Farther north he came to Brettanice (Britain), which he appears to have circumnavigated. The Sicilian historian Diodorus, an elder contemporary of Strabo, says [v. 21]: "Britain is triangular in form like Sicily; but the sides are not of equal length; the nearest promontory is Kantion [Kent], and according to what is reported it is 100 stadia distant (from the continent). The second promontory is Belerion [Cornwall], which is said to be four days' sail from the continent. The third lies towards the sea [i.e., towards the north] and is called Orkan.[44] Of the three sides the one which runs parallel to Europe is the shortest, 7500 stadia; the second, which extends from the place of crossing [Kent] to the point [i.e., Orkan], is 15,000 stadia; but the last is 20,000 stadia, so that the circumference amounts to 42,500 stadia."
These statements must originally have been due to Pytheas, even though Diodorus has taken them at second hand (perhaps from Timæus). But Pytheas cannot very well have acquired such an idea of the shape of the island without having sailed round it. It is true that the estimate attributed to him of the island's circumference is more than double the reality,[45] a discrepancy which is adduced by Strabo as a proof that Pytheas was a liar;[46] but neither Strabo nor Diodorus was acquainted with his own description, and there are many indications that the exaggeration cannot be attributed to himself, but to a later writer, probably Timæus. Pytheas in his work can only have stated how many days he took to sail along the coasts, and his day's sail in those unknown waters was certainly a short one. But the uncritical Timæus, who was moreover a historian and not a geographer, may, according to the custom of his time, have converted Pytheas's day's journeys into stadia at the usual equation of 1000 stadia (about 100 geographical miles) for one day's sailing.[47] Timæus served to a great extent as the authority for later authors who have mentioned Pytheas, and it is probably through him that the erroneous information as to the circumference of Britain reached Polybius, Strabo, Diodorus, Pliny, and Solinus. In this way geographical explorers may easily have gross errors attributed to them, when their original observations are lost.