Pedro Nuñes Salaciense (14921577 or 15021578) 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). 
The souvenir sheet from Portugal was issued
in 2002 to celebrate the 500th anniversary of the birth of Pedro
Nuñes. The first stamp shows loxodromic lines on the globe, the middle
stamp is a fictional portrait of Nuñes, and the last stamp shows the
nonius.

