Caption: The Fakhri mural sextant of the Samarkand Observatory, perhaps the greatest of Medieval Islamic observatories.

The Samarkand Observatory was founded by the Samarkand ruler Ulugh Beg (1394---1449; reigned 1447--1449)---who was a grandson of Timur Lang (1336--1405), AKA Tamerlane.

Fakhri sextant is aligned with the meridian (the great circle that runs from due north to due south through the zenith). The aperature looks due south??? I would guess.

By observing an astronomical object transiting the meridian through the aperature from a point along the arc, one can determine its declination (δ) and (with the help of a clock) its right ascension (RA).

Only astronomical objects that transit south of zenith can be observed with the Fakhri sextant. The declination is given by

    δ = latitude - θ

       where θ is the angle of the
       astronomical object south
       of zenith which is also the angle 
       measured from the bottom of the arc.  

The sidereal time (measured in sidereal hours, sidereal minutes, and sidereal seconds) between the transit of vernal equinox and the transit of the astronomical object is exactly equal to the right ascension (measured in angle hours, angle minutes, and angle seconds).

Because of its large size, very accurate/precise measurements are possible. However, bad technique can negate the effect of large size.

In the event, the Samarkand Observatory did produce new measurements of exceptional quality (see Wikipedia: Zij-i Sultani) that alas proved to have little effect on the development of astronomy---but one must keep trying.

Credit/Permission: © David Revelj (AKA User:Revelj), 2005 / Creative Commons CC BY-SA 3.0.
Image link: Wikimedia Commons: File:Ulughbek observatory interior.jpg.
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