Willebrord Snellius  

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Willebrord Snel van Royen (1580–1626)


Born  13 June 1580 "Leiden, "Dutch Republic 
Died  30 October 1626 "Leiden, "Dutch Republic 
(aged 46)
Nationality  "Dutch 
Alma mater  "University of Leiden 
Known for  "Snell's law 
Scientific career  
Fields  "Astronomer and "mathematician 
Institutions  "University of Leiden 
Academic advisors  "Ludolph van Ceulen "Rudolph Snellius 
Notable students  "Jacobus Golius 
Willebrord Snellius^{[1]}^{[2]} (born Willebrord Snel van Royen)^{[3]} (13 June 1580^{[4]} – 30 October 1626) was a "Dutch "astronomer and "mathematician, known in the Englishspeaking world as Snell. In the west, especially the English speaking countries, his name is attached to the law of refraction of light. (It is now known that this law was already known to "Ibn Sahl in 984. The same law was also investigated by "Ptolemy and in the Middle Ages by "Witelo, but due to lack of adequate "mathematical instruments (trigonometric functions) their results were saved as tables, not functions.)
The "lunar crater "Snellius is named after Willebrord Snellius. The Royal Netherlands Navy has named three survey ships after Snellius, including a "currentlyserving vessel.
Willebrord Snellius was born in "Leiden, "Netherlands. In 1613 he succeeded his father, "Rudolph Snel van Royen (1546–1613) as professor of "mathematics at the "University of Leiden.
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In 1615 Snellius, after the work of "Eratosthenes in "Ptolemaic Egypt in the 3rd century BC, probably was the first to try to do a largescale experiment to measure the "circumference of the earth using "triangulation. 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 ("regenten). In his work The terrae Ambitus vera quantitate (1617) under the author's name ("The Dutch Eratosthenes") Snellius describes the methods he used. He came up with an estimate of 28,500 Rhineland rods – in modern units 107.37 "km^{[5]} 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 high 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, with which he could accurately measure angles in tenths of degrees. 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's high points were nearly all "church spires. There were hardly any other 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: "Alkmaar : "St. Laurenskerk; "Haarlem : "SintBavokerk; "Leiden : a then new part (built in 1599) of the "City walls;^{[6]} "The Hague : "SintJacobskerk; "Amsterdam : "Oude Kerk; "Utrecht : "Cathedral of Utrecht; "Zaltbommel : SintMaartenskerk; "Gouda : "Sint Janskerk; "Oudewater : SintMichaelskerk; "Rotterdam : "SintLaurenskerk; "Dordrecht : "Grote Kerk; "Willemstad : Koepelkerk; "BergenopZoom : "Gertrudiskerk; "Breda : "Grote Kerk
The actual distance between the two church spires in "Alkmaar and "Breda, two places nearly on the same "meridian,^{[7]} is 116.1 kilometers.^{[8]} The difference in latitude between Alkmaar (52° 37' 57" N) and Breda (51° 35' 20" N) 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 SintLaurenskerk in Alkmaar and the Grote Kerk in Breda.
Snellius was also a distinguished 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 (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.
Snellius died at Leiden on October 1626, at the age of 46 from an illness diagnosed as "colic.^{[9]} His grave can be seen in the "Pieterskerk, Leiden, which has since been "deconsecrated.