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    Re: Ulugh Beg's sextant
    From: George Huxtable
    Date: 2006 Mar 24, 22:44 -0000

    Doug wrote-
    
    | Ok, I'm now reading a very good book dealing with observatories past and
    | present.
    |
    | There is an interesting chapter on the Samarkand Observatory in Uzbekistan
    | that Ulugh Beg built and used for astronomical observations in the 1400's.
    |
    | There is a crude description of the FAKHRI SEXTANT used by the Samarkand
    | Observatory that was used to measure the position of thousands of stars to
    | within a few seconds of arc. It has a radius of 120 ft.
    |
    | After a quick search on the web today for pictures or a better description
    | of this sextant I came up dry. There are a few pictures of parts of the
    | observatory but none that are identified as the Fakhri sextant.
    |
    | There is 1 picture of a section of a structure which has 1 vertical stone
    | wall on each side of a center groove and 2 stone tracks running in a
    | vertical arc between the 2 walls on either side of the center groove.
    |
    | I had pictured this "sextant" as kind of matching how Stonehenge is
    | constructed for celestial observations. But now I'm more interested than
    | ever to find what this "sextant" really looks like.
    |
    | Frank, because you're an astronomer, can you help or point me in the proper
    | direction to get a better understanding of what this looks like? Visually or
    | at least a better written description. Or can anyone else help out in this
    | quest?
    
    =========================
    
    Doug has asked an interesting question here. He is quite right in not expecting that instrument to
    resemble what we now think of as a sextant. It was called a sextant simply because its arc covered a
    sixth of a circle, 60 degrees.
    
    In the same way, the arc of a navigator's sextant covers 60 degrees, only in that case the doubling
    by reflection allows it to measure to 120 degrees.
    
    In Ulugh Beg's instrument, there was no such reflection, and no doubling. It was simply a sighting
    device, aligned North-South to the meridian, to observe altitudes of bodies as they crossed the
    meridian, as Fred has suggested. There were then no telescopes, and the instrument was made
    enormously large (40 metres
    radius!) in the hope of achieving great accuracy by naked-eye sighting, through a movable peep, with
    respect to some sort of pointer at the centre of the arc. It did a similar job to what a transit
    instrument would do, 350 years later.
    
    I've found a couple of books on my shelves with a bit of relevant information.
    
    The Cambridge illustrated history of astronomy (ed. Hoskin) of 1997 says that Ulugh Beg ascended to
    the throne in Samarkand in 1447, and was murdered in 1449, by which time his observatory had enjoyed
    three decades of existence. Construction of a three-story building took place in 1420. The major
    instrument was mounted out of doors, in a trench, between two marble walls aligned North-South and
    separated by some 20 inches. The instrument itself was a form of sextant (for measuring altitudes of
    celestial bodies over a range of one-sixth of a circle), its range being chosen so that the Sun,
    Moon, and other planets could all be observed. The radius of the sextant was over 130 feet, an
    illustration of Islamic astronomers' mistaken conviction that increased size would inevitably lead
    to increased accuracy.
    
    The great achievement of Ulugh Beg's observatory was a set of astronomical tables that included a
    catalogue of over 1,000 stars. Many of its star positions had been established by the Samarkand
    astronomers themselves, making this the one important star catalogue of the middle ages.
    
    I would add that calibration of that immense arc must have presented serious problems. That would
    limit the precision of any measured declinations. What puzzles me, however, is how they would get
    even a rough figure of right-ascension, in the absence of a timekeeper with any precision. How
    accurate could a water-clock be?
    
    I've found a bit more in "Astronomy before the telescope", ed. Walker, 1996. This refers (page 237)
    to "... instruments made by the best Asian astronomers, notably Ulugh Beg (1394-1449) in Samarqand,
    whose accuracy was between 5' and 10' of arc". That seems a far cry from the accuracy Doug quotes,
    of "a few seconds" of arc. The same book mentions that 300 years later, Jayasimha, (Jai Singh)
    constructed mammoth stone instruments, in imitation of those built by Ulugh Beg in Samarkand, whose
    substantial remains can still be studied in Delhi. And states that Ulugh Beg determined the
    obliquity, between the Earth's axis and the ecliptic, to be 23 deg 30' 17". And refers to his
    magnificent trigonometrical tables, which "display the values of sine and tangent to five
    sexagesimal digits for each minute of argument, and are generally accurate in the last digit".
    
    In the West at this time, however, astronomy still relied on astronomical observations that had been
    recorded by Ptolemy in 200 AD! It would be another 150 years before Tycho started proper
    measurement. The Muslims were way ahead.
    
    George
    
    ===========
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    George.
    
    
    

       
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