The axis of the Earth which is an imaginary line, makes an angle of 66° with its orbital plane. Due to Earth's spherical shape, only half of it gets light from the Sun at a time. The portion facing the Sun experiences day while the other half away from the Sun experiences night. This circle of illumination does not coincide with the axis. It is to be noted that throughout its orbit, the Earth is inclined in the same direction. Seasons change due to the change in the position of the Earth around the Sun. But if the Earth is not inclined towards its orbital plane the circle of illumination would match with the longitudinal lines and there would be no seasonal variation in a particular region.