(PORTRAIT) Frisius,  Gemma.  Gemma Frisius, Doccomiensis, Medicus et Mathematicus,  ca. 1725.  Fine condition.  Very fine head-and-shoulders portrait of the great mathematician, printed in the early 18th century. UNCOMMON.  $150

From the St. Andrews Site: (Article by: J J O'Connor and E F Robertson) Regnier Gemma Frisius was a native of Friesland, a coastal province in northern Netherlands, which explains his nickname of Frisius. He was educated at the University of Louvain, receiving a medical degree. He became the leading theoretical mathematician in the

Low Countries and became professor of medicine and mathematics at the University of Louvain. He was also a practicing physician in Louvain. Gemma Frisius applied his mathematical expertise to geography, astronomy and map making. In Louvain he cooperated with his student Gerardus Mercator and with the engraver and goldsmith Gaspar à Myrica in the construction of maps, globes and astronomical instruments. In 1530 Gemma Frisius wrote De Principiis Astronomiae Cosmographicae which he published in Louvain. Chapter 19 of this work describes, for the first time, how the longitude of a place may be found using a clock to determine the difference in local and absolute times. He says (see [5]):- ... it is with the help of these clocks and the following methods that longitude is found. ... observe exactly the time at the place from which we are making our journey. ... When we have completed a journey ... wait until the hand of our clock exactly touches the point of an hour and, at the same moment by means of an astrolabe... find out the time of the place we now find ourselves. ... In this way I would be able to find the longitude of places, even if I was dragged off unawares across a thousand miles. Aware of the difficulties of keeping exact time he writes:- ... it must be a very finely made clock which does not vary with change of air. In a second edition of the work three years later he added some notes about finding the longitude at sea, the first time anyone had attacked the problem. It is worth noting that although there were many methods of finding longitude proposed in the 250 years following Gemma Frisius's work, ultimately the methods he proposed were to become the solution to finding the longitude at sea. He described the theory of trigonometric surveying in the 1533 edition of De Principiis Astronomiae Cosmographicae and in particular he was the first to triangulation as a method of accurately locating places. In 1534 he wrote Tractatus de Annulo Astronomicae. His work on applying trigonometric methods to astronomical problems led him to note correctly that comets displayed a proper motion against the background stars. In 1535-36 Gemma Frisius cooperated with Myrica and Gerardus Mercator in constructing a terrestrial globe and, in 1537, they constructed a celestial globe. Gemma Frisius's work on astronomical instruments was described in several of his books. For example in De Radio Astronomico (1545) he describes his work constructing a cross-staff about 1.5 metres long with one cross piece about 3/4 of a metre in length. It had brass sighting vanes and a sliding vane. He also invented a new astrolabe which he described in De Astrolabio which was published in 1556, after his death. Gemma Frisius made many astronomical observations. In particular he recorded comets in July 1533, January 1538 and 30 April 1539. Some of these comet observations are described in works by his son, Cornelius Gemma Frisius, who was born in 1533 and went on to become professor of medicine and astronomy at Louvain.