Pytheas of Massalia: Thule, the Sea Lung and Linking the Tides to the Moon

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Pytheas of Massalia, a Greek geographer and astronomer from the colony that is now Marseille, set out around 325 BC to explore the northern Atlantic. Carthage blockaded the Strait of Gibraltar, and historians still debate whether he slipped past at night, sailed under a treaty, or crossed Gaul overland by river. His book On the Ocean is lost, surviving only in fragments quoted by later writers who mostly mocked him. He became the first documented Mediterranean mariner to reach and describe the British Isles, noting a triangular island with corners at Kent, Cornwall and Orkney, and he gave the earliest written form of the name Britain. In Cornwall he recorded how tin was mined, smelted and cast into knucklebone-shaped ingots.

Sailing six days north of Britain, he reached Thule and saw the midnight sun, confirming by observation what Greek theorists had only calculated. Beyond it the sea turned into a slushy mass he called a marine lung, likely pancake ice. On the Baltic coast he found amber so plentiful that locals burned it as fuel. Using a gnomon, he had fixed Massalia's latitude with remarkable accuracy, and by watching the sea he became the first recorded person to link the tides to the phases of the moon. Centuries later Strabo dismissed him as a poor private citizen telling tall tales, yet nearly everything Pytheas reported was accurate.

• Tin ingots were carted at low tide across causeways to an island called Ictis, then carried by horse on a 30-day trip to the Mediterranean.
• The name he recorded for Britain is linked to a Celtic word for the painted people.
• Candidates for Thule include Iceland, western Norway, Shetland and the Faroe Islands.
• Pliny the Elder reported Pytheas as saying British tides rose 80 cubits, likely a later misunderstanding.
• Strabo believed the Earth was uninhabitable north of Ireland and so rejected Thule outright.