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Willebrord Snellius[1][2] (born Willebrord Snel van Royen)[3] (13 June 1580[4] – 30 October 1626) was a Dutch astronomer and mathematician, commonly known as Snell. His name is usually associated with the law of refraction of light known as Snell's law.[5]
In 1615, Snellius, after the work of EratosthenesinPtolemaic Egypt in the 3rd century BC, probably was the first to try to do a large-scale experiment to measure the circumference of the earth using triangulation.[7][8] 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[9] 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.
The actual distance between the two church spires in Alkmaar and Breda, two places nearly on the same meridian,[11] is 116.1 kilometers.[12] 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 Sint-Laurenskerk 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 discovered the law of refraction in 1621.[13]
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.[6]
^For a reconstruction of this discovery see Hentschel 2001. 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 (i.e. trigonometric functions) their results were saved as tables, not functions.
Klaus Hentschel: Das Brechungsgesetz in der Fassung von Snellius. Rekonstruktion seines Entdeckungspfades und eine Übersetzung seines lateinischen Manuskriptes sowie ergänzender Dokumente. Archive for History of Exact Sciences 55,4 (2001), doi:10.1007/s004070000026.
This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Snell, Willebrord". Encyclopædia Britannica. Vol. 25 (11th ed.). Cambridge University Press. p. 293.
