The Circle of illumination divides Earth into two hemispheres known as :

The Circle of illumination, also known as the terminator, is the imaginary line that separates the portion of the Earth that is lit by the Sun from the portion that is in darkness [1]. Because the Earth is a spherical body, only half of it can be illuminated by the Sun at any given time, while the other half remains in shadow. This division creates two distinct hemispheres: the day hemisphere and the night hemisphere [2]. While the Equator divides the Earth into Northern and Southern hemispheres and the Prime Meridian divides it into Eastern and Western hemispheres, the circle of illumination specifically marks the boundary between day and night [1]. This line is dynamic, moving across the Earth's surface as the planet rotates on its axis [3].

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 11: Keeping Time with the Skies > p. 175
2. [2] https://education.nationalgeographic.org/resource/equator/
3. [3] https://sos.noaa.gov/catalog/datasets/daynight-terminator-daily/