Pedro Nunes (Portugal) b- 1502
Nunes is best known for being the first to approach navigation and cartography with mathematical tools. Among other accomplishments, he was the first to propose the idea of a loxodrome (a rhumb line), and was the inventor of several measuring devices, including the nonius (from which the Vernier scale was derived), named after his Latin surname
Abraham Zacuto (Spain)b-1452
His astrolabe of copper, astronomical tables, and maritime charts played an important role in the Spanish and Portuguese voyages of discovery, being used by both Vasco Da Gama and Christopher Columbus
Martin Behaim (Portugal): b-1459
a German textile merchant and cartographer. He served John II of Portugal as an adviser in matters of navigation and participated in a voyage to West Africa. He is now best known for his Erdapfel, the world’s oldest known globe, which he produced for the Imperial City of Nuremberg in 1492.
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The Wikipedia entry for Zacuto has an error:
Zacuto’s Almanach perpetuum (or Biur luḥot ) revolutionised ocean navigation. Prior to the Almanach , navigators seeking to determine their position on the high seas had to correct for “compass error” (the deviation of the magnetic north from the true north) by recourse to the quadrant and the Pole Star.
Navigation should be much easier and more affordable thanks to Harbor Freight. Only $14.97. 
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This is not correct.
navigators seeking to determine their position on the high seas had to correct for “compass error”
The almanac Zacuto created was a table of the sun’s declination. Once ship’s navigators lost sight of the north star they used the sun altitude to determine latitude.
The apparent sun moves between the Tropic of Cancer and the Tropic of Capricorn, the tropics.
“Tropics” is from the Greek meaning 'a turning" as that is where the sun direction appears to turn at the solstice.
“Troposphere” has the same root.
Conquering the Pacific: An Unknown Mariner and the Final Great Voyage of the Age of Discovery
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