The midnight sun is a natural phenomenon occurring in summer months at places north of the Arctic Circle and south of the Antarctic Circle where the sun remains visible at the local midnight. Around the solstice (June 21 in the north and December 21 in the south) and given fair weather the sun is visible for the full 24 hours. The number of days per year with potential midnight sun increases the farther towards either pole one goes. Although approximately defined by the polar circles, in practice midnight sun can be seen as much as 90 km outside the polar circle, as described below, and the exact latitudes of the farthest reaches of midnight sun depend on topography and vary slightly year-to-year. There are no permanent human settlements south of the Antarctic Circle, so the countries and territories whose populations experience it are limited to the ones crossed by the Arctic Circle, e.g. Canada (Yukon, Northwest Territories, and Nunavut), Denmark (Greenland), Finland, Sápmi, Norway, Russia, Sweden, the United States (Alaska), and extremities of Iceland. A quarter of Finland's territory lies north of the Arctic Circle and at the country's northernmost point the sun does not set at all for 60 days during summer. In Svalbard, Norway, the northernmost inhabited region of Europe, there is no sunset from approximately 19 April to 23 August. The extreme sites are the poles where the sun can be continuously visible for a half year. The opposite phenomenon, polar night, occurs in winter when the sun stays below the horizon throughout the day. Since the axial tilt of the Earth is considerable (approximately 23 degrees 27 minutes) the sun does not set at high latitudes in (local) summer. The duration of sunlight increases from one day during the summer solstice at the polar circle to several weeks only a hundred kilometres closer to the pole, to six months at the poles. At extreme latitudes, it is usually referred to as polar day. At the poles themselves, the sun only rises once and sets once each year. During the six months when the sun is above the horizon it spends the days continuously moving around the horizon, reaching its highest circuit of the sky at the summer solstice. Due to atmospheric refraction and also because the sun is a disk rather than a point, the midnight sun may be experienced at latitudes slightly below the polar circle, though not exceeding one degree (depending on local conditions). For example, it is possible to experience the midnight sun in Iceland, even though most of it (Grímsey being a notable exception) is slightly south of the Arctic Circle. The same mechanisms cause the period of sunlight at the poles to last slightly more than six months. Even the northern extremities of Scotland (and those places on similar latitudes such as St. Petersburg) experience a permanent twilight in the northern sky at these times. Observers at heights appreciably above sea level can experience extended periods of midnight sun as a result of the 'dip' of the horizon viewed from altitude.