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Pytheas (c.380 - c.310 BC) was a Greek merchant, geographer and explorer from the Phocaean colony Massilia (today Marseille). He made a voyage of exploration to northwestern Europe around 325 BC. He traveled over a considerable part of Britain, circumnavigating it between 330 and 320 BC. Pytheas was the first Graeco-Roman to describe the Midnight Sun, the aurora and Polar ice.

Pytheas described his travels in a book On the Ocean (Περι του Οκεανυ). It has not survived; only excerpts remain, quoted or paraphrased by later authors. Among them, Polybius and Strabo accused Pytheas of documenting a fictitious journey he could never have funded. His story is, however, plausible. The trip may have been underwritten by a wealthy patron. Pytheas estimated the circumference of Great Britain within 2.5% of modern estimates. There is some evidence he used the Pole Star to fix latitude and understood the relationships between tides and phases of the Moon. In northern Spain, he studied the tides, and may have discovered that they are caused by the Moon. This discovery was known to Posidonius.

Pytheas was not the first person to sail the seas around Britain. Trade between Gaul and Britain was already routine; fishermen and others would travel to the Orkneys, Iceland, Norway or Shetland. The Roman Avienus writing in the 4th century AD mentions an early Greek voyage, possibly from the 6th century BC. A recent conjectural reconstruction of the journey Pytheas documented has him traveling from Marseille in succession to Bordeaux, Nantes, Land's End, Plymouth, Isle of Man, Outer Hebrides, Orkneys, Iceland, Britain’s east coast, Kent, Helgoland, returning finally to Marseille.

The start of Pytheas's voyage is already a mystery. The Carthaginians had closed the Straits of Gibraltar to all ships from other nations. Some historians therefore believe that he travelled overland to the mouth of the Loire or the Garonne. Others believe that, to avoid the Carthaginian blockade, he may have stuck close to land and sailed only at night. It is also possible he took advantage of a temporary lapse in the blockade, known to have taken place around the time he travelled.

Cornwall was important because it was the main source of tin. Pytheas studied the production and processing of tin there. He circumnavigated Britain, and found that tides could be very high there. He recorded the local name of the islands in Greek as Prettanike, which Diodorus later rendered Pretannia. This supports theories that the inhabitants called themselves Pretani or Priteni, 'Painted’ or 'Tattooed’ people, a term Romans Latinised as Picti (Picts). He is quoted as referring to the British Isles as the ’’Isles of the Pretani’’.

Pytheas visited an island six days sailing north of Britain, called Thule. Probably Thule was (part of) the Norwegian coast, although Iceland, the Shetland Islands and Faeroe Islands have also been suggested by historians. Pytheas says Thule was an agricultural country that produced honey. Its inhabitants ate fruits and drank milk, and made a drink out of grain and honey. Unlike the people from southern Europe, they had barnss, and threshed their grain there rather than outside.

He said he was shown the place where the sun went to sleep, and he noted that the night in Thule was only two to three hours. One day further north the congealed sea began, he claimed. As Strabo says (as quoted in Chevallier 1984):

Pytheas also speaks of the waters around Thule and of those places where land properly speaking no longer exists, nor sea nor air, but a mixture of these things, like a "marine lung", in which it is said that earth and water and all things are in suspension as if this something was a link between all these elements, on which one can neither walk nor sail.

The term used for "marine lung" actually means jellyfish, and modern scientists believe that Pytheas here tried to describe the formation of pancake ice at the edge of the drift ice, where sea, slush, and ice mix, surrounded by fog.

After completing his survey of Britain, Pytheas travelled to the Shallows on the continental North Sea coast. He may also have visited the Baltic Sea, but he did visit an island which was a source of amber, probably Helgoland.

No record survives of his return voyage. He may have returned by the way he came; or perhaps by land, following the Rhine and Rhône rivers.

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