مَدار شُمالْگان (نام قدیمیتر: مدار قطب شمال) یکی از پنج مدار مهم زمین است که در نقشهها غالباً نمایش داده میشود. این مدار، مدار َ۳۳ ْ۶۶ + بر روی زمین است که مرز محدودهٔ قطبی را نشان میدهد، جایی که حداقل در یک شبانهروز از سال خورشید هرگز غروب نمیکند. به عبارت دیگر، مدار شمالگان، جنوبیترین عرض جغرافیایی نیمکرهٔ شمالی است که حداقل یک بار در سال خورشید به مدت ۲۴ ساعت بیوقفه بالای خط افق بر آن بتابد (خورشید نیمهشب).
در سال ۲۰۰۰، این مدار موازی با عرض جغرافیایی "۳۹ '۳۳ °۶۶ شمال استوا بودهاست. در سال ۲۰۱۱، این مدار به عرض جغرافیایی "۴۴ '۳۳ °۶۶ (°۶۶٬۵۶۲۲) شمالی رانده شدهاست.
The Arctic Circle is one of the two polar circles and the most northerly of the five major circles of latitude as shown on maps of Earth. It marks the northernmost point at which the centre of the noon sun is just visible on the December solstice and the southernmost point at which the centre of the midnight sun is just visible on the June solstice. The region north of this circle is known as the Arctic, and the zone just to the south is called the Northern Temperate Zone.
As seen from the Arctic, the Sun is above the horizon for 24 continuous hours at least once per year (and therefore visible at midnight) and below the horizon for 24 continuous hours at least once per year (and therefore not visible at noon). This is also true in the Antarctic region, south of the equivalent Antarctic Circle.
The position of the Arctic Circle is not fixed and currently runs 66°33′48.0″ north of the Equator. Its latitude depends on the Earth's axial tilt, which fluctuates within a margin of more than 2° over a 41,000-year period, due to tidal forces resulting from the orbit of the Moon. Consequently, the Arctic Circle is currently drifting northwards at a speed of about 15 m (49 ft) per year.
Midnight sun and polar night
The Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the centre of the sun can remain continuously above or below the horizon for twenty-four hours; as a result, at least once each year at any location within the Arctic Circle the sun is visible at local midnight, and at least once the centre is not visible at local noon.
Directly on the Arctic Circle these events occur, in principle, exactly once per year: at the June and December solstices, respectively. However, because of atmospheric refraction and mirages, and also because the sun appears as a disk and not a point, part of the midnight sun may be seen on the night of the northern summer solstice up to about 50 minutes (′) (90 km (56 mi)) south of the Arctic Circle; similarly, on the day of the northern winter solstice, part of the sun may be seen up to about 50′ north of the Arctic Circle. That is true at sea level; those limits increase with elevation above sea level, although in mountainous regions there is often no direct view of the true horizon.
Only four million people live north of the Arctic Circle due to the climate; nonetheless, some areas have been settled for thousands of years by indigenous peoples, who today make up 10% of the region's population. Tens of thousands of years ago, waves of people migrated from eastern Siberia across the Bering Strait into North America to settle.
The largest communities north of the Arctic Circle are situated in Russia, Norway, Finland and Sweden: Murmansk (population 295,374), Norilsk (178,018), Tromsø (75,638), Vorkuta (58,133), and Kiruna (16,936). Rovaniemi (62,667) in Finland is the largest settlement in the immediate vicinity of the Arctic Circle, lying 6 km (4 mi) south of the line.
In contrast, the largest North American community north of the Arctic Circle, Sisimiut (Greenland), has approximately 5,000 inhabitants. Of the Arctic communities in Canada and the United States, Utqiagvik, Alaska is the largest settlement with about 4,000 inhabitants.
The Arctic Circle passes through the Arctic Ocean, the Scandinavian Peninsula, North Asia, Northern America, and Greenland. The land within the Arctic Circle is divided among eight countries: Norway, Sweden, Finland, Russia, the United States (Alaska), Canada (Yukon, Northwest Territories, and Nunavut), Denmark (Greenland), and Iceland (where it passes through the small offshore island of Grímsey).
The climate inside the Arctic Circle is generally cold, but the coastal areas of Norway have a generally mild climate as a result of the Gulf Stream, which makes the ports of northern Norway and northwest Russia ice-free all year long. In the interior, summers can be quite warm, while winters are extremely cold. For example, summer temperatures in Norilsk, Russia will sometimes reach as high as 30 °C (86 °F), while the winter temperatures frequently fall below −50 °C (−58 °F).
Sites along the Arctic Circle
Starting at the prime meridian and heading eastwards, the Arctic Circle passes through: