Pytheas : biography
Diodorus does not mention Pytheas by name. The connection is made as follows: Pliny reports that "Timaeus says there is an island named Mictis … where tin is found, and to which the Britains cross."Natural History . Diodorus says that tin is brought to the island of Ictis, where there is an emporium. The last link is supplied by Strabo, who says that an emporium on the island of Corbulo in the mouth of the Loire was associated with the Britain of Pytheas by Polybius.Geographica . Assuming that Ictis, Mictis and Corbulo are the same, Diodorus appears to have read Timaeus, who must have read Pytheas, whom Polybius also read.
Pytheas on the tides
Pliny reports that "Pytheas of Massalia informs us, that in Britain the tide rises 80 cubits."Natural History The passage does not give enough information to determine which cubit Pliny meant; however, any cubit gives the same general result. If he was reading an early source, the cubit may have been the Cyrenaic cubit, an early Greek cubit, of 463.1 mm, in which case the distance was . This number far exceeds any modern known tides. The National Oceanography Centre, which records tides at tidal gauges placed in about 55 ports of the UK Tide Gauge Network on an ongoing basis, records the highest mean tidal change between 1987 and 2007 at Avonmouth in the Severn Estuary of . The highest predicted spring tide between 2008 and 2026 at that location will be on 29 September 2015. Even allowing for geologic and climate change, Pytheas’ 80 cubits far exceeds any known tides around Britain. One well-circulated but unevidenced answer to the paradox is that Pytheas is referring to a storm surge.
Matching fragments of Aëtius in pseudo-Plutarch and Stobaeus . Diels includes two matching fragments of Aëtius’ Placita, one from Pseudo-Plutarch Epitome Book III Chapter 17 often included in Moralia and the other from Stobaeus’ Extracta Book I Chapter 38 . attribute the flood tides ( plēmmurai) to the "filling of the moon" ( plērōsis tēs sēlēnēs) and the ebb tides ( ampōtides) to the "lessening" ( meiōsis). The words are too ambiguous to make an exact determination of Pytheas’ meaning, whether diurnal or spring and neap tides are meant, or whether full and new moons or the half-cycles in which they occur. Different translators take different views.
That daily tides should be caused by full moons and new moons is manifestly wrong, which would be a surprising view in a Greek astronomer and mathematician of the times. He could have meant that spring and neap tides were caused by new and full moons, which is partially correct in that spring tides occur at those times. A gravitational theory (objects fall to the center) existed at the time but Pytheas appears to have meant that the phases themselves were the causes (αἰτίαι aitiai). However imperfect or imperfectly related the viewpoint, Pytheas was the first to associate the tides to the phases of the moon.
Pytheas’ measurements of latitude
Latitude by the altitude of the sun
In discussing the work of Pytheas, Strabo typically uses direct discourse: "Pytheas says …" In presenting his astronomical observations, he changes to indirect discourse: "Hipparchus says that Pytheas says …" either because he never read Pytheas’ manuscript (because it was not available to him) or in deference to Hipparchus, who appears to have been the first to apply the Babylonian system of representing the sphere of the earth by 360°.
Strabo uses the degrees, based on Hipparchus.Geographica : "If, then, we cut the greatest circle of the Earth into three hundred and sixty sections, each of these sections will have seven hundred stadia." Neither say that Pytheas did. Nevertheless Pytheas did obtain latitudes, which, according to Strabo, he expressed in proportions of the gnōmōn ("index"), or trigonometric tangents of angles of elevation to celestial bodies. They were measured on the gnōmōn, the vertical leg of a right triangle, and the flat leg of the triangle. The imaginary hypotenuse looked along the line of sight to the celestial body or marked the edge of a shadow cast by the vertical leg on the horizontal leg.