Mercury starts the month as a magnitude 0.0 evening object, visible from southern and equatorial latitudes and setting up to 2 hours after sunset. The fast moving planet then draws rapidly back to the Sun until inferior conjunction is reached on December 13th. On the previous day, Mercury reaches perihelion at 0.307 AU (approx. 45.9 million kilometres or 28.5 million miles).
During the 3rd week of the month, Mercury re-appears in the pre-dawn sky for observers at northern locations. The planet brightens quickly and by months end will have reached magnitude -0.3. It will be well placed in the sky, just before sunrise.
The long morning apparition of Venus finally comes to an end in December. At the start of the month the planet remains a brilliant magnitude -3.9 object, low down above the eastern horizon just before dawn. However, it's not long before Venus is lost to the glare of the Sun. For those at southern latitudes, the planet will only be visible at the very beginning of December although observers at northern temperate latitudes should be able to spot it for a few days more.
Mars remains a morning object towards the east before sunrise. The planet starts the month in Virgo just 3 degrees north of slightly brighter star, Spica (α Vir - mag. +1.0). From northern latitudes, Mars rises about 4 hours before the Sun, although considerably less for those located further south. As the month progresses, the planet improves in magnitude from +1.7 to +1.5 with its angular size increasing from 4.2 to 4.8 arc seconds. However, this is still too small to telescopically make out any significant surface details. On December 13th, the waning crescent Moon passes 4 degrees north of Mars. The planet then crosses into Libra on December 21st. At the end of the year, much brighter Jupiter (mag. -1.7) will be nearby.
Jupiter is a morning object in Libra. The dominant planet in the Solar System rises some 2 hours before the Sun on December 1st, improving to up to nearly 4 hours by months end. Its magnitude increases from -1.7 to -1.8 and its apparent diameter from 31.4 to 33.0 arc seconds as the month progresses.
On December 7th, asteroid Vesta (mag. +7.9) passes 4 degrees north of Jupiter. Later on the 14th, the waning crescent Moon also passes 4 degrees north of Jupiter. As previously mentioned, Mars and Jupiter are close together towards month's end, although the "Red planet" is 3 magnitudes fainter than its giant neighbour.
Saturn reaches solar conjunction on December 21st. As a result, it won't be visible this month from northern temperate latitudes. From southern latitudes, the planet (mag. +0.6) may be glimpsed during the first few days of the month, low down above the southwestern horizon just after sunset. Brighter Mercury is a few degrees nearby at the start of the month.
Uranus, mag. +5.8, continues to move slowly retrograde. The distance ice giant is two months past opposition but remains well placed for observation in the evening sky in Pisces. During December, it's visible as soon as it's dark enough, remaining so until after midnight.
Uranus is faintly visible to the naked eye and very easy to spot with binoculars and small telescopes. A small 80mm (3.1-inch) refractor at high magnifications shows a small greenish disk, 3.7 arc seconds in diameter.
Neptune, mag. +7.9, is moving direct in Aquarius and a few degrees from the Pisces constellation border. The most distant planet in the Solar System can be seen with binoculars and small telescopes for a few hours towards the west after dark. It’s currently positioned about 30 degrees southwest of the centre of the Great Square of Pegasus and just south of lambda Aqr (λ Aqr - mag. +3.7).
On December 24th, the waxing crescent Moon passes 1.4 degrees south of Neptune.
Solar System Data Table December 2017
|Date||Right Ascension||Declination||Mag.||App. Size||Illum. (%)||Dist. (AU)||Constellation|
|Sun||Dec 05||16h 45m 23.2s||-22d 19m 46.3s||-26.8||32.5'||100||0.985||Ophiuchus|
|Sun||Dec 15||17h 29m 17.8s||-23d 14m 56.6s||-26.8||32.5'||100||0.984||Ophiuchus|
|Sun||Dec 25||18h 13m 39.6s||-23d 23m 59.4s||-26.8||32.5'||100||0.984||Sagittarius|
|Mercury||Dec 05||17h 54m 47.7s||-24d 16m 27.7s||0.7||8.7"||23||0.771||Sagittarius|
|Mercury||Dec 15||17h 10m 17.8s||-20d 41m 49.3s||4.3||9.8"||2||0.683||Ophiuchus|
|Mercury||Dec 25||16h 47m 58.3s||-19d 33m 41.9s||0.0||7.9"||39||0.847||Ophiuchus|
|Venus||Dec 05||16h 09m 46.2s||-20d 25m 16.1s||-3.9||9.9"||99||1.684||Scorpius|
|Venus||Dec 15||17h 03m 05.8s||-22d 35m 26.4s||-3.9||9.8"||99||1.697||Ophiuchus|
|Venus||Dec 25||17h 57m 41.9s||-23d 38m 08.1s||-4.0||9.8"||100||1.705||Sagittarius|
|Mars||Dec 05||13h 41m 46.2s||-09d 26m 14.1s||1.7||4.3"||95||2.178||Virgo|
|Mars||Dec 15||14h 05m 38.6s||-11d 40m 27.1s||1.6||4.5"||94||2.099||Virgo|
|Mars||Dec 25||14h 29m 50.2s||-13d 46m 39.8s||1.5||4.6"||94||2.016||Libra|
|Jupiter||Dec 05||14h 38m 07.4s||-14d 21m 16.1s||-1.7||31.5"||100||6.255||Libra|
|Jupiter||Dec 15||14h 45m 54.4s||-14d 56m 33.2s||-1.7||32.0"||100||6.160||Libra|
|Jupiter||Dec 25||14h 53m 17.1s||-15d 28m 37.5s||-1.8||32.6"||100||6.047||Libra|
|Saturn||Dec 05||17h 51m 15.1s||-22d 29m 37.2s||0.6||15.1"||100||11.012||Sagittarius|
|Saturn||Dec 15||17h 56m 17.9s||-22d 31m 10.8s||0.6||15.1"||100||11.042||Sagittarius|
|Saturn||Dec 25||18h 01m 24.4s||-22d 31m 59.8s||0.6||15.0"||100||11.047||Sagittarius|
|Uranus||Dec 05||01h 32m 14.8s||09d 00m 32.1s||5.7||3.7"||100||19.234||Pisces|
|Uranus||Dec 15||01h 31m 30.3s||08d 56m 32.9s||5.8||3.6"||100||19.372||Pisces|
|Uranus||Dec 25||01h 31m 03.7s||08d 54m 19.2s||5.8||3.6"||100||19.526||Pisces|
|Neptune||Dec 05||22h 52m 16.0s||-08d 11m 35.7s||7.9||2.3"||100||29.955||Aquarius|
|Neptune||Dec 15||22h 52m 38.1s||-08d 09m 06.1s||7.9||2.3"||100||30.127||Aquarius|
|Neptune||Dec 25||22h 53m 12.7s||-08d 05m 21.2s||7.9||2.3"||100||30.293||Aquarius|