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
A similar great circle on any sphere, especially on a celestial body, or on other reasonably symmetrical three-dimensional body.
The midline of any generally spherical object, such as a fruit or vegetable, that has identifiable poles.
An imaginary great circle around Earth, equidistant from the two poles, and dividing earth's surface into the northern and southern hemisphere.
The celestial equator.