The Arctic Circle marks the latitude where the sun doesn't set on summer solstice or rise on winter solstice. Eight countries have territory north of this line:
Yukon, Northwest Territories, Nunavut—~40% of landmass, 160,000 km Arctic coastline (world's longest). Inuit Nunangat homeland.
Greenland (Kalaallit Nunaat) autonomous; 80% ice sheet; majority Inuit population. Thule Air Base (US).
Nordland, Troms, Finnmark counties + Svalbard (special treaty status). North Cape tourism, oil/gas, fisheries.
Lapland (Lappi) region; Sámi homeland; Santa Claus Village (Rovaniemi); EU's only Arctic territory.
Norrbotten county + Swedish Lapland; Kiruna mining, ICEHOTEL, Sami culture, space research (Esrange).
Vast Siberian coastline (Murmansk to Chukotka); Novaya Zemlya, Severnaya Zemlya, New Siberian Islands. Major military, resource, Northern Sea Route interests.
North Slope, Brooks Range, Arctic Coastal Plain; Prudhoe Bay oil, ANWR, Utqiaġvik (Barrow), indigenous Iñupiat communities.
Mainland just south of Circle; two small northern islands cross it. Grímsey inhabited (~60 people); Kolbeinsey uninhabited, eroding.
Axial precession shifts the Circle ~14 m/year northward. Over millennia, national inclusions change slightly.