# Mr Howie Chu**Honours graduate**
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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 Ptolemyâs 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 planetâs period and its distance
from the Sun. Newton would later mathematically prove many of Keplerâs 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. Newtonâs law of gravitation would eventually be supplanted by Einsteinâs general
relativity which proposed gravity as a manifestation of the curvature of space around massive
bodies. General relativity also successfully explained the discrepancy between Mercuryâs anomalous
orbit and Newtonâs theory, and marks the final chapter in our understanding of planetary motion.