Mercury the innermost and smallest planet reaches greatest eastern elongation (20 degrees) on December 29th and therefore is visible as an early evening object during the latter part of the month. For observers at tropical and southern latitudes it appears low down above the western horizon from about the middle of December. Try looking for the planet from about 30 minutes after sunset. Mercury is more difficult to spot from mid-latitude northern locations. Only during the last few evenings of December will observers have their latest change to glimpse this elusive World, low down towards the southwest, 35 minutes after sunset.
During December the magnitude of Mercury fades slightly from -0.8 to -0.5 with the thin waxing crescent Moon passes 7 degrees north of the planet on the 12th.
Venus remains a superb morning object although its period of visibility is now decreasing. At the start of December from northern temperate latitudes the planet rises over 4 hours before the Sun decreasing to about 3 hours by months end. For observers located further south, Venus is visible for less time. For example from Sydney, Australia (33.5 S) it rises 2.5 hours before the Sun on December 1st, decreasing to 2 hours by December 31st.
Venus fades slightly from magnitude -4.2 to -4.0 with its illuminated phase increases from 67% to 77% this month. It continues its rapid eastwards motion crossing the constellation boundary from Virgo into Libra on December 11th. By the end of the month, Venus is now 30 degrees southeast of much fainter Mars (mag. +1.4).
On December 7th, the waning crescent Moon passes 0.7 degrees north of Venus and an occultation is visible from North and Central America at 16:56 UT.
Mars is located in Virgo and continues to gradually improve in brightness and apparent size as the distance between the "Red planet" and Earth continues to decrease. It remains a morning object throughout December, starting it at magnitude +1.5 and ending it at magnitude +1.3. The apparent size increases from 4.8 to 5.5 arc seconds.
From northern temperate latitudes you can almost set you watch by Mars this month. As a consequence of its current eastwards and southwards motion Mars rises at approx. the same time each morning; between 2 am and 3 am depending on location. For observers at tropical and southern latitudes the planet rises slightly earlier each morning and by the end of the year it's visible just after midnight.
It's generally regarded among amateur astronomers that an apparent size of 5 arc minutes is the minimum required to begin serious Martian telescopic work. Mars crosses that threshold this month and although obviously still small in diameter, a medium 150mm (6-inch) scope at high powers on nights of good seeing should reveal the more prominent markings.
The waning crescent Moon passes 0.1 degrees south of Mars on December 6th with an occultation visible from Australia, Central and East Africa, Indonesia, Southern India (2:42 UT). Later on December 21st, Mars passes 4 degrees north of Spica (α Vir - mag +1.0).
Jupiter the major dominant planet in the Solar System is now a brilliant morning object moving direct in southeastern Leo. The giant planet rises after midnight at the start of the month improving to before midnight by months end and remains visible until dawn. It increases in magnitude from -2.0 to -2.2 with the apparent size growing from 36 to 39 arc seconds as the month progresses.
Through a pair of binoculars the four large Galilean moons, Io, Europa, Ganymede and Callisto are easily visible. They constantly change position as they orbit the planet and sometimes all four can be seen at once but more often than not some are hidden from view as they pass behind or in front of Jupiter's disk. On rare alignment occasions none of the four can be seen at all, just the planet itself.
When viewed through a telescope, Jupiter is a stunning sight. Even a small 80mm (3.1 inch) refractor will show the main Northern and Southern equatorial cloud belts. Larger telescopes reveal much more finer details, including smaller belts, ovals, festoons, darker regions and of course the famous "Great Red Spot".
The waning crescent and last quarter Moon pass 2 degrees and 1.5 degrees south of Jupiter on December 4th and 31st respectively.
Saturn passed through solar conjunction late last month and subsequently re-appears low down in the southeastern sky before sunrise during the second half of December. The "Ringed" planet is located in Ophiuchus, shinning at magnitude +0.6.
Uranus, mag +5.8, is now two months past opposition but remains well placed for observation in the evening sky among the stars of Pisces. During December, the second furthest planet from the Sun is visible as soon as it's dark enough, remaining so until around midnight. It's easy to spot with binoculars.
Uranus starts the month moving slowly retrograde. It then reaches its second stationary point on December 26th - 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 2 degrees south of epsilon Psc (ε Psc - mag. +4.3).
