![]() Then, as Earth completely overtakes and zooms past, the passenger must look back at Mars. To the passenger, it seems like Mars has moved backwards a little bit, relative to the crowd. At the moment Earth overtakes, our passenger sees Mars against a different crowd-a lady draped in a flag, sitting a whole block of seats before hotdog guy. We’ll use a person as a landmark: say there’s a guy with a hotdog.īut Earth is moving so fast that it overtakes Mars before it reaches the hotdog guy itself. ![]() As a passenger in Earth’s car looks forward at Mars, they see-in a snapshot of time-Mars set against a particular part of the crowd. ![]() At first, as Earth approaches to overtake, Mars looks like it’s moving the same way as Earth is. So let’s think about what this overtaking motion looks like from Earth. ![]() The trick with retrograde motion is that it’s all in the perspective. Earth is travelling more quickly, so at regular intervals, it overtakes Mars on the inside and zooms on around the Sun, while Mars continues along behind, never to catch up. Imagine Earth and Mars are racecars, with Earth on the inside lane and Mars on the outside. Because Mars is further out than Earth, it orbits slower. Simple physics tells us that planets further from the sun orbit more slowly than those closer to the sun. So if planets aren’t moving in orbits within orbits, what’s happening when they loop around? They could scrap the epicycles entirely, and with Kepler’s refinements-realising that planets orbit in ellipses, and not circles-we suddenly had a precise model that explained away retrograde motion. Nonetheless, it was the best model they had for centuries.Ĭue Copernicus, who suggested a teeny adjustment to model: how about we put the Sun at the centre of the solar system, instead of the Earth?Īfter much grumbling, astronomers realised that this actually simplified all their complications. Astronomers realised that it wasn’t entirely accurate, because it often failed to predict the movements of the planets as precisely as it should. As you can imagine, this model got extremely complicated very quickly, creating layers upon layers of epicycles to explain the complication of the heavens. Ptolemy proposed the concept of epicycles, where the planets completed orbits within their orbits, moving in smaller, looping paths on their way around the Earth. This is known as retrograde motion, and astronomers tried extremely hard to make it fit into their geocentric model of the universe. Sometimes, their motion even seemed to reverse direction for a while, and they’d loop around before continuing on their normal path. Although they looked like points of light just like the stars, the planets wandered in their own paths against the background sky. However, something threatened this perfect view: the planets. It appeared that the heavens orbited the Earth, so it was only natural to suppose that we are the centre of the universe, and everything moves in perfect circles around us. To our ancestors, the stars moved from East to West across the sky just like the sun, completing one crossing every night. In our simple model of coplanar orbits, we need not make this fine distinction.Racecars, Retrograde Motion, and the Red Planet it is at the moment of changing direction) in right ascension, and when it is stationary in ecliptic longitude. The second small point to notice is that, for precise work, it is necessary to distinguish between when a planet is stationary (i.e. This is not the case, and indeed there is a small exercise on this point in the penultimate paragraph of this chapter. It is sometimes believed by the unwary that the stationary points in the orbit of an inferior planet occur when the planet is at greatest elongation from the Sun. ![]() We would believe the same today if we hadn’t read differently in books and on this web site. It is small wonder that the ancient astronomers, believing that the Earth was at the centre of the solar system, believed in their system of deferents and epicycles. As seen from Earth, a planet moves generally eastward relative to the stars, except for a short time near opposition (for a superior planet) or inferior conjunction (for an inferior planet) when it briefly retrogrades towards the west. Therefore at some point in its orbit a planet will be stationary relative to the stars at the moment when its proper motion changes from direct to retrograde. It will, however, be obvious that a superior planet at conjunction, or an inferior planet at superior conjunction, will move eastward (“direct” or “prograde”) relative to the stars. Thus a superior planet at opposition moves westward (it “retrogrades”) relative to the stars, and an inferior planet at inferior conjunction also moves westward (it “retrogrades”) relative to the stars. ![]()
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