The Naked Eye

From before the dawn of history until less than 400 years ago, our Universe consisted of what we could see with our eyes: a sky filled with objects in constant motion. The Earth seemed to rest at the center of a starry sphere, and the Sun, Moon, planets, and stars appeared to move around the Earth. People used measuring instruments to map the stars and to plot the changing positions of the Sun, Moon, and planets in order to understand and predict their motions. Our early ideas about the Universe were largely based on what we learned from these ancient tools.