Willebrord Snell (1580 – October 30, 1626), also known as Snell van Royen or Snellius, was a Dutch astronomer and mathematician, most famous for the law of refraction now known as Snell's law.

Snell was born at Leiden. In 1613 he succeeded his father Rudolph Snell (1546—1613) as professor of mathematics in 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 yds (107.395 km). The actual distance is approximately 111 km. Snell 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. He died at Leiden on October 30, 1626.

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) by him was published a year after his death.

This article incorporates text from the public domain 1911 Encyclopædia Britannica.

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