What causes the midnight sun?

Answer 1

The midnight sun is a phenomenon occurring in latitudes north and nearby to the south of the Arctic Circle and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight.

Since the Earth's axis is tilted with respect to the ecliptic by approximately 23 degrees 27 minutes, the sun does not set at high latitudes in (local) summer. The duration of the midnight sun increases from one day during the summer solstice at the polar circle to approximately six months at the poles. At extreme latitudes, it is usually referred to as polar day. The length of the time the sun is above the horizon varies from 20 hours at the Arctic Circle and Antarctic Circle to 186 days at the poles.

Overall, it's the earth axis inclination to its course around the sun which causes the seasons, midnight sun and the midwinter darkness. It's also the same inclination that defines where the Arctic circle goes.

---- Source:

http://fairbanks-Alaska.com/midnite-sun.htm

http://en.wikipedia.org/wiki/Midnight_sun

Answer 2

The midnight sun occurs in polar regions when the tilt of the Earth's axis leans toward the Sun during summer months. This causes certain areas to experience sunlight throughout the night, as the Sun never fully sets below the horizon.