Willebrord Snellius

Willebrord Snellius, born Willebrord Snel van Royen in Leiden on 13 June 1580 and died on 30 October 1626, was a Dutch astronomer and mathematician. He was the eldest of three children of the mathematician Rudolph Snel van Royen, studied at the University of Leiden under the influence of Ludolph van Ceulen, and succeeded his father as professor of mathematics there after 1613. He is best known for the law of refraction of light now called Snell law, for his pioneering survey work known as Snellius triangulation, and for the Snellius-Pothenot problem of finding an unknown point from known ones in plane trigonometry. In 1615 he used triangulation for a large-scale arc measurement to determine the circumference of the Earth, and described it in a work published in 1617. The refraction law had been found earlier by the Persian scientist Ibn Sahl around 984.