<P> To tourists, one of the fascinations of the islands is their "nightless" summers . On the longest day in Shetland there are over 19 hours of daylight and complete darkness is unknown . This long twilight is known in the Northern Isles as the "simmer dim". Winter nights are correspondingly long with less than six hours of daylight at midwinter . At this time of year the aurora borealis can occasionally be seen on the northern horizon during moderate auroral activity . </P> <P> There are numerous important prehistoric remains in Orkney, especially from the Neolithic period, four of which form the Heart of Neolithic Orkney UNESCO World Heritage Site that was inscribed in 1999: Skara Brae; Maes Howe; the Stones of Stenness; and Ring of Brodgar . The Knap of Howar Neolithic farmstead situated on the island of Papa Westray is probably the oldest preserved house in northern Europe . This structure was inhabited for 900 years from 3700 BC but was evidently built on the site of an even older settlement . Shetland is also extremely rich in physical remains of the prehistoric eras and there are over 5,000 archaeological sites all told . Funzie Girt is a remarkable Neolithic dividing wall that ran for 4 kilometres (2.5 mi) across the island of Fetlar, although the Iron Age has provided the most outstanding archaeology on Shetland . Numerous brochs were erected at that time of which the Broch of Mousa is the finest preserved example of these round towers . In 2011 the collective site, "The Crucible of Iron Age Shetland" including Broch of Mousa, Old Scatness and Jarlshof joined the UK's "Tentative List" of World Heritage Sites . </P> <P> The culture that built the brochs is unknown, but by the late Iron Age the Northern Isles were part of the Pictish kingdom . The main archaeological relics from these times are symbol stones . One of the best examples is located on the Brough of Birsay; it shows three warriors with spears and sword scabbards combined with traditional Pictish symbols . The St Ninian's Isle Treasure was discovered in 1958 . The silver bowls, jewellery and other pieces are believed to date from approximately 800 AD . O'Dell (1959) stated that "the treasure is the best survival of Scottish silver metalwork from the period" and that "the brooches show a variety of typical Pictish forms, with both animal - head and lobed geometrical forms of terminal". </P> <P> Christianity probably arrived in Orkney in the 6th century and organised church authority emerged in the 8th century . The Buckquoy spindle - whorl found at a Pictish site on Birsay is an Ogham--inscribed artefact whose interpretation has caused controversy although it is now generally considered to be of Irish Christian origin . </P>

What other kingdom claimed ownership of the northern islands of scotland