Celestial sphere

From Glossary of Meteorology

celestial sphere

The apparent sphere of infinite radius, having Earth as its center.

It is upon the "inner surface" of this sphere that all heavenly bodies, the ecliptic, and the celestial equator appear. Disregarding the effects of topography and refraction near the horizon, for practical purposes half of this sphere may be considered visible from any point on the earth's surface at any time.