Gemma Frisius

Gemma Frisius, born Jemme Reinerszoon, who lived from 1508 to 1555, was a Dutch physician, mathematician, cartographer, philosopher and instrument maker. Born in Dokkum in Friesland to poor parents who died when he was young, he began his studies at the Old University of Leuven in about 1525 and qualified there as a doctor of medicine in 1536, remaining on its faculty for the rest of his life and teaching medicine, mathematics, astronomy and geography. He helped produce globes and mathematical instruments noted for their accuracy, notably a terrestrial globe in 1536 and a celestial globe in 1537, with Gerardus Mercator as his pupil and engraver. In 1533 he described the method of triangulation still used in surveying, and in about 1553 he was the first to describe how an accurate clock could be used to determine longitude. Gemma's rings, an astronomical instrument, are named after him, and his students included Mercator, Johannes Stadius, John Dee, Andreas Vesalius and Rembert Dodoens.