Ulugh Beg's Buried Sextant
The Islamic Golden AgeMarble Under a Hillside
In 1908 a Russian archaeologist in Samarkand, Vasily Vyatkin, was working through a basket of seventeenth-century waqf deeds — endowment records of who owned which plot of land. One of them described a parcel lying beside "the observatory". Muslim writers had mentioned Ulugh Beg's observatory for centuries, but nobody could say where it had stood. Vyatkin took the deed's bearings to a low hill north of the old city, dug, and struck marble: two parallel walls faced in polished stone, running down into the earth in a trench roughly two metres wide. Between them ran a curved graduated scale, the surviving stretch covering solar altitudes from 57° to 80°. He had found the buried lower arc of the largest instrument of its kind ever built.
The Prince Who Preferred Instruments
Its patron was born in March 1394 at Sultaniyya in Persia: Muhammad Taraghay, called Ulugh Beg, "great prince", and grandson of Timur, whose armies had emptied cities from Delhi to Damascus. He governed Samarkand from his teens and held Transoxiana for some forty years — long enough that his reputation as a mediocre ruler is well earned — and was sultan for only the last two and a half. What he was genuinely good at was recruitment. He founded a madrasa on the Registan in 1417 where mathematics and astronomy were core subjects, and drew in perhaps sixty scholars, among them Jamshid al-Kashi from Kashan and Qadizada al-Rumi from Bursa. Al-Kashi, writing home to his father, rated his patron's own command of the mathematics unusually high.
A Four-Hundred-Year-Old Design
Building began around 1420: a cylindrical structure some 46 metres across and three storeys high, wrapped around one enormous instrument. It was a meridian arc of roughly 40 metres radius, so large that a single degree of sky occupied about 70 centimetres of marble, ruled to minutes of arc and by some accounts finer still. Rather than raise a tower, the astronomers sank the lower half into a trench cut into the hillside. The design was not theirs. It is called a Fakhri sextant after Fakhr al-Dawla, a Buyid ruler in northern Iran who paid for al-Khujandi to build one at Rayy around 994. Samarkand inherited the instrument, Ptolemy's Almagest in Arabic, al-Sufi's star descriptions, and the working habits of Maragha.
A Thousand and Eighteen Stars
The result, given an epoch of 1437, was the Zij-i Sultani: tables, a treatise, and a catalogue of 1,018 stars. Twenty-seven of them lie too far south ever to rise at Samarkand's latitude and were copied from al-Sufi's catalogue of 964; the brightnesses and star descriptions were inherited too. The positions were measured afresh, and specialists say so without hedging: it is the only star catalogue built mainly on new observation between Ptolemy and Tycho Brahe. Tested against modern Hipparcos data its scatter is about 22 arcminutes against Ptolemy's 27 — better, not transformed — while gross errors fall from roughly 2% of entries to 0.1%. Two different year lengths circulate under his name: the tropical year of 365 days 5 hours 49 minutes 15 seconds, out by under half a minute, and the sidereal year of 365 days 6 hours 10 minutes 8 seconds, nearer a minute long. Both are remarkable for naked-eye work; neither is the flawless figure sometimes claimed.
Beheaded, Buried, Still in Print
In 1447 his father died and Ulugh Beg became sultan; two years later his son Abd al-Latif defeated him in the field. Given leave to set out for Mecca, he did not know a court had already condemned him. He was killed near Samarkand on 27 October 1449, though some sources place it two days earlier. When a Soviet team opened the Gur-e Amir tombs in June 1941, they found his head laid beside his body. The observatory, though, was not levelled by a mob that year: Babur, in Samarkand in 1497, describes it still standing three storeys high. Ruin came slowly, across the sixteenth century. Ali Qushji carried the method to Ottoman Istanbul; the tables were printed at Oxford in 1665, and were still being checked against the sky in eighteenth-century Jaipur.
Key Dates to Remember
- c. 1420 — Construction of the Samarkand observatory begins on a hill north of the city, with Jamshid al-Kashi as its first director.
- 1437 — Epoch of the Zij-i Sultani, whose catalogue gives positions for 1,018 stars — all but 27 of them newly measured.
- 27 October 1449 — Ulugh Beg is beheaded near Samarkand after a court convened by his son Abd al-Latif condemns him; a few sources date it two days earlier.
- 1665 — Thomas Hyde prints the star catalogue at Oxford in Persian with a Latin translation, more than two centuries after it was compiled.
- 1908 — Vasily Vyatkin, guided by a seventeenth-century endowment deed, excavates the buried lower arc of the great meridian instrument.