Sea in culture of barbarian invaders and knowledge of geography took a backward step. In the seventh century, Isidore of Seville produced a "wheel map" in which Asia, Africa and Europe were arranged like segments in an orange, separated by the "Mare Mediterranean", "Nilus" and "Tanais" and surrounded by "Oceanus". It was not until the fifteenth century that Ptolemy's maps were used again and Henry the Navigator of Portugal initiated ocean exploration and maritime research. Encouraged by him, Portuguese navigators explored, mapped and charted the west coast of Africa and the Eastern Atlantic and this knowledge prepared the way for the great voyages
United States presidential visits to Eastern Europe and Northern Asia United States presidential visits to Eastern Europe and Northern Asia Ten United States presidents have made presidential visits to Eastern Europe and Northern Asia. The first trip by an incumbent president to Eastern Europe was made by Franklin D. Roosevelt in 1945, to the Soviet Union, and was an offshoot of Allied diplomatic interactions during World War II. The first trip by an incumbent president to Northern Asia was made by Gerald Ford in 1974, also to the Soviet Union, and was an offshoot of U.S.–Soviet Détente during the Cold War. The first presidential visits to other Eastern European countries