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:''“Snellius” redirects here. For the lunar crater named Snellius, see Snellius (crater).''
'Willebrord Snellius'^[1] (born 'Willebrord Snel van Royen'^[2]) (1580–October 30, 1626, Leiden) was a Dutch astronomer and mathematician, most famous for the law of refraction now known as Snell's law.
Snellius was born in Leiden, Holland. In 1613 he succeeded his father, Rudolph Snel van Royen (1546–1613) as professor of mathematics at the University of Leiden. In 1615 he planned and carried into practice a new method of finding the radius of the earth, by determining the distance of one point on its surface from the parallel of latitude of another, by means of triangulation. His work ''Eratosthenes Batavus'' ("The Dutch Eratosthenes"), published in 1617, describes the method and gives as the result of his operations between Alkmaar and Bergen op Zoom—two towns separated by one degree of the meridian—which he measured to be equal to 117,449 yards (107.395 km). The actual distance is approximately 111 km. 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 that is named after him in 1621. The lunar crater Snellius is also named after him.

In addition to the ''Eratosthenes Batavus'', he published ''Cyclometria sive 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 trigonometry (''Doctrina triangulorum'') authored by Snellius was published a year after his death.

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1. Willebrord Snellius at the Leiden Digital Family Tree.

2. Encarta Winkler Prins, Grote Oosthoek

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