what is the circle of illumination

The circle of illumination is the boundary between day and night on Earth. It is the line that separates the illuminated half of the Earth from the dark half. The circle of illumination always bisects the equator and moves as the Earth rotates on its axis. The circle of illumination is different from Earths axis, which is a line along which the planet rotates. While the circle of illumination divides light from darkness and day from night, the axis is a line along which the planet rotates.