Willebrord Snell was the Dutch mathematician who developed the triangulation method of measuring distances. He first began determining distances using this method in 1615, using church spires and roof tops for reference points. He soon began to extend this technique, estimating distances between towns and eventually estimating the radius of the Earth quite accurately. He is best remembered for Snell's Law (1621) which explains the relationship between the angles of incidence and refraction of a ray of light crossing the boundary between two media. This states that the ratio of the sines of the angles of incidence and refraction of a ray is a constant, known as the refractive index of the two media.
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