On December 20th, the waxing gibbous Moon passes 1.2 degrees south of Uranus with an occultation visible from Southern South America, Falkland Islands at 0:50 UT.
Neptune, mag. +7.9, is located in Aquarius and is visible with binoculars and small telescopes. The Solar System's distant far planet can be seen towards the west for a few hours after sunset. It's positioned some 30 degrees southwest of the "Great Square of Pegasus" and two degrees northeast of sigma (σ) Aqr (mag. +4.8).
On December 17th, the waxing crescent Moon passes 3 degrees north of Neptune.
Solar System Data Table December 2015
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||5th Dec 2015||16h 43m 15.1s||-22d 15m 55.8s||-26.8||32.5'||100||0.986||Ophiuchus|
|Sun||15th Dec 2015||17h 27m 08.5s||-23d 13m 19.9s||-26.8||32.5'||100||0.984||Ophiuchus|
|Sun||25th Dec 2015||18h 11m 28.9s||-23d 24m 40.1s||-26.8||32.5'||100||0.984||Sagittarius|
|Mercury||5th Dec 2015||17h 24m 12.1s||-25d 03m 20.8s||-0.7||04.8"||96||1.394||Ophiuchus|
|Mercury||15th Dec 2015||18h 32m 13.3s||-25d 30m 18.2s||-0.6||05.3"||90||1.281||Sagittarius|
|Mercury||25th Dec 2015||19h 34m 53.3s||-23d 32m 23.3s||-0.7||06.1"||73||1.094||Sagittarius|
|Venus||5th Dec 2015||13h 53m 12.5s||-09d 14m 00.7s||-4.2||16.9"||68||0.985||Virgo|
|Venus||15th Dec 2015||14h 38m 43.9s||-13d 04m 29.7s||-4.1||15.8"||72||1.054||Libra|
|Venus||25th Dec 2015||15h 26m 11.5s||-16d 31m 08.9s||-4.1||14.9"||75||1.121||Libra|
|Mars||5th Dec 2015||12h 49m 46.3s||-03d 44m 36.5s||1.5||04.8"||93||1.934||Virgo|
|Mars||15th Dec 2015||13h 11m 19.1s||-05d 57m 28.4s||1.4||05.1"||92||1.845||Virgo|
|Mars||25th Dec 2015||13h 32m 39.2s||-08d 04m 18.5s||1.3||05.3"||92||1.751||Virgo|
|Jupiter||5th Dec 2015||11h 29m 30.2s||04d 30m 38.9s||-2.0||36.0"||99||5.475||Leo|
|Jupiter||15th Dec 2015||11h 32m 48.5s||04d 12m 11.9s||-2.1||37.1"||99||5.316||Leo|
|Jupiter||25th Dec 2015||11h 35m 04.3s||04d 00m 33.3s||-2.1||38.2"||99||5.157||Leo|
|Saturn||5th Dec 2015||16h 25m 19.2s||-20d 00m 01.1s||0.6||15.1"||100||10.989||Ophiuchus|
|Saturn||15th Dec 2015||16h 30m 15.8s||-20d 11m 15.4s||0.6||15.2"||100||10.963||Ophiuchus|
|Saturn||25th Dec 2015||16h 35m 05.8s||-20d 21m 27.9s||0.6||15.2"||100||10.911||Ophiuchus|
|Uranus||5th Dec 2015||01h 01m 54.5s||05d 53m 37.3s||5.8||03.6"||100||19.407||Pisces|
|Uranus||15th Dec 2015||01h 01m 23.7s||05d 50m 49.4s||5.8||03.6"||100||19.557||Pisces|
|Uranus||25th Dec 2015||01h 01m 11.4s||05d 49m 57.5s||5.8||03.6"||100||19.720||Pisces|
|Neptune||5th Dec 2015||22h 35m 37.0s||-09d 43m 54.5s||7.9||02.3"||100||30.037||Aquarius|
|Neptune||15th Dec 2015||22h 36m 04.3s||-09d 41m 01.6s||7.9||02.3"||100||30.207||Aquarius|
|Neptune||25th Dec 2015||22h 36m 43.9s||-09d 36m 57.0s||7.9||02.2"||100||30.369||Aquarius|