![]() ![]() This mechanism works for all the superior planets. ![]() The same thing happens when you pass by a slower-moving car on the highway – for a moment, it appears to move in the opposite direction. When we pass Mars, it seems to be moving “backward” because we're moving faster than it is. This superior planet moves slower in its orbit than the Earth. Retrograde motion is an optical illusion caused by differences in the planets’ orbital speed. What causes the apparent retrograde motion of the planets? This westward movement is called apparent retrograde motion. This motion shouldn’t be confused with the daily motion of the planets and the Sun in the sky which goes from east to west and is caused by the Earth’s rotation on its axis.Īt specific periods of time, a planet can start moving “backward” – from east to west. Astronomers call it direct or prograde motion. When a planet seems to reverse its direction in the sky, it’s called retrograde motion (from the Latin word retrogradus – "going backward").ĭay to day and week to week, as the Earth revolves around the Sun, the planets in the sky typically move in the same direction as the Sun – from west to east. See Infographic What does it mean when a planet is in retrograde? 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.Want to know what causes the apparent retrograde motion of the planets? Check out this infographic to learn how retrograde motion works. 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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