![]() For Aristotle, this meant that the Earth had to be stationary, and the planets, the Sun, and the fixed dome of stars rotated around Earth.įor nearly 1,000 years, Aristotle’s view of a stationary Earth at the center of a revolving universe dominated natural philosophy, the name that scholars of the time used for studies of the physical world. He saw no sign that the Earth was in motion: no perpetual wind blew over the surface of the Earth, and a ball thrown straight up into the air doesn’t land behind the thrower, as Aristotle assumed it would if the Earth were moving. One camp thought that the planets orbited around the Sun, but Aristotle, whose ideas prevailed, believed that the planets and the Sun orbited Earth. The ancient Greek philosophers, whose ideas shaped the worldview of Western Civilization leading up to the Scientific Revolution in the sixteenth century, had conflicting theories about why the planets moved across the sky. The world has scarcely become known as round and complete in itself when it was asked to waive the tremendous privilege of being the center of the universe.” -Johann Wolfgang von Goethe “Of all discoveries and opinions, none may have exerted a greater effect on the human spirit than the doctrine of Copernicus. 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.“We revolve around the Sun like any other planet.” -Nicolaus Copernicus 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. Galileo’s telescopic observations of the planets, including the phases of Venus, demonstrated that planets travel around the Sun. 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 Copernican Revolution placed the sun at the centre of our solar system. Planets even temporarily reverse direction, exhibiting " retrograde motion". Planets appear to speed up and slow down as they cross the sky. Indeed "planet" is derived from the Ancient Greek for "wandering star".Īnd planetary motion isn't simple. From night to night, the planets gradually moved with respect to the stars. In antiquity, what really distinguished planets from stars was their motion through the sky. They're a bit brighter than most stars and twinkle less, but otherwise look like stars. 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. But this shift in view didn't happen overnight.
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