Willebrord Snellius (born Willebrord Snel van Royen, also Willebrord van Roijen Snell (13 June 158030 October 1626), commonly known simply as Snellius and Snell, was a Dutch astronomer and mathematician.
Snell is best known for discovering the law of refraction of light, now known as Snell's law, his pioneering work in survey known as Snellius's triangulation, and the Snellius–Pothenot problem, a means in planar trigonometry of finding an unknown point from known ones.
Despite being commonly attributed to Snell, the law of refraction was first discovered by the Persian scientist Ibn Sahl around 984 AD.
Early life
Willebrord Snellius was born Willebrord Snel van Royen on 13 June 1580 in Leiden, in the Dutch Republic. He was the eldest of three children of the mathematician Rudolph Snel van Royen, a professor of mathematics at the University of Leiden. His mother, Machteld Cornelisdochter, came from a leading family in Oudewater. He was named after his paternal grandfather.
In his work The terrae Ambitus vera quantitate (1617) under the author's name ("The Dutch Eratosthenes"), Snellius describes achieving his result by calculating the distances between a number of high points in the plain west and southwest of the Netherlands using triangulation. By necessity Snellius's high points were nearly all church spires, virtually the only tall buildings at that time in the west of the Netherlands. More or less ordered from north to south and/or in successive order of measuring, Snellius used a network of fourteen measure points to make a total of 53 triangulation measurements.
These cities were: Alkmaar: St. Laurenskerk; Haarlem: Sint-Bavokerk; Leiden: a then new part (built in 1599) of the city walls; The Hague: Sint-Jacobskerk; Amsterdam: Oude Kerk; Utrecht: Cathedral of Utrecht; Zaltbommel: ; Gouda: Sint Janskerk; Oudewater: ; Rotterdam: Sint-Laurenskerk; Dordrecht: Grote Kerk; Willemstad: ; Bergen-op-Zoom: Gertrudiskerk; Breda: Grote Kerk.
Snellius was helped in measuring by two of his students, the Austrian barons Erasmus and Casparus Sterrenberg. In several cities he also received support of friends among the civic leaders (regenten).
In order to carry out these measurements accurately Snellius had a large quadrant built, with which he could accurately measure angles in tenths of degrees. This instrument can still be seen in the Museum Boerhaave in Leiden. In his calculations Snellius made use of a solution for what is now called the Snellius–Pothenot problem.
He came up with an estimate of 28,500 Rhineland rods – in modern units 107.37 km for one degree of latitude. 360 times 107.37 then gives a circumference of the Earth of 38,653 km. The actual circumference is 40,075 kilometers, making Snellius' estimate 3.5% low.
Mathematics and physics
Snellius was also a distinguished mathematician, producing a new method for calculating π—the first such improvement since ancient times. He discovered the law of refraction in 1621.
Other works
thumb|Cyclometricus, 1621
In addition to the Eratosthenes Batavus, he published (1621), and Tiphys Batavus (1624). He also edited Coeli et siderum in eo errantium observationes Hassiacae (1618), containing the astronomical observations of Landgrave William IV of Hesse. A work on trigonometry (Doctrina triangulorum) authored by Snellius was published a year after his death.
Death
thumb|Coeli et siderum in eo errantium observationes Hassicae, 1618
Snellius died in Leiden on 30 October 1626, aged 46, from an illness diagnosed as colic. He was buried in the Pieterskerk, Leiden.