The midnattssol - the midnight sun - is the phenomenon, observed only north of the Arctic Circle (66°33' N), of the sun remaining above the horizon for twenty-four hours during the high summer. The exact dates depend on latitude. In Bodø, just north of the Circle, it runs from about 31 May to 12 July. In Tromsø, close to 70 degrees north, the window runs from about 18 May to 25 July. In Hammerfest it is 14 May to 29 July, and at Nordkapp on the island of Magerøya, near the northern tip of Europe, 12 May to 1 August. By the time you reach Longyearbyen on Svalbard at 78°N, the sun stays above the horizon from around 19 April and will not set again until 23 August.
The astronomical explanation is straightforward and worth stating: the Earth's rotational axis is tilted approximately 23.4 degrees relative to its orbital plane around the Sun. During the northern summer, the Arctic regions are tilted toward the Sun continuously, so any point above the Arctic Circle experiences a period of weeks during which the Sun does not set below the local horizon. The phenomenon has its mirror image in the polar night (mørketid) of the same regions during the northern winter.
Below the Arctic Circle the trick is gentler but still strange. Further south, in Trondheim, midsummer night is a long blue civil twilight rather than a black sky. In Oslo and Bergen, even in late June, the sky barely darkens. The practical effect for travel planning is that any Norwegian summer trip - Arctic or sub-Arctic - happens under unusually long days and that the rhythm of the holiday tends to reorganise itself around appetite for daylight rather than around the clock.
The aurora borealis (nordlys) is invisible during the midnight sun period because the sky is too bright. The two phenomena occur in opposite halves of the year. For a longer treatment of how to plan a midnight-sun trip, see our editorial Living in the light.