Mind-Benders of Islamic History
Pi to sixteen decimals, by hand, in 1424
The Persian mathematician al-Kashi (c. 1380–1429), working at Samarkand, computed pi to an accuracy equivalent to sixteen decimal places in his Treatise on the Circumference (1424). It was the most precise value known anywhere, and it held the record for roughly 170 years, until Ludolph van Ceulen surpassed it in 1596.
How we know
Jamshīd al-Kāshī (b. c.1380 Kashan – d. 22 June 1429 Samarkand); Risāla al-muḥīṭiyya (Treatise on the Circumference), July 1424, π to ~16 decimal digits via a 3×2^28-sided polygon; unsurpassed until van Ceulen's 20 digits in 1596 (172 yrs). Refs: Britannica, MacTutor (St Andrews), Wikipedia.
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