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January 2020

Mr Howie Chu

Honours graduate


Office: 721 |

Honours thesis

Planetary motion: a historical perspective

Ever since ancient times, human beings have been observing the skies and motion of the stars. With beginnings in geometry and algebra in Egypt and Babylonia, these observations of nature were developed by the Ancient Greeks into the first known ideas of how the universe could possibly be structured. The Greeks initially proposed a cylindrical Earth, later refining it to a sphere, with the Sun and planets revolving around it. They had some measure of relative distances between and volumes of Earth, Sun and the Moon, and made some attempt to explain the phenomenon of retrograde. The Greeks eventually settled on Ptolemy’s geocentric model of the universe, employing the use of epicycles in the model. Stagnation in astronomical developments followed the Greek era, with geocentrism retained as the standard model throughout this period. The next significant discovery would occur over 1400 years later, when Copernicus put forth heliocentrism, combined with a rotating Earth, to successfully explain the different retrograde characteristics of the inferior and superior planets together with other observations. The revolution would continue with Kepler who discarded what was then considered to be the perfect orbital shape of a circle and took the ellipse instead, and empirically discovered a relationship between a planet’s period and its distance from the Sun. Newton would later mathematically prove many of Kepler’s findings, and introduce new concepts of mass and momentum, laws of motion and the method of calculus in his famous book Principia. He applied them to celestial orbits and gave the world a deeper understanding of gravity. Newton’s law of gravitation would eventually be supplanted by Einstein’s general relativity which proposed gravity as a manifestation of the curvature of space around massive bodies. General relativity also successfully explained the discrepancy between Mercury’s anomalous orbit and Newton’s theory, and marks the final chapter in our understanding of planetary motion.