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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Neptune at Opposition – 2020

   Friday September 11th the outer planet Neptune reaches a position in its orbit around the Sun when it is on the opposite side of the Earth from the Sun. This coincidentally is known as opposition, and it is an orbital position which only the planets further from the Sun than the Earth may reach.
   At opposition an object orbiting the Sun beyond Earth’s orbit rises and sets in a fashion similar to our Moon when it is at full phase, in that the object at opposition rises at sunset and sets at sunrise.
   Currently Neptune rises at sunset and by late evening is over the southeastern horizon. Neptune has an apparent magnitude of 7.88 so it is beyond unaided-eye visibility but could be visible with large aperture telescopes or with a camera.

   
   
   

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Planetary Line-Up Ecliptic Style

   Look at the arrangement of the planets in the graphics below. One of the defining aspects of our solar system is the arrangement of the planets outward from the Sun. Not by size or distance but rather how their respective orbital paths around the Sun are all vertically arranged near the Earth’s orbital path, or as it is typically referred to as the Ecliptic or the Plane of the Ecliptic. The 8 classic planets all orbit the Sun with an orbital path that is up to about 8o from the ecliptic. This is called inclination. The table shows inclination relative the Earth’s orbit and also relative to the Sun’s center, its equator.
    Click on this link to read a previous posting (Tales Along the Ecliptic) about the ecliptic and inclination.
   During this week as the Moon moves eastward it will pass by the outer ringed planet Neptune Tuesday and Wednesday evenings September 1st and 2nd as the graphics show. However given the tremendous difference in apparent magnitude between the two (full Moon: -12.64 ; Neptune: 7.81) Neptune will not be visible, at least not while the Moon in nearby.


   Keep following the Moon as it orbits eastward toward the planet Mars when on September 5th Mars and the waning gibbous Moon will be less than 1o apart.
   
   
   

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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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Venus at Western Elongation

inner-planets-positions   Wednesday August 12th the inner planet Venus reaches the point in its orbit called greatest western elongation. As this graphic shows the inner planet Venus, or Mercury, is more or less at a right angle (90o) from the Sun and Earth at western elongation. From the surface of the Earth, your backyard, for example, Venus is to the right, or western side of the Sun and is rising before the Sun.

   On the day of the elongation Venus will be very visible over the southeastern horizon at or before sunrise local time. Venus is joined by a few other planets, Jupiter, Uranus, Mars, and Dwarf Planet Ceres. The waning crescent Moon is nearly straight overhead (at least from 40oN) and a few degrees from the open star cluster the Pleiades. In this graphic the orbital path of Venus has been added, and also an arrow to show the direction Venus will now be moving toward (eastward).

   At western elongation Venus, or for that matter Mercury the other inner planet, is as far out from the Sun as we see them and as a result Venus or Mercury will rise at the earliest time for this orbit. Locally Venus rises that morning at around 3:00 am CDT. On the day of the western elongation Venus will be 45.8o from the Sun. From western elongation forward Venus or Mercury will be moving eastward toward the Sun and each day rising closer and closer to the time of sunrise. As the planet moves eastward it is moving further away from the Earth toward superior conjunction on the opposite side of the Sun.


    As the distance between the Earth and Venus, or Mercury, increases combined with the decreasing angle between the planet, the Earth, and the Sun, Venus or Mercury decreases in apparent size and also waxes through gibbous phase shapes but we never see it at a full phase since that is at superior conjunction.

   
   
   

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Moon – Mars Conjunction

   Early mornings before sunrise the waning gibbous Moon is working its way eastward toward new Moon phase. Along the way the 20-day old waning gibbous Moon will be about 1o from the ‘Red Planet’ Mars on Sunday morning August 9th . Both will fit within the field of view of binoculars and should fit within the field of view of a low-power widefield type telescope eyepiece.
   The contrast in apparent magnitude is quite a range, from the Moon’s -12.0 to the -1.22 apparent magnitude of Mars.

   
   
   

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Uranus at Western Quadrature – 2020

   Sunday August 2nd the position of the planet Uranus, with respect to the Earth and the Sun, places this ringed planet at what is called western quadrature. At that orbital position Uranus, and actually any outer planet, is at a 90 degree angle from the Earth as this graphic shows, and also this graphic. Think third quarter Moon as that is a fair comparison of the relative positions of Earth, Sun, and Uranus.
    At western quadrature Uranus leads the Sun across the sky from east to west as the Earth is rotating, meaning that Uranus rises before the Sun and also sets before the Sun.

   
   
   This is a short video clip from a much longer video that I made as part of a live musical performance called “Orbit” at the Gottleib Planetarium in Kansas City Missouri during May 2011.

   
   
   

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Have Some ‘Pie’ – View Some Planets

   Saturday March 14th, 3/14/2020, is perhaps better known as Pi Day given that 3.14, the value for Pi, is also the month and day number. Learn more about Pi and see what NASA has planned for this special day at the NASA Pi Day Challenge web site.
   Saturday the 14th, weather depending, is another day this year when you have a choice of planet viewing – all of the six visible planets in fact. In the early morning skies look east and southeast for a line-up of planets ranging from Mercury, to Saturn, Mars, Jupiter, and the 19-day old waning gibbous Moon. In the evening after sunset Venus shines brightly over the western horizon. Venus is near the outer ringed planet Uranus, but Uranus is not considered a naked-eye visible planet in most skies.
   Mercury, Venus, Mars, Jupiter, and Saturn are all visible, but that’s only five visible planets. So where is the sixth visible planet?

   
   
   

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Orion, Batter Up!

   Late evening Tuesday March 3rd the 9.5-day old waxing gibbous Moon will be near the stars that make up the weapon held in Orion’s upraised right arm. Sort of looks like Orion is swinging a baseball bat at the Moon. And given the shape of the Moon’s orbit the pitch must be a curve ball!

   
   
   

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The Moon and the Sisters

   Sunday evening March 1st the 7.5-day old nearly first quarter Moon, but still a waxing crescent Moon, will be 7-8o from the open star cluster the Pleiades. A few degrees from the Pleiades is another open star cluster the Hyades.
   These two open star clusters are part of the constellation Taurus the Bull. The Hyades make up the face of the Bull, while the Pleiades are located on the Bull’s shoulder. The Pleiades are also known as the “Seven Sisters”.
   
   
   

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