An abstract sphere of infinite radius which serves as the imaginary backdrop for celestial objects, and of which the visible sky is one hemisphere.
the figure of the universe as an imaginary sphere whose center is the Earth
The great circle on the celestial sphere that is the mean apparent path of the sun as viewed from the earth.
A great circle drawn on a terrestrial globe, used for illustrating and solving astronomical problems.
Also, the plane in which this great circle lies.
Relating to an eclipse or the ecliptic.