While Columbus was sailing west the Portuguese were sailing south around Africa. They were the first westerners to cross the equator which provided a special problem. As they crossed into the Southern Hemisphere they could no longer see the North Star to tell the latitude of where they were. Since they were the first to have this problem they had to create a new way to tell their latitude. They used the sun by measuring the suns height at noon. Unfortunately clocks were not yet perfected for telling time at sea (which meant the longitude still could not be determined with accuracy). So they had to take measurements throughout the day to determine at which point the sun was the highest. They also had to create tables to take into account the fact the the sun did not make a perfect path in the sky throughout the year.

(Book – The Last Crusade: The Epic Voyages of Vasco Da Gama – Nigel Cliff)