Pytheas, (fourth century B.C.), a Greek navigator and explorer. He was a native of the Greek colony of Massalia (Marseille, France). Pytheas was the first Greek to explore the coast of northern Europe. Surviving fragments of his writings, and secondhand accounts of his voyages by Greek and Roman historians, indicate that he explored parts of Britain a nd sailed north to a point near the Arctic Circle. He described a land called Thule, now believed to be Iceland or Norway, where days and nights were each six months long.