Willebrord Snellius

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. After his father's death in 1613, Snellius succeeded him as professor of mathematics at the University of Leiden. == Surveying ==

of Snellius, Museum Boerhaave, Leiden on Snellius's house in Leiden In 1615, Snellius, became the first known surveyor since Eratosthenes in 3rd century BC Ptolemaic Egypt to use triangulation to make a large-scale arc measurement to determine the Earth's circumference. 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 ==

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 ==

Snellius died in Leiden on 30 October 1626, aged about 46, from an illness diagnosed as colic. He was buried in the Pieterskerk, Leiden. == Legacy ==

Snellius Glacier in Antarctica is named after Willebrord Snellius. The lunar crater Snellius is named after Willebrord Snellius. The Royal Netherlands Navy has named three survey ships after Snellius, including a currently-serving vessel. == Works ==