Hello,

YES, the Circle of Illumination is an imaginary line that separates the day-side and night-side.

If we imagine this is 2-d plane, then it is a line.

But in 3-d plane, when you look from the point of view of the Sun, then this will appear to be a Circle. For imagination, take a cricket ball and keep it infront of you and try to look at the finger grip.. doesn't it appear like a circle??

You can always post such thought-provoking questions on Careers360 to get expert answers.

Hope this helps.

Thanks.