Final answer:
The equator is the imaginary circle that divides the Earth into the Northern and Southern hemispheres. It is a great circle with 0 degrees latitude, essential for geographical and celestial navigation.
Step-by-step explanation:
The imaginary circle around the middle of the Earth that separates the Northern and Southern hemispheres is known as the equator. The equator is a great circle on the Earth's surface and represents 0 degrees latitude. Lines of latitude are measured from 0 degrees at the equator to 90 degrees at the poles, with the Northern Hemisphere labeled as 'N' and the Southern Hemisphere labeled as 'S'.
Understanding the concept of the equator is essential for knowledge about the Earth's rotation and its implications on the celestial sphere. As the Earth rotates, the sky appears to revolve around the celestial poles, mirroring the terrestrial poles. The celestial equator is the projection of the Earth's equator onto the celestial sphere, and just like the terrestrial equator, it is halfway between the celestial poles.