Thursday September 24th the second largest of the known Dwarf Planets, Pluto, ends its apparent retrograde motion and resumes its regular direct or eastward motion in its 248 year (Earth years) long revolution around the Sun. Retrograde motion is an apparent westward, or backward motion relative to the regular eastward orbital motion nearly all solar system Sun orbiting objects follow. Any of these objects further from the Sun than the Earth seem to slow down, stop, and then reverse their direction for a period of time lasting from several days to a few weeks. The apparent reverse in direction, retrograde motion, happens as the faster moving Earth passes by the more distant object.
While retrograde motion is generally thought of in terms of outer planets the two inner planets, Mercury and Venus, also have retrograde motion but not as a result of the Earth’s orbital motion. From eastern elongation through inferior conjunction to western elongation these two inner planets move westward. And then from western elongation through superior conjunction to eastern elongation they move eastward.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.