Mercury reaches greatest elongation east (20.8 degrees) on December 11th. From southern and tropical latitudes the closest planet to the Sun remains visible after sunset above the southwestern sky for a short time for about the first 3 weeks of the month. Its brightness fades from mag. -0.5 to +0.5 during this time.
Those living further north will find it much more difficult to spot the planet this month due to its declination of -24 degrees. Fast moving Mercury then reaches inferior conjunction on December 28th.
Venus is now a magnificent early evening object that continues to improve in brightness and visibility as the month progresses. The unmistakable planet completely dominates the western sky after sunset and by months end sets up to 4 hours after the Sun.
Venus is now gradually moving northwards in declination, starting the month in Sagittarius before moving into Capricornus on December 8th where it remains for the rest of the year. On December 3rd, the waxing crescent Moon passes 6 degrees north of the planet providing nice early evening viewing.
Venus increases from mag. -4.2 to -4.4 during December with its illuminated phase decreasing from 69% to 57%.
Mars is now well past its best but remains an evening object towards the southwest / west throughout December. The planet, which peaked earlier this year at mag. -2.0, starts the month at mag. +0.6 in Capricornus. By months end it has moved into Aquarius and dimmed down to mag. +0.9. Despite that Mars is still much brighter than any surrounding stars and as it continues to move eastwards and northwards becomes more favourable placed from northern latitudes than recently. From such locations it sets up to 5 hours after the Sun on New Years Eve although slightly less from further south.
The waxing crescent Moon passes 3 degrees north of Mars on December 5th. At months end, the planet moves to within a degree of Neptune (mag. +7.9).
Jupiter remains a bright morning object moving direct in Virgo. During December, the giant planet slightly increases in brightness from mag. -1.8 to -1.9. Its period of visibility continues to slowly improve and by months end it rises before 2 am from northern temperate latitudes and slightly earlier from those located further south.
The waning crescent Moon passes 2 degrees north of Jupiter on December 22nd.
Saturn reaches solar conjunction on December 10th. As a result it won't be visible at all this month from northern temperate latitudes. At the very end of December, from tropical and southern latitudes the planet may be glimpsed low down above the east-southeast just before dawn.
Uranus, mag +5.8, is two months past opposition but remains well placed for observation in the evening sky among the stars of Pisces. During December it's visible as soon as it's dark enough, remaining so until around midnight. Uranus is easy to spot with binoculars.
Uranus starts the month moving slowly retrograde. It then reaches its second stationary point on December 29th - signaling the end of this year's opposition period - after which direct motion is once again resumed. The planet is positioned 15 degrees south and 20 degrees east of the centre of the "Great Square of Pegasus" and less than a degree southeast of zeta Psc (ζ Psc - mag. +5.2).
On December 9th, the waxing gibbous Moon passes 3 degrees south of Uranus.
Neptune, mag. +7.9, is moving direct in Aquarius and is visible with binoculars and small telescopes. The most distant planet in the Solar System can be seen for a few hours towards the west after dark.
Neptune is currently located about 30 degrees southwest of the centre of the Great Square of Pegasus and 3 degrees southwest of star lambda Aqr (λ Aqr - mag. +3.7). Positioned 20 degrees further south of the planet is first magnitude star Fomalhaut (α Psc - mag. +1.2). On the last day of the year, Mars (mag. +0.9) moves to within a degree of Neptune. However, there is no comparison in brightness, the "Red planet" is 250x brighter than much more distant Neptune!
On December 6th, the waxing crescent Moon passes 0.7 degrees north of Neptune with an occultation visible from USA and western Europe (22:04 UT).
Solar System Data Table December 2016
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||5th Dec 2016||16h 46m 30.0s||-22d 21m 44.5s||-26.8||32.5'||100||0.985||Ophiuchus|
|Sun||15th Dec 2016||17h 30m 25.3s||-23d 15m 47.2s||-26.8||32.5'||100||0.984||Ophiuchus|
|Sun||25th Dec 2016||18h 14m 46.5s||-23d 23m 36.7s||-26.8||32.5'||100||0.984||Sagittarius|
|Mercury||5th Dec 2016||18h 11m 37.3s||-25d 46m 58.7s||-0.5||05.9"||77||1.142||Sagittarius|
|Mercury||15th Dec 2016||18h 58m 25.6s||-24d 12m 06.8s||-0.4||07.3"||49||0.916||Sagittarius|
|Mercury||25th Dec 2016||18h 51m 33.9s||-21d 35m 45.2s||2.9||09.6"||06||0.703||Sagittarius|
|Venus||5th Dec 2016||19h 57m 01.1s||-23d 10m 06.2s||-4.2||17.3"||67||0.964||Sagittarius|
|Venus||15th Dec 2016||20h 45m 15.3s||-20d 21m 03.1s||-4.3||18.7"||64||0.893||Capricornus|
|Venus||25th Dec 2016||21h 30m 20.3s||-16d 42m 19.5s||-4.4||20.3"||60||0.821||Capricornus|
|Mars||5th Dec 2016||21h 27m 08.4s||-16d 26m 04.0s||0.7||06.4"||88||1.276||Capricornus|
|Mars||15th Dec 2016||21h 56m 23.3s||-13d 48m 03.8s||0.8||06.1"||89||1.337||Capricornus|
|Mars||25th Dec 2016||22h 25m 03.5s||-10d 58m 13.1s||0.8||05.9"||90||1.400||Aquarius|
|Jupiter||5th Dec 2016||13h 05m 27.4s||-05d 40m 14.5s||-1.8||33.1"||99||5.948||Virgo|
|Jupiter||15th Dec 2016||13h 11m 11.1s||-06d 13m 29.1s||-1.9||33.9"||99||5.808||Virgo|
|Jupiter||25th Dec 2016||13h 16m 11.8s||-06d 41m 48.0s||-1.9||34.9"||99||5.657||Virgo|
|Saturn||5th Dec 2016||17h 08m 14.7s||-21d 35m 48.1s||0.5||15.1"||100||11.027||Ophiuchus|
|Saturn||15th Dec 2016||17h 13m 18.3s||-21d 42m 23.7s||0.5||15.1"||100||11.028||Ophiuchus|
|Saturn||25th Dec 2016||17h 18m 20.2s||-21d 48m 09.6s||0.5||15.1"||100||11.003||Ophiuchus|
|Uranus||5th Dec 2016||01h 16m 56.4s||07d 27m 33.3s||5.8||03.6"||100||19.328||Pisces|
|Uranus||15th Dec 2016||01h 16m 19.6s||07d 24m 13.5s||5.8||03.6"||100||19.473||Pisces|
|Uranus||25th Dec 2016||01h 16m 01.1s||07d 22m 44.8s||5.8||03.6"||100||19.632||Pisces|
|Neptune||5th Dec 2016||22h 43m 58.0s||-08d 58m 02.6s||7.9||02.3"||100||30.004||Aquarius|
|Neptune||15th Dec 2016||22h 44m 23.3s||-08d 55m 17.1s||7.9||02.3"||100||30.175||Aquarius|
|Neptune||25th Dec 2016||22h 45m 01.0s||-08d 51m 18.6s||7.9||02.3"||100||30.339||Aquarius|