But every once in a while a planet will also appear to move in a retrograde direction. A composite of Comet NEOWISE captured over Dizin, Iran, by Reyhaneh Valipour on 21 July 2020. It’s generally only comets, with eccentric orbits, that travel around the Sun in a clockwise, or retrograde, direction. Below are two pictures of time-lapsed retrograde motion of Mars. This is known as normal or direct motion. With heliocentrism, explaining retrograde motion was simple. For the purposes of determining when retrograde motion occurs, motion of the planets perpendicular to the ecliptic plane does not matter. It’s worth noting that one of the reasons heliocentrism replaced geocentrism (the notion that the Earth is in the center of the Solar System) is that geocentrism couldn’t explain the retrograde motion of planets. ![]() (This thought experiment works with different modes of transportation, from walking to cycling to flying in airplanes.) As the faster car overtakes the slower, it will appear (from the vantage point of the faster car) as though the slower car stops, then moves backward, even though its actual speed has not changed. One travels slightly faster than the other. The planets inner to Mars: Mercury, Venus, and Earth will move retrograde as they overtake Mars. They move retrograde when they are opposite the sun in the sky. Imagine two cars traveling in the same direction in two different lanes. For outer planets: Jupiter, Saturn, Uranus and Neptune, they will move retrograde as Mars passes them at opposition. The planets Venus and Uranus spin clockwise, which is considered retrograde motion. We can actually see this effect on Earth. Rotation in the opposite direction of what is traditionally seen in the cosmos is called retrograde. ![]() Notice how Mars seems from Earth to travel in a loop. “It just appears to do so because of the relative positions of the planet and Earth and how they are moving around the Sun.” Graphic showing the retrograde motion of Mars. Retrograde motion refers to the change of direction of the planets as they wander through the fixed background of the stars. ![]() “It is not REAL in that the planet does not physically start moving backwards in its orbit,” NASA states.
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