Gemma Frisius (1508–1555)

Dutch mathematician, astronomer, and geographer. He is best known for improving mapmaking and for showing (before it was feasible to carry out) how to calculate longitude using portable clocks. He also wrote a mathematics textbook that was widely used in the 17th century, and was well known as a maker of globes, astrolabes, and other mathematical and astronomical instruments.

Born at Dockum, East Friesland, he became a pupil of Peter Apian and was educated at the university of Louvain, where he was appointed professor of medicine in 1541. In his Libellus de locorum describendorum ratione/Little Book on a Method for Delineating Places, incorporated in his 1533 edition of Apian's Cosmographia, Frisius published the first clear description of how maps could be constructed more accurately using triangulation.

Less immediate in its application (on account of the lack of sufficiently reliable timepieces) was his proposal in De principiis astronomiae et cosmographiae/The Principle of Astronomy and Cosmography (1530) that longitude at sea and elsewhere could be determined with the aid of portable clocks. His mathematics text book, Arithmeticae practicae methodus facilis was published in 1540 and went through 59 editions