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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Uranus is Backing Up

July 22nd to December 22nd :  Uranus at 30-day intervals   Well that’s a good to know!
   Saturday August 15nd the outer ringed planet Uranus appears to halt its regular eastward orbital motion and begins moving toward the west, in retrograde motion. Retrograde motion is an apparent motion to the west that any outer planet relative to the Earth appears to do whenever the faster moving Earth passes by. Sort of like passing a car on the highway. You know both vehicles are moving in the same direction but from your perspective it could appear that the other car is moving backward as you go by.
   Regardless, retrograde motion for the outer planets happens at regular intervals as the Earth pass each one. It is always more than a year and always a little further to the east when the retrograde motion begins each time. This is because the outer planet is also moving eastward.
   Mars, another outer planet, begins its retrograde motion next month.
   
   

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Two More Go Retro!

   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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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.

   
   
   

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Mercury Ends Retrograde Motion

   
mars-retrograde-ani   When the Earth’s orbital speed is compared to an outer planet there will be a time period when the faster moving Earth passes the outer planet. This sets up a temporary illusion where it appears as if the outer planet has reversed direction and is now moving backward toward the west, or in retrograde motion.
   On the other hand the two inner planets, Mercury and Venus, also revolve around the sun toward the east and go through a period of retrograde motion. However theirs is not as a result of the Earth’s faster orbital speed but rather it is their faster orbital speed compared with the Earth’s that gives them their retrograde motion. And unlike the apparent retrograde motion for an outer planet, Mercury and Venus do actually move in retrograde motion as this animated graphic is showing.
   So, on Saturday December 23rd the innermost planet, Mercury, ends its retrograde motion for this particular orbit and resumes its eastward motion. In the animated graphic the size for Mercury is exaggerated and the animation is set to a 1-day interval. It is showing Mercury moving westward as Venus is moving eastward. As Mercury moves along its orbit it eventually curves around and starts moving toward the east. This may be close to the day when Mercury reaches its furthest separation from the Sun on the west or right side of the Sun. That point is known as Greatest Western Elongation and is on January 1st. The counterpoint to this is when Mercury, or Venus, reaches their respective Greatest Eastern Elongation on the left or east side of the Sun.

   
   
   
   

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.

Mercury Begins Retrograde Motion


   Sunday April 9th the innermost planet Mercury reaches a point in its orbit around the Sun where it moves toward the west as we view Mercury from the Earth. Typically when one hears this the term Retrograde Motion comes to mind, but with regard to an outer planet (relative to Earth). However while an outer planet’s retrograde motion is apparent the westward motion of an inner planet, Mercury or Venus, is very real. For Mercury or Venus retrograde motion begins around the time of eastern elongation and continues through inferior conjunction to around the time of western elongation. For Mercury this particular retrograde motion will end next month on the 17th.

   
   
   

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.

Mars and Neptune Conjunction, inlcuding the Moon and Venus


   A previous post described the daily motion of Mars as it moves toward the planet Neptune. Well Monday evening January 2nd the planets Mars and Neptune, and our Moon (a 5-day old waxing crescent Moon) will all fit within the field of view of binoculars with Mars and Neptune separated by about 1.5o.

   In that post I had also mentioned that were Neptune observed from the surface of Mars then Neptune would be starting its retrograde motion. Actually I was not entirely correct as I was basing this on how the two planets looked from Earth rather than Neptune as seen from Mars. Compare the distance between the Earth and Mars and the difference between the orbital speed of the two planets. Then consider Mars and Neptune where there is a greater difference between the orbital speeds of Mars and Neptune as well as the distance between the Mars and Neptune then there is between the Earth and Mars.

   What this means is that Neptune, as viewed from Mars, will not actually begin to retrograde until around the middle of June.

   
   
   

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.

Neptune Moves Forward and Sees Red

   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.
But wait…
still more cosmic thrills…

mars-neptune-tele   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.

Mercury Begins to Retrograde

mercury-retrograde-ani   The innermost planet Mercury is about 2 weeks after its Greatest Eastern Elongation on August 16th. As the name suggests Mercury has been on the east (left) side of the Sun and visible in the evening skies after sunset. From a combination of the Earth’s and Mercury’s orbital speed and angle relative to each other Mercury is now seen as moving westward toward Inferior Conjunction on September 12th. Mercury will continue moving westward until it reaches its Greatest Western Elongation on September 28th.
   Retrograde or westward motion is probably best known as it relates to the apparent motion an outer planet relative to the Earth has. mars-retrograde-aniThis apparent westward motion comes about as the Earth, moving faster than an outer planet, catches up with and passes the slower moving outer planet. As this happens the outer planet appears to slow down, pause, then move westward for a period of time lasting from days to weeks. At some point the angle between the Earth and the outer planet has shifted enough so that it appears as if the westward motion has ended and the outer planet resumes its direct or eastward motion around the Sun.
   For an outer planet this is an apparent motion relative to the stars in the background, while for the two inner planets they really do move westward as they orbit the Sun between eastern and western elongation, and along the way passing between the Earth and the Sun at inferior conjunction.

   
   
   

Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to go to bobs-spaces.

Mars Moves Forward

   Thursday June 30th Mars ends its apparent westward motion (retrograde motion) relative to the stars in the background and resumes its eastward or direct motion. The location of mars for this particular retrograde has the ‘Red Planet’ near a reddish star with a name suggestive of how similar it appears to Mars. This is the star Antares, the heart of Scorpius the Scorpion.
   Antares could be thought to literally translate to ‘anti-Ares’ suggesting that this reddish celestial object should not be confused with Ares, the Greek god of War. Many stars have several names originating several centuries ago and in different languages however many of the translations have Antares meaning ‘equal of Mars’.
   Right now Mars is just past its greatest apparent magnitude, its brightest. This typically happens at around Mars opposition, but Mars is even brighter if its opposition is around the time of its perihelion and the time when Earth is at aphelion.
   Mars is now again moving toward the east and it will gradually move closer to Antares coming within about 1o from Antares on August 24th. However the Earth is also revolving around the Sun so at the same time that Mars is closing in on Antares that part of the sky will be moving further west and setting earlier.

   This year Mars and Antares will still be above the horizon during August and by then local time for sunset will also be earlier.

   
   
   
   
   
   
   
   
   

Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to go to bobs-spaces.net.