Question

Define circle of illumination.

Answer 1

Hint:Our earth undergoes rotation as well as revolution. Rotation is the movement of the earth on its axis while revolution is earth's movement around the sun in its orbit. Now earth’s axis, which is an imaginary line that goes through the centre of earth from top to bottom and along which the earth rotates, forms an angle of 66.5 degrees with the orbital plane.

Complete Answer:
The earth is spherical in shape and rotates along its axis which forms an angle of 66.5 degrees with the orbital plane. Due to this, the sun rays do not fall evenly on the planet. This is why it is day on one part of the world while its night on the other.
The part of the earth that receives sunlight is day and the part that does not, is night.
The imaginary line that separates day from night on earth is called the circle of illumination.
It is basically the region that is experiencing sunshine.
The circle of illumination cuts all latitudes into half on the spring and autumnal equinoxes. Another fact is that the circle of illumination passes through the equator in a way that it bisects it.

Note:It is important to note that the circle of illumination is not the same as the axis. The axis,which is tilted at an angle of 23.5 degrees relative to our orbital plane, is the line along which the earth rotates whereas the circle of illumination is the line that separates day from night on earth. These two lines do not coincide.