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Eratosthenes calculated the tilt of the Earth's axis. This helped future astronomers understand the seasons.
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Claudius Ptolemy synthesized Greek knowledge of the known Universe. His work enabled astronomers to make accurate predictions of planetary positions and solar and lunar eclipses.
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Nicolaus Copernicus created the heliocentric(sun-centered) solar system. This established the fact that the planets revolved around the sun, planets orbited at different speeds, and faster planets would pass slower ones.
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Tycho Brahe discovered the Supernova. When Tycho Brahe discovered the supernova he proved it was possible for the universe beyond the sun to change.
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Johannes Kepler created three laws of planetary motion. This established that planets orbit in ellipses, not in circles, as was the common belief at the time.
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Sir Isaac Newton created The universal law of gravitation. It explains the motion of planets around the sun. It helps in determining the trajectory of astronomical bodies and to predict their motion.
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Albert Einstein created the Theory of special relativity. This determined that the laws of physics are the same for all non-accelerating observers, and he showed that the speed of light within a vacuum is the same no matter the speed at which an observer travels.