Labels:text | screenshot | font OCR: In Leonardo's time, there were thought to be four building blocks of nature: earth, water, air, and fire. These four cosmic elements were arranged in concentric spheres according to their relative densities, with earth, the heaviest, at the center. Next came water, then air, and finally, in the upper atmosphere, fire. These beliefs were predicated on centuries of scientific thinking about the universe. Scientists, including Leonardo, accepted Ptolemy's revision of Aristotle's scheme of a universe composed of 55 concentric spheres. According to Aristotle,
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