![]() ![]() ![]() Scientific models are used to test our understanding of the laws of science by predicting the behavior of a system. (Image credit: via Getty Images) Assumptions of the geocentric model The geocentric model was refined by Claudius Ptolemaeus (also known as Ptolemy) in his treatise Almagest. Related: What's the story behind the stars? According to NASA, The geocentric model also conformed to religious beliefs at the time as many Greek philosophers and astronomers believed that the gods created man and the heavens are divine then of course we must lie at the center of it all. It could explain why stars appear to rotate around Earth once per day and why planets move differently to stars. As stars appeared to move much slower than the planets, they were placed in the outermost sphere, furthest away from Earth, according to Lumen Learning.ĭespite the complexity of the model, this view was widely accepted for many years. The order of the solar system with regards to the geocentric model, according to Penn State University is Earth (stationary and at the center), moon, Mercury, Venus, sun, Mars, Jupiter and Saturn. They believed that the Earth was at the center with the sky or 'heavens' rotating around it in a series of layered spheres.Īccording to the educational website Lumen Learning, Ptolemy's complicated geocentric model stated that a planet moves in a small circle (known as an epicycle), the epicycle then moves around Earth in a larger circle (known as a deferent). Something’s almost always in retrograde.When gazing up at the night sky from what feels like a fixed reference point and witnessing the stars and planets dance across the sky we can understand why the ancient Greeks adopted the geocentric view of the universe. Furthermore, at any one time, 40% of the outer planets are in retrograde motion anyway. Of course, the planet isn’t really changing its motions at all, it’s only our perspective of the planet that’s changing. When this happens, astronomers say that Mercury is in retrograde.Īstrologers seem to think that retrograde motion is an unlucky or bad situation because it goes against a planet’s natural movements. It’s still going down the highway at a high speed, but it seems to be going back and forth compared to you. Imagine you’re driving next to a car in the freeway which is speeding up and slowing down. When Mercury is furthest from the Sun, it’s at the slowest point in its orbit, and this gives the Earth a chance to “catch up”. Mercury moves faster than the Earth as it travels around the Sun however, Mercury has a highly elliptical orbit, so the speed of its orbit changes. The curve of the fingers indicates the direction all the planets orbit around the Sun. The direction of the thumb points in the direction of the Sun’s northern pole. Hold your right hand out, make a fist and stick the thumb up. The motion of the planets around the Sun follow the right-hand rule. So let’s understand retrograde motion, and look at what is Mercury retro in particular. The retrograde motion of Mercury and the other planets is due to our relative positions in orbit. The Earth is just another planet, and they’re all orbiting the Sun together. It wasn’t until Nicolaus Copernicus developed his Sun-centered model of the Solar System that the bizarre retrograde motion of Mercury and the rest of the planets finally made sense. If the planets were orbiting the Earth, why would they go backwards? Why would Mercury go retrograde? They developed elaborate models where the planets followed a spiraling path around the Earth to account for this retrograde motion. One complication of this model was the planets which took these very predictable retrograde paths in their orbit. In ancient times, people thought the Earth was the center of the Universe, and all objects in the night sky orbited around us. When a planet goes backwards, it’s called retrograde, and one of the best planets for this is Mercury. Some times the planets would slow down, go backwards, and then go forwards again. But the movements of the planets were puzzling to ancient astronomers. These moving stars were known as planets, and there were 5: Mercury, Venus, Mars, Jupiter and Saturn. Some stars were brighter than the rest, and seemed to move across the sky from night to night. Ancient people have known about the planets since we were able to look up.
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