the sun's location, modern reproductions of Viking "sky-polarimetric" navigation have found these sun compasses to be highly inaccurate, and not usable in cloudy or foggy weather. The archaeological find known as the Visby lenses from the Swedish island of Gotland may be components of a telescope. It appears to date from long before the invention of the telescope in the 17th century. Recent evidence suggests that the Vikings also made use of an optical compass as a navigation aid, using the light-splitting and polarisation-filtering properties of Iceland spar to find the location of the sun when it was not directly visible. Religion Trade centres Some of the most important trading ports founded by the Norse during the period include both existing and former cities such as Aarhus (Denmark), Ribe (Denmark), Hedeby (Germany), Vineta (Pomerania), Truso (Poland), Bjørgvin (Norway), Kaupang (Norway), Skiringssal (Norway), Birka (Sweden), Bordeaux (France), York (England), Dublin (Ireland) and Aldeigjuborg (Russia). One important centre of trade was at Hedeby. Close to the border with the Franks, it was effectively a crossroads between the cultures, until its eventual destruction by the Norwegians in an internecine dispute around 1050. York was the centre of the kingdom of Jórvík from 866, and discoveries there (e.g., a silk cap, a counterfeit of a coin from Samarkand, and a cowry shell from the Red Sea or the Persian Gulf) suggest that Scandinavian trade connections in the 10th century reached beyond Byzantium. However, those items could also have been Byzantine imports, and there is no reason to assume that the Varangians travelled significantly beyond Byzantium and the Caspian Sea. Genetics A genetic study published at bioRxiv in July 2019 and in Nature in September 2020 examined the population genomics of the Viking Age. 442 ancient humans from across Europe and the North Atlantic were surveyed, stretching from the Bronze Age to the Early Modern Period. In terms of Y-DNA composition, Viking individuals were similar to present-day Scandinavians. The most common Y-DNA haplogroup in the study was I1 (95 samples), R1b (84 samples) and R1a, especially (but not exclusively) of the Scandinavian R1a-Z284 subclade (61 samples). It was found that there was a notable foreign gene flow into Scandinavia in the years preceding the Viking Age and during the Viking Age itself. This gene flow entered Denmark and eastern Sweden, from which it spread into the rest of Scandinavia. The Y-DNA of Viking Age samples