Jupiter Corrects Its Reversal

jupiter-ani   On Saturday September 12th in a move that comes about annually, Jupiter will once again reverse its direction from moving toward the west to moving toward the east.
   For about the past 4 months Jupiter has appeared to be moving westward (toward the right) rather then toward the east. This happens as the faster moving Earth catches up with and then passes by Jupiter. This apparent westward motion of Jupiter is known as retrograde motion and for Jupiter it’s retrograde motion comes to an end on September 12th when Jupiter resumes its normal orbital motion toward the east, or to the left as we view Jupiter from the Earth.
   Since retrograde is a reference to motion and means backward or reverse motion, then the opposite of retrograde would be prograde motion. So it would be correct to describe Jupiter as having resumed prograde motion. However you would probably get a blank look from those you say this to! So stick with “direct motion” or just say that Jupiter orbits the Sun to the east as we see it from Earth.
   Jupiter is located over the southern horizon after sunset local time. It is about 7-8o to the west from Saturn.

   
   
   
   

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Saturn Goes Retro!

   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.

   Take a short tour of Saturn and some of its moons in this video clip from the longer Orbits video. These were part of a live performance by the group Dark Matter.

   
   
   

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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Uranus is Stationary

   I suppose if you are into twerking then the title of this posting should cause a pause. However.
    All planets orbit the Sun and relative to Earth’s orbit the direction is to the east. Each planet periodically appears to go backward to the west, or in retrograde motion. The outer planets appear to retrograde but it is an illusion created by the faster moving Earth as it passes the outer planet. The two inner planets, on the other hand (or orbit) do actually move westward as they orbit from eastern elongation to western elongation. At the transition between regular or direct motion and retrograde motion, and the transition from retrograde back to direct motion the planet appears to pause in its orbital motion. This apparent pause is referred to as when the planet is stationary.
   Retrograde motion of an outer planet like Uranus will vary in the duration. Uranus began its retrograde motion on July 26th and ends that apparent motion and resumes its east, direct, or prograde motion on December 26th. The animated graphic shows the orbital motion of Uranus during most of 2015 including its retrograde motion. The graphic is based on an Atlas view of the sky and like any map shows some of the coordinate lines. The stars are part of the constellation Pisces the Fishes. The green line is the ecliptic, the blue line is part of the constellation shape pattern..
   
   
   
Caution: Objects viewed with an optical aid are further than they appear.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

Pluto Shifts Out Of Reverse

   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.
inner-planets   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.

   
   
   
   

Caution: Objects viewed with an optical aid are further than they appear.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

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The 2014 MaRetrograde Motion

The 2014 Mars Retrograde Motion

   The ‘Red Planet’ Mars begins its retrograde Motion today. Retrograde motion is the apparent backward, or westward motion that a planet will appear to make at a regular point in its revolution around the Sun. We typically think of retrograde motion as being done by an outer planet from the Earth. This happens when the faster orbiting Earth catches up and passes by an outer planet. As this is happening the outer planet appears to slow down and then reverse its orbital direction toward the west. After a period of time (days to months) the outer planet again appears to slow down and then return to its regular eastward, or direct motion.
An inner planet orbit

An Inner Planet Orbit

   However the two inner planets Mercury and Venus also undergo retrograde motion. Approximately one-half of their respective orbits is eastward as is with all the other planets. This then brings the inner planets to what is know as eastern elongation in the evening skies. The other half of the orbit for Mercury and Venus is toward the west as they move from eastern elongation through inferior conjunction toward western elongation in the morning skies.

Coordinate Positions for Mars

 Date             RA	    Dec
  		 h  m  	  deg  m          
Feb  4		13 32	  -6 54
Feb 14		13 40	  -7 34
Feb 24  	13 45 	  -7 55
Mar  6  	13 45 	  -7 53
Mar 16  	13 41 	  -7 27
Mar 26  	13 32 	  -6 39
Apr  5  	13 19 	  -5 35
Apr 15  	13 04 	  -4 26
Apr 25  	12 51 	  -3 27
May  5  	12 40 	  -2 50
May 15  	12 35 	  -2 43
May 25  	12 34 	  -3 05
Jun 4  		12 38 	  -3 53
Jun 14  	12 46 	  -5 02
Jun 24  	12 57 	  -6 29

   
   
   
Caution: Objects viewed with an optical aid are further than they appear.
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.