All Sun orbiting objects further from the Sun than the Earth will have orbital speeds slower than the Earth’s orbital speed. From our perspective on Earth orbital directions are toward the east.
There will come a time when the faster moving Earth will overtake a slower mover and pass by that object much like a car passes a slower moving car on the highway.
During the time that Earth is passing by an outer object there is an appearance that the outer object slows down and then orbits in the reverse direction, toward the west.
This is known as retrograde motion. After a few weeks to a few months, depending on the outer object, the apparent westward motion slows to a stop and then the outer object resumes its regular motion toward the east known as prograde motion.
Such is the situation for outer ringed planet Saturn. It begins retrograde motion this month and will resume its regular eastward, prograde, motion during September.
Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to return to bobs-spaces.
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