Ulugh Beg Observatory

A ruler who observed

Ulugh Beg (1394–1449) governed Samarkand for his father Shah Rukh and gathered mathematicians and astronomers around a madrasa he founded in the city. Around 1428 he completed an observatory on a hill north-east of the city, a circular building of three storeys that held a giant instrument. Jamshid al-Kashi, who wrote of the building, and Qadi Zada al-Rumi worked there, and after al-Kashi's death in about 1429 Ali Qushji continued the work.

The programme produced the Zij-i Sultani, completed in 1437: a table of planetary motions and a catalogue of 1,018 stars with positions measured afresh rather than copied from Ptolemy and al-Sufi. It was widely copied, and a Latin edition by Thomas Hyde appeared in Oxford in 1665.

Scale as a technology

In the age before the telescope, the way to fine angles was a larger instrument. The Fakhri sextant, a meridian arc of about 40 metres radius, was set in a trench cut into the hill, so that only the upper part rose above ground. A bigger arc meant each degree was a longer length that could be divided finely, and a fixed, buried arc could not warp or shift as a free-standing frame might.

The results were remarkable. The sidereal year was given as 365 days 6 hours 10 minutes, and the tropical year as 365 days 5 hours 49 minutes 15 seconds, within about half a minute of the true value.

Loss and rediscovery

Ulugh Beg was killed in 1449, and the observatory was destroyed soon after. The site was lost until 1908, when the archaeologist Vassily Vyatkin located the base of the arc. A museum opened on the site in 1970. Vyatkin's find, a trench cut into the rock, is among the clearest surviving traces of a pre-telescopic instrument.

A precise catalogue, a short life

Ulugh Beg's catalogue gave positions for about a thousand stars with an accuracy of a few minutes of arc, better than most earlier lists, and was copied across Asia and Europe. The observatory itself lasted only about two decades, and after the prince's murder in 1449 the work was dispersed by his students. Its scientific life continued through the tables and through Ali Qushji, who carried a copy to Constantinople. The pattern recurs in the atlas: an institution is fragile, while its data, if well published, travel. The record of the observatory's building is also archaeological, since the arc had to be rediscovered from a trench in the rock.

Instruments and technologies

• Fakhri sextant A meridian arc of about 40 metres radius, partly underground, with marble-faced walls graduated for solar and stellar meridian observations.
• Circular building A three-storey structure housing the arc and supporting observers at several levels around it.

The people

• Ulugh Beg Timurid ruler and astronomer who founded and led the observatory.
• Jamshid al-Kashi Mathematician and astronomer who worked at the observatory and described it.
• Qadi Zada al-Rumi Mathematician and teacher at Ulugh Beg's madrasa and observatory.
• Ali Qushji Astronomer who completed the work and carried the tables abroad.

Timeline

• 1428 Observatory completed in about 1428 (sources differ on exact dates).
• 1437 The Zij-i Sultani tables and star catalogue are published.
• 1449 Ulugh Beg is assassinated and the observatory is destroyed.
• 1908 Archaeologist Vassily Vyatkin rediscovers the site.
• 1970 The Ulugh Beg Observatory Museum is built.

Key lessons for observation today

• Precision came from scale and stability The arc was big and buried. Observation quality still depends on the stability of the mount and the long baseline, in a mast, a pier or a seismic vault.
• Re-measure, do not only copy The catalogue replaced inherited values with measured ones. Modern calibration against a traceable reference does the same for every instrument.
• Publish in a form others can reuse The Zij-i Sultani travelled because it was a table anyone could use. Data that follow a common structure and units travel the same way.

Sources

• Wikipedia, Ulugh Beg Observatory
• MacTutor History of Mathematics, Ulugh Beg
• Wikipedia, Jamshid al-Kashi