The study of the universe is called cosmology. Cosmologists study the structure and changes in the present universe. The universe contains all of the star systems, galaxies, gas and dust, and all the matter and energy that exist. The universe also includes all of space and time. The part of the entire universe that we can see (because light from objects has had time to reach us) is called the observable universe. Evolution of Human Understanding of the Universe What did the ancient Greeks recognize as the universe? In their model, the universe contained Earth at the center, the Sun, the Moon, five planets, and a sphere to which all the stars were attached. This idea held for many centuries until new ideas and better observing instruments allowed people to recognize that Earth is not the center of the universe. Galileo's telescope revealed four moons orbiting Jupiter (not Earth), the phases of Venus, the mountains of the Moon, and many more stars than are visible to the naked eye. More importantly, Galileo's experiments established the principle of inertia which countered the physical arguments the Greeks used against a rotating and moving Earth.

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