Pedro Nuñes Salaciense (1492-1577 or 1502-1578) was born in in Alcácer do Sal. He studied at the University of Salamanca where he studied mathematics and medicine. In 1547 Pedro Nuñes was appointed Chief Royal Cosmographer of Portugal. Among his contributions to mathematics and navigation two stand out.

     Perhaps his greatest discovery is the “rhumb line,” or loxodrom. This represents the course a ship traces when it holds a steady compass course. In 1569 Gerard Mercator devised a map projection in which a straight line between any two points would describe the loxodrom between them, the famous “Mercator’s projection.” This was a great advance in navigation because it made it possible for a navigator to determine his position and then draw a straight line between that position and his destination which would indicate the compass course to be followed. This course is not be the shortest route (a great circle route) but it is a close approximation of it.
     Nuñes second contribution was the invention of the nonius, which made it possible to measure the fractional parts of a degree. He described it as:

drawing on the face of a quadrant for measuring angles 45 concentric arcs, one of which was divided into 90 equal parts or degrees, and the remainder into 89, 88, 87, 86, etc., successively, the last being divided into 46 equal parts. When the index did not exactly cut one of the divisions of the arc of degrees, it passed through or near to one of the divisions of one or other of the other arcs; and by noting the place of that division the fractional parts of a degree were calculated.  

     Applied to the astrolabe this allowed a navigator to be much more accurate in his observations of latitudes and calculations of courses. A century later Pierre Vernier invented the movable part that was added to measuring instruments of many kinds in use today (slide sticks, calipers, etc.).
     The stamps were issued to celebrate the 400th anniversary of the death of Pedro Nuñes. They show the nonius and a diagram from Nuñes’ Tratado da Rumapao do Giobo (Treatise on the Way to Delineate the Globe for the Art of Sailing) (about 1544).