According to the ancients, the stars traveled around Earth on the transparent, hollow celestial sphere. It is a projection of an imaginary gigantic bubble.
The celestial sphere can be defined as a projection of an imaginary gigantic bubble (or sphere) upon which celestial bodies can be projected.
This imaginary sphere (celestial sphere) is concentric on the Earth planet.
All celestial bodies (including stars) may be imagined as projected upon the inner surface of this celestial sphere.
Learn more about the celestial sphere here:
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