Aristotle (384-322 BC) was among the first to recognize the fact of our planet being a round sphere. He observed lunar eclipses and noticed that only a round sphere could imply a circular shadow. Isaac Newton first proposed that Earth was not perfectly round. Instead, he suggested it was an oblate spheroid—a sphere that is squashed at its poles and swollen at the equator. The distance from Earth’s centre to sea level is roughly 21 kilometres (13 miles) greater at the equator than at the poles. Earth’s shape also changes over time due to a menagerie of other dynamic factors. Mass shifts around inside the planet, altering those gravitational anomalies.