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Willebrord Snellius
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Willebrord Snellius was a Dutch astronomer and mathematician, known in the English-speaking world as Snell. The same law was also investigated by Ptolemy and in the Middle Ages by Witelo, the lunar crater Snellius is named after Willebrord Snellius. The Royal Netherlands Navy has named three ships after Snellius, including a currently-serving vessel. Willebrord Snellius was born in Leiden, Netherlands, in 1613 he succeeded his father, Rudolph Snel van Royen as professor of mathematics at the University of Leiden. He was helped in his measurements by two of his students, the Austrian barons Erasmus and Casparus Sterrenberg, in several cities he also received support of friends among the city leaders. In his work The terrae Ambitus vera quantitate under the authors name Snellius describes the methods he used and 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, so Snellius underestimated the circumference of the earth by 3. 5%. Snellius came to his result by calculating the distances between a number of points in the plain west and southwest of the Netherlands using triangulation. In order to carry out these measurements accurately Snellius had a large quadrant built and this quadrant can still be seen in the Museum Boerhaave in Leiden. In a network of fourteen cities a total of 53 triangulation measurements were made, in his calculations Snellius made use of a solution for what is now called the Snellius–Pothenot problem. By necessity Snellius his high points were nearly all church spires, there were hardly any other tall buildings in Snellius 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 points, Alkmaar. The difference in latitude between Alkmaar and Breda is 1.0436 degree, assuming Snellius corrected for this he must have calculated a distance of 107.37 *1.0436 =112.05 kilometers between the Sint-Laurenskerk in Alkmaar and the Grote Kerk in Breda. Snellius was also a mathematician, producing a new method for calculating π—the first such improvement since ancient times. He rediscovered the law of refraction in 1621, in addition to the Eratosthenes Batavus, he published Cyclometricus, de circuli dimensione, and Tiphys Batavus. He also edited Coeli et siderum in eo errantium observationes Hassiacae, a work on trigonometry authored by Snellius was published a year after his death. Snellius died at Leiden on October 1626, at the age of 46 from an illness diagnosed as colic, oConnor, John J. Robertson, Edmund F. Willebrord van Royen Snell, MacTutor History of Mathematics archive, University of St Andrews. This article incorporates text from a now in the public domain, Chisholm, Hugh, ed. Snell