The Earth, with it's axis of rotation at 23.5 degrees relative to the viewer's vertical, showing Northern summer illumination. The illumination source moves round as Earth continues to rotate, to show the Northern winter situation.

The Earth's axis of rotation is tilted 23.5 degrees from the plane of its orbit around the Sun. This means that the Southern hemisphere receives more energy in the form of sunlight than the Northern hemisphere for half of the Earth's orbit around the Sun. Six months later, the situation is reversed. Above the Arctic Circle, the sun doesn't set in summer and doesn't rise in winter, giving rise to "the land of the midnight sun" and the long polar night.

Credit : Planetary Visions Ltd.