Thursday December 29th the outermost of the 8 planets, Neptune, ends its westward retrograde motion around the Sun and will resume direct motion, moving eastward as we view Neptune from the Earth. In this graphic the location of Comet 45P/Honda-Mrkos-Pajdušáková is shown. Keep watching that space as on the 31st the thin waxing crescent Moon will be near the comet.
still more cosmic thrills…
Currently located in the constellation Pisces the Fishes Neptune will be joined by the red planet Mars on New Year’s Eve when the two will be separated by less than 0.5o as seen in this simulated view through a 25mm eyepiece on an 8″ Reflector. Mars has an apparent magnitude of 0.88 compared with Neptune’s apparent magnitude of 7.8. And for a real comparison check out Venus shining at an apparent magnitude of -4.32.
Retrograde motion is an apparent motion that the outer planets, relative to the Earth, have. It is an apparent motion that looks as if the outer planet stops it normal direct motion to the east and reverses direction to the west. After a period of time the apparent westward motion ends and the planet resumes its normal orbital path to the east. Retrograde motion happens as the faster moving Earth catches up with and then passes by the outer planet. It is during this time that the backward apparent motion happens.
The two inner planets also have retrograde motion but it is a result of their orbit around the Sun and not the Earth passing them by. For approximately one-half of their orbit they move east, from western elongation through superior conjunction to eastern elongation. Then at eastern elongation the inner planet starts moving westward through inferior conjunction to western elongation.
Read a little more about retrograde motion in my February 2012 Scope on the Skies column, drawkcab planets, in Science Scope Magazine.
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.