Willebrord Snellius (June 13, 1580 - October 30, 1626) was a Dutch astronomer and mathematician. He was a 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. He also produced a new method for calculating pi, he first such improvement since ancient times. He rediscovered the law of refraction in 1621. It is now known that this law was first discovered by Ibn Sahl in 984. The same law was also investigated by Ptolemy, but due to lack of adequate mathematical instruments (trigonometric functions) their results were saved as tables, not functions. Snell's law (AKA Snell-Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass. He died in 1626 at the age of 66.