The universe is the totality of all space, time, matter, and energy.
Astronomy is the study of the universe.
Early observers grouped the thousands of stars visible to the naked eye into patterns called constellations. These patterns have no physical significance, although they are a very useful means of labeling regions of the sky. The nightly motion of the stars across the sky is the result of Earth's rotation on its axis. Early astronomers, however, imagined that the stars were attached to a vast celestial sphere centered on Earth and that the motions of the heavens were caused by the rotation of the celestial sphere about a fixed Earth. The points where Earth's rotation axis intersects the celestial sphere are called the north and south celestial poles.