[45] Allowing for the greater bays, and putting a degree of latitude at 700 stadia, the sides of Great Britain are about 4000, 7800 and 12,000 stadia; altogether 23,800 stadia, or about 2375 miles.
[46] Strabo erred just as much on his side in making the circumference of Britain much too small.
[47] Cf. Hergt, 1893, p. 44. This hypothesis is supported by the round numbers which answer to 7½, 15, and 20 days’ sail.
[48] The Greeks divided the day into twelve hours at all times of the year; it was thus only at the equinoxes, when the day was really twelve hours long, that the hours were of the same length as ours. These are, therefore, called equinoctial hours.
[49] A similar statement in Cleomedes [i. 7], after Eratosthenes and Posidonius [i. 10], may also be derived from Pytheas: “the longest day in Britain has eighteen hours.”
[50] If we assume that the length of the day was found by a theoretical calculation of the time between the rising and setting of the sun’s centre above the horizon, without taking account of refraction, then a longest day of nineteen hours answers to 60° 52′ N. lat.; but if we suppose that the length of the day was found by direct observation and was calculated from the first appearance of the sun’s limb in the morning until its final disappearance in the evening, then horizontal refraction will be of importance (besides having to take the sun’s semi-diameter into account), and a longest day of nineteen hours then answers to 59° 59′ N. lat. Now the Shetland Isles lie between 59° 51′ and 60° 51′ N. lat.; while the northern point of the Orkneys lies in 59° 23′ N. lat., and has a longest day, theoretically of 18 hours 27 minutes, and actually of 18 hours 36 minutes. A longest day of 18 hours answers theoretically to 57° 59′, actually to fully 57° N. lat. Professor H. Geelmuyden has had the kindness to work out several of these calculations for me. Hipparchus said that at the winter solstice the sun attained to a height of less than three cubits above the horizon in the regions where the longest day was of nineteen hours. If we take one cubit as equal to two degrees these regions will then lie north of 60° N. lat.
[51] It may be possible, as many think, that it was the Shetlands that he called Orkan (or Orkas); but the more reliable of the known quotations from him seem rather to show that it was really the northernmost point of Britain, or the neighbouring Orkneys that were thus called by him, and have thenceforward been known by that name; while it is later authors who have extended the name also to Shetland. If this supposition be correct: that the islands north of Britain mentioned by Pliny [Nat. Hist. iv. 104] are originally derived from Pytheas, which may be doubtful, and that Berricen (or Nerigon) is Mainland of Shetland, then Orkan cannot apply to these. But, as we shall see later, it is very doubtful what Pliny’s islands may have been originally.
[52] Cf. Strabo [ii. 114] and Cleomedes [i. 7]. The Arctic Circle (or Circle of the Bear) was, as already mentioned, the circle round the celestial pole which formed the limit of the continuously visible (circumpolar) stars, and it had been given this name because in Asia Minor (and Greece) it ran through the Great Bear (Arctus). Its distance in degrees from the north celestial pole is equal to the latitude of the place of observation, and consequently increases as one goes farther north. At the polar circle, as mentioned above, it coincides with the Tropic of Cancer, and at the North Pole with the Equator. Cleomedes has also the remarkable statement that the latitude for a summer day of one month in length runs through Thule.
[53] It may be thought that Pytheas is merely relating a legend current among the barbarians that the sun went to its resting-place during the night, a myth which is moreover almost universal. But it seems more probable that as an astronomer he had something else in his mind. If he had had the two points accurately indicated to him, where the sun set and rose on the shortest night of the year, he must easily have been able, by measuring the angle between them, to ascertain how long the sun was down.
[54] These figures are kindly supplied by Professor H. Geelmuyden.