The line which separates the darkened portion and the illuminated one is called “Circle of Illumination”.
On June 21, the northern half of the earth is inclined towards the sun and southern half is away from it. The sun rays fall vertically on the Tropic of Cancer (23½° N) due to which the Northern Hemisphere becomes extremely hot and warm. There is high temperature condition in the Northern Hemisphere and it is due to the two facts. Firstly, inclination of earth’s axis and secondly, direct angle of sun rays at the Tropic of Cancer.
It is summer season and the days are longer and nights are shorter in the north of the equator. Whereas in the south, the sun rays can reach up to only Antarctica circle (66½° S), after that there is complete darkness for six months.