Pytheas of Massalia
Around the year 330 BC, it is believed that a Greek explorer--Pytheas--from the colony of Massalia (modern Marseilles) sailed to the ends of what was the known world for the Greeks--to the British Isles, and possibly to Iceland.
It was known not only to the Greeks but to the Phonecians that the world was round (records going back to the sixth century BC), and that the Phonecians traded for tin in Cornwall. Later, it was the Greeks who needed tin--kasseritos--for bronze, and it was believed by the historian Polybius that Pytheas was sent to the "Kasiterides Islands" (British Isles) to find if he could obtain this tin.
In the process, Pytheas sailed not only to Britain--"Pretanike"--but also to the Orkneys (Orcadia), Ireland (Ivernia), and finally "a congealed sea" and lands where the sun shown for only three hours a day. He set foot on these islands, describing the inhabinates as farmers and fierce warriors in horse-drawn chariots (much like the later Irish sagas). Moreover, he called these people "Keltoi".1
More incredible is his description of the Arctic:
He reported sailing six days northwest towards Iceland (Thule) where he encountered dense fog described as so thick and eerily quiet that the ship and the sea seemed suspended in a void. He recorded the presence of water and slush ice that "binds all together, and can be traveled neither on foot nor by boat". This condition would not permit him to go further and forced him to turn back.
Not until the beginning of the 20th century and to Arctic explorers like Fridtjof Nansen and Vilhjalmur Stefansson were we able to explain and confirm his descriptions of the many strange and fascinating accounts he had experienced including the strange mixture of fog, air, ice and water in those wild, windy and frigid seas. 2
He called this land Thule--as in Ultima Thule, the legendary northern lands later claimed by the Nazis.
His book does not survive, but excerpts are found--often with the ridicule of the author--in the works of Strabo, Polybius, and Dikaiarkhos of Messina. However, Eratosthenes and Hipparkhos did believe, and use his astronomical calculations, such as that the Pole Star was not truely north, and that the moon affects the tides. It is also believed that Pytheas was the first person to calculate the latitude of Marseilles.
SOURCES
http://www.wikipedia.org/wiki/Pytheas
1. The Extraordinary Voyage of Pytheas the Greek Barry Cunliffe
2. http://www.ahepafamily.org/d5/pytheas.html