Retrograde motion is an apparent motion of any Sun-orbiting object as viewed from one object to another object further from the Sun than the one the viewing is made from. Since we are based on the Earth our views of the planets and other objects orbiting the Sun come from that perspective. As I posted the other day, outer planets. for example, orbit the Sun more slowly than the Earth. So there are times when, as the Earth passes an outer planet, the outer planet appears to slow down and then move backward, to the west. After a period of time the planet resumes its regular eastward, or prograde, motion.
The retrograde motion of an outer planet is easy to understand and even visualize, however the two inner planets also undergo retrograde motion. Half of each their respective orbit is eastward, prograde, but when they reach the opposite side of the Sun their orbit carries the inner planet around the Sun through inferior conjunction (between the Earth and Sun) toward the west, retrograde, for the other half of the orbit.
For the record each ‘side’ of the orbit is known as an elongation. So at western elongation the inner planet is on the west side of the Sun and rises before the Sun rises – a morning planet. Half an orbit later the inner planet is at eastern elongation and rises and sets after the Sun – an evening planet.
Venus is currently very prominent as an evening planet over the western horizon at sunset. The planet Mercury has recently passed through superior conjunction is gradually moving into the evening skies. Mercury is moving in prograde motion while Venus is moving in retrograde motion.
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