have normal signs of age and wear commensurate with their age; these issues will likely not be mentioned in the condition report. Please contact Eldred's before the auction with any condition questions. Questions about condition will not be answered after purchase. Condition reports are provided as a courtesy, and we are not responsible for any errors or omissions. Important note on frames: Frames are not guaranteed to be in the same condition as they are in the item photograph. Due to handling and shipping, many frames, especially antique ones, are prone to losses. If you have questions about the condition of a frame, please contact us prior to the auction. 

  • 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 with the later bubble sextant), this instrument 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.
    The 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 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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March 1, 2023 9:30 AM EST
East Dennis, MA, US


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