Navigator, geographer, astronomer and the first Greek to visit and describe the British Isles and the Atlantic coast of Europe.
Though his principal work On the Ocean is lost something is known of his ventures through the Greek historian Polybius (c. 200-c. 118 BC).
Sailing from the Mediterranean into the Atlantic he stopped at the Phoenician city of Gades (Cádiz, Spain) probably followed the European shoreline to the tip of Brittany and eventually reached Belerium (Land´s End, Cornwall) where he visited the tin mines famous in the ancient world.
He claimed to have explored a large part of Britain on foot. He estimated its circumference at 4,000 miles (6,400 kilometres) an accurate estimate. He also estimated the distance between northernmost Britain and Massalia (Marseille) at 1,050 miles; the actual distance is 1,120 miles.
He visited some northern European countries and may have reached the mouth of the Vistula River on the Baltic Sea. He also told of Thule, the northernmost inhabited island, six days´ sail from northern Britain and extending at least to the Arctic Circle.
Thule cannot be precisely identified. The region he visited may have been Iceland or Norway.
His comments on small points -on the native drinks made of cereals and honey and the use of threshing barns (contrasted with open-air threshing in Mediterranean regions) -show acute observation.
His scientific interest appear from his calculations made with a sundial at the summer solstice and from notes on the lengthening days as he travelled northward.
He also observed that the Pole Star is not at the true pole and that the Moon affects tides.
Pytheas´ findings were attacked by the ancient historians Strabo (1st centuries BC and AD) and Polybius but modern scholarship generally has thought better of him.