Why is the Arctic Circle located at 66.5 degrees N?

Answer 1

The Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours (at the June solstice and December solsticerespectively).

Answer 2

These lines of latitude mark the end of the area on earth where sunrises and sunsets occur during every 24-hour period.
Beyond these lines, there are periods of no sunrise/ set, and at the circles, this period is one of 24 hours. Farther beyond these lines -- at the poles, for example, the period is six months.

Answer 3

The Arctic Circle is the line of latitude located 66.5 degrees north of the Equator. There are only 23.5 degrees north of the Arctic Circle before you start going south.