Mercury at Eastern Elongation

orbital-positions    On Thursday October 1st Mercury, the innermost planet, will reach its orbital position known as greatest eastern elongation. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows.
   From our perspective the orbits of Mercury and Venus appear to move from one side of the Sun to the other – from superior conjunction, behind the Sun, out to the left (east) from the Sun to eastern elongation, then reverse and move westward through (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!

   The planet Mercury is currently over the western horizon at sunset local time and will remain visible for most of the month.

   
   
   

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Saturn Corrects Its Reversal

   On Tuesday September 29th in a move that comes about annually, Saturn will once again reverse its direction from moving toward the west to moving toward the east.
   Since around May 11th Saturn 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 Saturn. This apparent westward motion of Saturn is known as retrograde motion and for Saturn it’s retrograde motion comes to an end during mid-September when Saturn resumes its normal orbital motion toward the east, or to the left as we view Saturn 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 Saturn 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 Saturn orbits the Sun to the east as we see it from Earth.

   Saturn is located over the southern horizon after sunset local time. It is about 7-8o to the east from Jupiter.

   
   
   
   

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Moon – Jupiter & Saturn Conjunctions

   The waxing gibbous Moon, over the next two evenings (September 24th and 25th, will pass by two of the giant outer planets. The Moon will pass within about 2-3o from Jupiter and about 1-2o from Saturn.
   Off toward the eastern horizon is the innermost planet Mercury and the bluish-white star Spica in Virgo the Harvest Maiden.


   
   
   

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September Moon at Descending Node

   Wednesday September 23rd the 7-day old first quarter Moon crosses the plane of the ecliptic moving south. This is known as the descending node, one of two intersections the Moon’s orbital path (dark green line) has with the ecliptic.

   On the date of the descending node the first quarter Moon will be about 20o to the east from the heart of Scorpius the Scorpion, the reddish star Antares, and about 15-16o west from Jupiter.
   Mercury and Spica are still a couple of degrees apart but low above the horizon as the Sun is setting.

   
   
   
   
   

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Mercury- Spica & Moon – Antares

   Tuesday September 22nd shortly after sunset local time look toward the western horizon for the innermost planet Mercury to be about 1o from the bluish-white star Spica in Virgo the Harvest Maiden. The two should make for an interesting comparison in apparent magnitudes with Spica at 0.96 and Mercury with a -0.01 apparent magnitude.
   The nearly first quarter but still waxing crescent Moon will be about 7-8o from the reddish star Antares in Scorpius the Scorpion. Both are positioned over the southern horizon.

   
   
   

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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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The Red Object Tour: From Mars to Aldebaran


   Following a close encounter (conjunction) on September 5th-6th with the ‘Red Planet’ Mars our Moon will orbit eastward waning toward last quarter phase on September 9th. On the 9th the Moon will have a not as close encounter with another red celestial object. This being the reddish star Aldebaran, the ‘angry eye’ of Taurus the Bull.
   Aldebaran is at one of the open ends of the v-shaped open star cluster, the Hyades, the shape making up the Bull’s face.

   You can follow the Moon’s changing daily position with the graphic sequence below.
   
   
   
   
   
   
   
   


   
   
   

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September Apogee Moon

   Our Moon reaches apogee, (furthest from Earth), for this orbit, on Sunday September 6th. For this apogee the 19-day old waning gibbous Moon will be at a distance of 31.79 Earth diameters, 252,031 miles (405,606 km) from the Earth.

   On the date of the apogee the 19-day old waning gibbous Moon will be about 5o to the east from the planet Mars and both very visible through the night hours. Not visible to the naked-eye because of the bright Moonshine is the outer planet Uranus with a 5.8 apparent magnitude about 2-3o from the Moon.

   Does our Moon actually go around the Earth as this graphic shows? 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)

   
   
   
   
   
   
   
   
   

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.


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