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Ptolemy (100-170 CE) was a mathematician, astronomer, and geographer. He believed that a celestial body traveled at a constant speed in a perfect circle.
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Copernicus (1473-1543) was a mathematician and astronomer. He formulated a model of the universe that placed the sun, rather than the Earth at the center. This idea is known as heliocentric.
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Galileo (1564-1642) was an astronomer, physicist, and engineer. He improved the telescope and he observed planetary motion that supported the heliocentric model.
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Now, with rockets, we have begun to explore space. Heliocentric is what we know is true today. While some ancient astronomers believed in a geocentric universe. We have sent humans on orbital missions and on missions to the moon, but recently, we have relied on unmanned probes, like the Mars robotic explorers. We use satellites to gather information about our planet and others. Also, there are the outer planets - Jupiter, Saturn, Uranus and Neptune, which we still know very little about.
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Aristotle (384-322 BCE) was a Greek philosopher. He was the first genuine scientist. His model placed the moon, sun, planets, and stars on a series of circles that surrounded Earth.
