Pytheas - Pytheas On The Tides

Pytheas On The Tides

Pliny reports that "Pytheas of Massalia informs us, that in Britain the tide rises 80 cubits." 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 37 metres (121 ft). 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 6.955 m (22.82 ft). The highest predicted spring tide between 2008 and 2026 at that location will be 14.64 m (48.0 ft) 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 attribute the flood tides (πλήμμυραι plēmmurai) to the "filling of the moon" (πλήρωσις τῆς σελήνης plērōsis tēs sēlēnēs) and the ebb tides (ἀμπώτιδες ampōtides) to the "lessening" (μείωσις 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.