Mercury at Western Elongation

   On Saturday December 15th Mercury, the innermost planet, will reach its orbital position known as greatest western elongation at 21.3o. 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!


   Mercury is visible in the morning skies just before sunrise local time, as this graphic shows. Venus, to the west from Mercury rises about 2 hours before Mercury, and Jupiter rises shortly after Mercury.

   
   
   
   
   

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Mars – Neptune – Moon in Conjunction

   Friday evening December 14th the first quarter Moon, Mars, and Neptune will all be within the field of view of binoculars.

   
   
   

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Mars – Uranus at Heliocentric Conjunction

   Thursday December 13th the two outer planets, Mars and Uranus will reach a point in their respective orbits called heliocentric conjunction, both having the heliocentric longitude of 31o.

   
   
   

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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December Moon at Apogee

   Our Moon reaches apogee, (greatest distance from Earth), on Wednesday December 12th. At that time the Moon will more or less be at a distance of 31.76 Earth diameters 251,765 miles (405,177 km) from the Earth.

   Does our Moon actually go around the Earth? 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*

   On the day of the apogee the 6-day old waxing crescent Moon, an hour after sunset local time (4:55 pm CST), will be over the southwestern horizon and joined by several planets. Over the southern horizon is the planet Mars. Neptune and Uranus are also shown however at 8th Neptune requires optical assistance or a camera to become visible, while Uranus at around 6th magnitude could be visible with binoculars or telescope.

*Click here to read my 2006 Scope on the Sky column “The Real Shape of the Moon’s Orbit”. (PDF)

Read this very informative article about the Earth-Moon system and their orbital motions, written by Joe Hanson. “Do We Orbit the Moon?”

   
   
   

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.

December Moon at Descending Node


   Monday December 10th the 3.5-day old waxing crescent Moon will be crossing 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 day of the node crossing the 3.5-day old waxing crescent Moon will be over the southwest horizon about an hour after the Sun sets.

   
   
   
   
   

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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Thin Waxing Moon in Conjunction with Saturn

   Saturday evening December 8th the 1.6-day young waxing crescent Moon will be in conjunction with the outer ringed planet Saturn. The two will be within about 3o from each other. Joining the Moon and Saturn is the Dwarf Planet Pluto, and further to the east are the planets Uranus, Neptune, and Mars.

   Within the field of view of 7×50 binoculars, in addition to the Moon and Saturn, are several Messier Objects, M-22, M-25, and M-26. These may be a challenge to see given that they are low over the western horizon and by the time the sky is darker they will have set or are just about to set.

   
   
   
   
   
   
   
   
   
   
   

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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Thin Moon Conjunction with Mercury

   Wednesday morning December 5th the very thin 28-day old waning crescent Moon will be within 4-5o from the innermost planet Mercury. Venus shines very brightly above or west from the Moon. About an hour after the Moon and Mercury have risen the outer planet Jupiter will rise above the eastern horizon.

   
   
   

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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