tend to blow from colder regions to warmer ones. Pythagoras was perhaps the first to propose a spherical world, arguing that the sphere was the most perfect form. This idea was embraced by Plato, and Aristotle presented empirical evidence to verify this. He noted that the Earth's shadow during a lunar eclipse is curved from any angle (near the horizon or high in the sky), and also that stars increase in height as one moves north. Eudoxus of Cnidus used the idea of a sphere to explain how the sun created differing climatic zones based on latitude. This led the Greeks to believe in a division of the world into five regions. At each of the poles was an uncharitably cold region. While extrapolating from the heat of the Sahara it was deduced that the area around the equator was unbearably hot. Between these extreme regions both the northern and southern hemispheres had a temperate belt suitable for human habitation. Hellenistic period These theories clashed with the evidence of explorers, however, Hanno the Navigator had traveled as far south as Sierra Leone, and Egyptian Pharaoh Necho II of Africa is related by Herodotus and others as having commissioned a successful circumnavigation of Africa by Phoenician sailors. While they were sailing west around the Southern tip of Africa, it was found that the sun was to their right (the north). This is thought to have been a key trigger in the realization that the earth is spherical, in the classical world. In the 4th century BC the Greek explorer Pytheas traveled through northeast Europe, and circled the British Isles. He found that the region was considerably more habitable than theory expected, but his discoveries were largely dismissed by his contemporaries because of this. Conquerors also carried out exploration, for example, Caesar's invasions of Britain and Germany, expeditions/invasions sent by Augustus to Arabia Felix and Ethiopia (Res Gestae 26), and perhaps the greatest Ancient Greek explorer of all, Alexander the Great, who deliberately set out to learn more about the east through his military expeditions and so took a large number of geographers and writers with his army who recorded their observations as they moved east. The ancient Greeks divided the world into three continents, Europe, Asia, and Libya (Africa). The Hellespont formed the border between Europe and Asia. The border between Asia and Libya was generally considered to be the Nile