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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September Moon at Ascending Node

   Wednesday September 10th the 22-day old last quarter Moon will be crossing the plane of the ecliptic moving north. This is known as the ascending node, one of two intersections the Moon’s orbital path has with the ecliptic. The ecliptic is actually the Earth’s orbit, and the Moon’s orbit is inclined about 6o from the ecliptic. So there are two node intersections, the ascending and descending nodes.
   On the morning of the node crossing the 22-day old last quarter crescent Moon will be to the east of the reddish star Aldebaran in Taurus the Bull. If you are a late night type watch for the Moon to rise around midnight local time giving you an opportunity to see Mars, Saturn and Jupiter arranged across the sky.

   On the other hand if you are like me and an early morning type then look for the Moon to be high above the southern horizon an hour or two before the Sun rises. Venus will be over the eastern horiozn and Mars over the southwestern horizon.

   Does our Moon actually go around the Earth as many graphics show? From our perspective on the Earth the Moon appears to circle around the Earth. However, in reality, the Moon orbits the Sun together with the Earth*
*Click here to read my 2006 Scope on the Sky column “The Real Shape of the Moon’s Orbit”. (PDF)
   
   
   
   
   
   
   

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Moon – Hyades Conjunction

   Spoiler Alert: This is an early morning thing! The 21-day old Waning gibbous Moon rises late tonight (September 8th) just before midnight and then will be visible above the horizon through the rest of the morning pre-dawn hours on Wednesday September 9th. During that time the Moon will be passing the open star cluster, the Hyades, and will be about 4-5o from the reddish star Aldebaran in Taurus the Bull. From mythology the v-shaped open star cluster is the face of the Bull while Aldebaran, with its reddish color, represents the ‘angry eye’ of Taurus as it prepares to attack.

   An open star cluster, like the v-shaped Hyades and the dipper-shaped Pleiades make for interesting views using binoculars, and especially when another celestial object passes by.

   
   
   

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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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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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Mercury at Western Elongation

   On Wednesday July 22nd Mercury, the innermost planet, will reach its orbital position known as greatest western elongation at 20.1o. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows. Even though it sounds confusing at western elongation for either Mercury or Venus the inner planet will be to the right of the Sun as we view them, meaning that at western elongation an inner planet rises in the east before the Sun rises. And at eastern elongation with the inner planet on the left side of the Sun the inner planet follows the Sun across the sky setting after the Sun sets.

   From our perspective the orbits of Mercury and Venus appear to move from one side of the Sun to the other – out to the left (east) from the Sun to eastern elongation, then reverse and move westward (inferior conjunction) between the Earth and the Sun to western elongation. From there the inner planet moves eastward going behind the Sun (superior conjunction) and eventually reappearing on the eastern side of the Sun for an eastern elongation. Repeat over and over – do not stop!

   There is a lot to see and a few things you cannot see on the morning of Mercury’s western elongation. Going by relative apparent magnitudes Mercury (0.33), Venus (-4.43), Mars (-0.89) are all easily visible as bright to very bright star-like objects. On the other hand those not seen with the naked-eye but are still above the horizon in the morning skies include: Uranus (5.80), Eris (18.64), Neptune (7.84), Ceres (7.40)

   
   
   
   
   

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Comet in the Clouds

   This morning, July 13th, the sky was generally overcast with thin status type clouds in most directions, including the northeast where the comet was just starting to appear over the trees marking my local horizon. Fortunately the clouds were still transparent enough for the comet to be just barely visible to the naked-eye, but very visible with time exposure pictures.
   I was hoping to position my camera so that the Baseball player would look as if he were swinging at the comet but the clouds started to thicken in that direction as I moved off the road and into some tall grasses.
   The other planets that were very visible yesterday morning were hidden or blurred by the clouds. Jupiter shined through the clouds but not Saturn or Mars. The Moon light was reflecting off clouds brightening the sky in that direction. And Venus and Aldebaran were somewhat visible but it took a time exposure picture to catch the light from the rest of the stars making the v-shaped part of the Hyades.

   
   
   

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