A major advance was the introduction of the caravel in the mid-15th century, a small ship able to sail windward more than any other in Europe at the time. Evolved from fishing ships designs, they were the first that could leave the coastal cabotage navigation and sail safely on the open Atlantic. For celestial navigation the Portuguese used the Ephemerides, which experienced a remarkable diffusion in the 15th century. These were astronomical charts plotting the location of the stars over a distinct period of time. Published in 1496 by the Jewish astronomer, astrologer, and mathematician Abraham Zacuto, the Almanach Perpetuum included some of these tables for the movements of stars. These tables revolutionized navigation, allowing the calculation of latitude. Exact longitude, however, remained elusive, and mariners struggled to determine it for centuries. Using the caravel, systematic exploration continued ever more southerly, advancing on average one degree a year. Senegal and Cape Verde Peninsula were reached in 1445 and in 1446, Álvaro Fernandes pushed on almost as far as present-day Sierra Leone.