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.