What determines the latitude of Arctic Circle?

The latitude of the Arctic Circle is determined by the Earth's axial tilt. The Earth's axis of rotation is tilted away from the Sun by an angle of approximately 23.4 degrees. This means that the Earth's Northern and Southern Hemispheres are alternately tilted towards and away from the Sun as the Earth orbits the Sun.

At the summer solstice, the Northern Hemisphere is tilted towards the Sun and the Arctic Circle is at its most northerly point, at 66.5622°N latitude. At the winter solstice, the Northern Hemisphere is tilted away from the Sun and the Arctic Circle is at its most southerly point, also at 66.5622°N latitude.

The Arctic Circle is the latitude at which the Sun does not set on the summer solstice and does not rise on the winter solstice. This is because the Sun's rays are parallel to the Earth's surface at these latitudes.