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Mercury

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.

Venus

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

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, Mars, Spica and the thin crescent Moon as seen just before sunrise from mid-latitude northern latitudes on December 14th (credit:- freestarcharts)

Jupiter

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

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

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.

On December 27th, the waxing gibbous Moon passes 4.5 degrees south of Uranus.

Uranus during December 2017 (credit:- freestarcharts)

Uranus during December 2017 - pdf format (credit:- freestarcharts)

Neptune

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.

Neptune during December 2017 (credit:- freestarcharts)

Neptune during December 2017 - pdf format (credit:- freestarcharts)

Solar System Data Table December 2017

 DateRight AscensionDeclinationMag.App. SizeIllum. (%)Dist. (AU)Constellation
SunDec 0516h 45m 23.2s-22d 19m 46.3s-26.832.5'1000.985Ophiuchus
SunDec 1517h 29m 17.8s-23d 14m 56.6s-26.832.5'1000.984Ophiuchus
SunDec 2518h 13m 39.6s-23d 23m 59.4s-26.832.5'1000.984Sagittarius
MercuryDec 0517h 54m 47.7s-24d 16m 27.7s0.78.7"230.771Sagittarius
MercuryDec 1517h 10m 17.8s-20d 41m 49.3s4.39.8"20.683Ophiuchus
MercuryDec 2516h 47m 58.3s-19d 33m 41.9s0.07.9"390.847Ophiuchus
VenusDec 0516h 09m 46.2s-20d 25m 16.1s-3.99.9"991.684Scorpius
VenusDec 1517h 03m 05.8s-22d 35m 26.4s-3.99.8"991.697Ophiuchus
VenusDec 2517h 57m 41.9s-23d 38m 08.1s-4.09.8"1001.705Sagittarius
MarsDec 0513h 41m 46.2s-09d 26m 14.1s1.74.3"952.178Virgo
MarsDec 1514h 05m 38.6s-11d 40m 27.1s1.64.5"942.099Virgo
MarsDec 2514h 29m 50.2s-13d 46m 39.8s1.54.6"942.016Libra
JupiterDec 0514h 38m 07.4s-14d 21m 16.1s-1.731.5"1006.255Libra
JupiterDec 1514h 45m 54.4s-14d 56m 33.2s-1.732.0"1006.160Libra
JupiterDec 2514h 53m 17.1s-15d 28m 37.5s-1.832.6"1006.047Libra
SaturnDec 0517h 51m 15.1s-22d 29m 37.2s0.615.1"10011.012Sagittarius
SaturnDec 1517h 56m 17.9s-22d 31m 10.8s0.615.1"10011.042Sagittarius
SaturnDec 2518h 01m 24.4s-22d 31m 59.8s0.615.0"10011.047Sagittarius
UranusDec 0501h 32m 14.8s09d 00m 32.1s5.73.7"10019.234Pisces
UranusDec 1501h 31m 30.3s08d 56m 32.9s5.83.6"10019.372Pisces
UranusDec 2501h 31m 03.7s08d 54m 19.2s5.83.6"10019.526Pisces
NeptuneDec 0522h 52m 16.0s-08d 11m 35.7s7.92.3"10029.955Aquarius
NeptuneDec 1522h 52m 38.1s-08d 09m 06.1s7.92.3"10030.127Aquarius
NeptuneDec 2522h 53m 12.7s-08d 05m 21.2s7.92.3"10030.293Aquarius