Today, we know better: the huge sphere is an illusion. Nonetheless, it is still useful as a scientific model of the sky. A scientific model is a mental picture, often with a corresponding mathematical description, that allows us to think about an aspect of nature.
Many models in science are truly attempts to describe what we believe to be an underlying reality. But many scientific models are just convenient constructs that make thinking about nature simpler. The model of the sky, called the celestial sphere, does not describe an underlying reality; there really isn't a huge sphere that encloses the earth! But it is a very useful way to think about the sky.
 | The
Celestial Sphere
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| Magnitudes | ||||||||||||||||||||
| Precession
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| north circumpolar constellation |
