Pytheas : biography
Seen from the equator the celestial North Pole (boreios polos) is a point on the horizon. As the observer moves northward the pole rises and the circumpolar stars appear, now unblocked by the Earth. At the Tropic of Cancer the radius of the circumpolar stars reaches 24°. The edge stands on the horizon. The constellation of mikra arktos (Ursa Minor, "little bear") was entirely contained within the circumpolar region. The latitude was therefore called the arktikos kuklos, "circle of the bear". The terrestrial Arctic Circle was regarded as fixed at this latitude. The celestial Arctic Circle was regarded as identical to the circumference of the circumpolar stars and therefore a variable.
When the observer is on the terrestrial Arctic Circle and the radius of the circumpolar stars is 66° the celestial Arctic Circle is identical to the celestial Tropic of Cancer.. That is what Pytheas means when he says that Thule is located at the place where the Arctic Circle is identical to the Tropic of Cancer. At that point, on the day of the Summer Solstice, the vertical rod of the gnōmōn casts a shadow extending in theory to the horizon over 360° as the sun does not set. Under the pole the Arctic Circle is identical to the Equator and the sun never sets but rises and falls on the horizon. The shadow of the gnōmōn winds perpetually around it.
Latitude by longest day and shortest solar elevation
Strabo uses the astronomical cubit (pēchus, the length of the forearm from the elbow to the tip of the little finger) as a measure of the elevation of the sun. The term "cubit" in this context is obscure; it has nothing to do with distance along either a straight line or an arc, does not apply to celestial distances, and has nothing to do with the gnōmōn. Hipparchus borrowed this term from Babylonia, where it meant 2°. They in turn took it from ancient Sumer so long ago that if the connection between cubits and degrees was known in either Babylonia or Ionia it did not survive. Strabo states degrees in either cubits or as a proportion of a great circle. The Greeks also used the length of day at the summer solstice as a measure of latitude. It is stated in equinoctial hours (hōrai isēmerinai), one being 1/12 of the time between sunrise and sunset on an equinox.
Based partly on data taken from Pytheas, Hipparchus correlated cubits of the sun’s elevation at noon on the winter solstice, latitudes in hours of a day on the summer solstice, and distances between latitudes in stadia for some locations.. Pytheas had proved that Marseilles and Byzantium were on the same parallel (see above). Hipparchus, through Strabo,Strabo . adds that Byzantium and the mouth of the Borysthenes, today’s Dnepr river, were on the same meridian and were separated by 3700 stadia, 5.3° at Strabo’s 700 stadia per a degree of meridian arc. As the parallel through the river-mouth also crossed the coast of "Celtica", the distance due north from Marseilles to Celtica was 3700 stadia, a baseline from which Pytheas seems to have calculated latitude and distance.However, Srabo implies 3800, still attributed to Hipparchus. Eratosthenes has quite a different view. See under Thule.
Strabo says that Ierne (Ireland) is under 5000 stadia (7.1°) north of this line. These figures place Celtica around the mouth of the Loire river, an emporium for the trading of British tin. The part of Ireland referenced is the vicinity of Belfast. Pytheas then would either have crossed the Bay of Biscay from the coast of Spain to the mouth of the Loire, or reached it along the coast, crossed the English channel from the vicinity of Brest, France to Cornwall, and traversed the Irish Sea to reach the Orkney Islands. A statement of Eratosthenes attributed by Strabo to Pytheas, that the north of the Iberian Peninsula was an easier passage to Celtica than across the Ocean,. is somewhat ambiguous: apparently he knew or knew of both routes, but he does not say which he took.
At noon on the winter solstice the sun stands at 9 cubits and the longest day on the summer solstice is 16 hours at the baseline through Celtica.Strabo II.1.18. The notes of the Loeb Strabo summarize and explain this information. At 2500 stadia, approximately 283 miles, or 3.6°, north of Celtica, are a people Hipparchus called Celtic, but whom Strabo thinks are the British, a discrepancy he might not have noted if he had known that the British were also Celtic. The location is Cornwall. The sun stands at 6 cubits and the longest day is 17 hours. At 9100 stadia, approximately 1032 miles, north of Marseilles, 5400 or 7.7° north of Celtica, the elevation is 4 cubits and the longest day is 18 hours. This location is in the vicinity of the Firth of Clyde.