![]() Kepler, using astronomer Tycho Brahe’s pre-telescopic observations, was able to trace out the elliptical paths of the planets as they orbited the sun. For example, Mars returns to the same position in its orbit every 687 days.Īs Kepler knew the dates when a planet would be at the same position in space, he could use the different positions of the Earth along its own orbit to triangulate the planets’ positions, as illustrated above. Planets (approximately) repeat the same path as they orbit the sun, so they return to the same position in space once every orbital period. Johannes Kepler devoted years of his life to understanding the motion of Mars, and he cracked this problem with a most ingenious weapon. The circular motions of Ptolemaic and Copernican models resulted in large errors, particularly for Mars, whose predicted position could be in error by several degrees. Johannes Kepler triangulated the position of Mars by using observations of Mars when it returned to the same position in its orbit. Furthermore, the original Copernican model was no simpler than the earlier Ptolemaic model.Īs 16th Century astronomers did not have access to telescopes, Newtonian physics, and statistics, it wasn’t obvious to them that the Copernican model was superior to the Ptolemaic model, even though it correctly placed the sun in the centre of the solar system. In some cases the position of Mars is in error by 2 degrees or more (far larger than the diameter of the moon). Copernicus disposed of the equant, which he despised, but replaced it with the mathematically equivalent epicyclet.Īstronomer-historian Owen Gingerich and his colleagues calculated planetary coordinates using Ptolemaic and Copernican models of the era, and found that both had comparable errors. The Copernican planets still travelled around the solar system using motions described by the superposition of circular motions. ![]() Unfortunately, the original Copernican model was loaded the Ptolemaic baggage. The original Copernican model has similarities to Ptolemaic models, including circular motions and epicycles. And it thus became the primary means of explaining planetary motion for over a millennium. But, to his credit, Ptolemy’s model predicted the positions of planets in the night sky with an accuracy of a few degrees (sometimes better). Ptolemy explained planetary motion using the superposition of two circular motions, a large “ deferent” circle combined with a smaller “ epicycle” circle.įurthermore, each planet’s deferent could be offset from the position of the Earth and the steady (angular) motion around the deferent could be defined using a position know as an equant, rather than the position of the Earth or the centre of the deferent. This model, from an Arabic copy of Ptolemy’s Almagest, is illustrated above. Qatar National LibraryĪncient Greek astronomers produced geocentric (Earth-centred) models of the solar system, which reached their pinnacle with the work of Ptolemy. They’re a bit brighter than most stars and twinkle less, but otherwise look like stars.Ī page of an Arabic copy of Ptolemy’s Almagest, illustrating the Ptolemaic model for a planet moving around the Earth. At first the planets don’t really distinguish themselves from the stars. ![]() Imagine you’re an astronomer from antiquity, exploring the night sky without the aid of a telescope. These notes give us a clue to the labour, insights and genius that drove the Copernican revolution. We can gain insights into how this profound shift unfolded by looking at the actual notes left by the astronomers who contributed to it. Rather, it took almost a century of new theory and careful observations, often using simple mathematics and rudimentary instruments, to reveal our true position in the heavens. In antiquity people believed the Earth was the centre of the solar system and the universe, whereas now we know we are on just one of many planets orbiting the sun.īut this shift in view didn’t happen overnight. It’s not a stretch to say the Copernican revolution fundamentally changed the way we think about our place in the universe. ![]()
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