The sky as seen from our Earth appears as a hemisphere. If we could see the entire sky at a time, it would appear to us as a globe or sphere seen from inside. In other words, we may think of the Earth as situated within a much larger sphere, the surface of which is the sky. The 'sky' here is, of course, only imaginary because around Earth (and its atmosphere) there is only empty space. And the stars are billions of kilometres away. But if we want to get familiar with the stars, this imaginary sky, or 'celestial sphere' as astronomers call it, offers a convenient framework for locating stars in the night sky.
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