Mercury reaches greatest elongation west on December 4th (21 degrees) and is visible as a morning object for most of the month. This apparition is a good one for northern hemisphere observers, although not as favourable for their southern hemisphere counterparts.
Opportunities to observer Mercury are usually limited at best to a few days either side of the date when the planet reaches either greatest elongation east or west. Even then, if the angle of the ecliptic is not favourable the planet won't be readily placed for observation. What makes this elongation interesting is that brilliant Venus lies close by, as does Saturn and first magnitude star Spica.
For observers located at northern latitudes such as London, England (51.5N), the altitude of Mercury is nearly 10 degrees above the southeastern horizon, 45 minutes before sunrise on December 4th, the date of greatest elongation west. The planet then moves back towards the Sun appearing a little lower each morning until about the last week of December, when it becomes lost to the bright twilight glare. From southern latitudes Mercury is also visible during this time but not quite as high in the morning sky.
Also visible in the morning sky is Venus, Saturn and first magnitude star Saturn. The morning of December 5th is a good opportunity to spot the three planets and Spica, as they will appear almost in line. On this day, Mercury is relatively bright at magnitude -0.4, Venus brilliant at magnitude -3.9, Saturn magnitude 0.8 and Spica at magnitude 1.0.
Although not as well placed as last month, Venus is still a magnificent morning object. From northern temperate latitudes at the start of the month, the planet rises about 2.5 hours before the Sun, decreasing to less than 2 hours by months end. The period of visibility for southern hemisphere observers is considerably less. For example from Sydney, Australia, Venus rises less than 1.5 hours before sunrise.
Venus shines at magnitude -3.9 with its phase increasing from 88% to 94% during December. The planet starts the month in Libra before moving through Scorpius and finally ending the month in Ophiuchus. On December 11, the waning crescent Moon passes 1.6 degrees south of Venus.
Mars is a difficult evening object located towards the southwestern horizon. The planet (magnitude 1.2) now sets just under two hours after the Sun for northern hemisphere observers and remains inconveniently low down and is located only a few degrees above the horizon in the early evening twilight sky. For example, from London, England (51N) on December 15th, Mars is located only 7 degrees above the southwestern horizon one hour after sunset. The planet appears slightly higher from southern latitudes - from Sydney, Australia (34S), it is 11 degrees above the southwestern horizon one hour after sunset on December 15th.
The "Red planet" starts the month in Sagittarius before moving into Capricornus on December 26th. It is now more than 2.1 AU distant from Earth, resulting in an apparent size of only 4 arc seconds. On December 15th, the waxing crescent Moon will pass 6 degrees north of Mars.
Jupiter is at its magnificent best this month when it reaches opposition on December 3rd and hence is visible all night. At magnitude -2.8, the giant planet is unmistakable - a dazzling beacon of light among the stars of the constellation of Taurus. It is currently moving retrograde and is located a few degrees to the north of orange/red star Aldebaran (mag. 0.9) which is surrounded by the large sprawling Hyades open cluster.
This year's opposition of Jupiter favours northern hemisphere observers since the plant is located in the northern section of the sky. From southern hemisphere latitudes, Jupiter appears lower down, but still unmistakable due to its brightness. As well as been bright, Jupiter is also large. The apparent diameter at opposition is an impressive 48 arc seconds, allowing ample surface details to be observed even through a small telescope. Also easily visible, but not always at the same time are the four brightest moons of Jupiter.
On December 26, the almost full Moon passes 0.4 degrees south of Jupiter and an occultation is visible from central South America and the southern Africas.
After passing through superior conjunction in October, Saturn reappeared as an early morning object low down among the stars of Virgo in late November. This month the planet gradually climbs higher in the sky and by the end of December it rises approx. 5 hours before sunrise.
Saturns shines at magnitude 0.8 and the distance between the "Ringed planet" and Earth is currently decreasing. On the last day of the year it stands at just over 10.2 AU, resulting in an apparent diameter of more than 16 arc seconds. The rings are now wide open and nicely situated at 19 degrees tilt. There is also a good opportunity this month to spot Saturns moon Iapetus, which is famous for its "two-tone" colouration; one side of the moon being much darker in colour than the other. As with all moons, Iapetus is tidally locked and hence when positioned on the western side of Saturn (when viewed from Earth) it appears brighter than when it appears on the opposite side. This is due to the lighter side facing the Earth at this time. A good date to try and spot Iapetus is on December 23rd when it is at greatest western elongation and has an apparent magnitude of 11.5.
Earlier in the month on December 10th, the waning crescent Moon passes 4 degrees south of Saturn.
Uranus shines at magnitude 5.8 and is located in Pisces just south of the "Great Square of Pegasus". The second most distant planet from the Sun is visible as soon as it gets dark until around midnight. At the beginning of December, Uranus moves retrograde before reaching its stationary point on the 13th. It then returns to direct motion.
The planet is located a couple of degrees to the west of star 44 Piscium, which at magnitude 5.8 is of equal brightness.
Neptune is an early evening object that is visible for a few hours as soon as it gets dark. The planet is located in Aquarius. To locate Neptune, imagine a line connecting stars Ancha (θ Aquarii) mag. 4.2 and ι Aquarii (mag. 4.3). Just over halfway along this line is e Aqr (38 Aquarii), a mag. 5.4 star. Positioned about 0.5 degrees from e Aqr is Neptune.
Neptune shines at mag. 7.9 and is too faint to be seen with the naked eye, but is easily visible with binoculars or a small telescope.
Solar System Data Table December 2012
|Date||Right Ascension||Declination||Mag.||Size||Illum. (%)||Distance (AU)||Constellation|
|Sun||5th Dec 2012||16h 46m 36.1s||-22d 21m 54.2s||-26.8||32.5'||100||0.985||Ophiuchus|
|Sun||15th Dec 2012||17h 30m 32.7s||-23d 15m 49.5s||-26.8||32.5'||100||0.984||Ophiuchus|
|Sun||25th Dec 2012||18h 14m 54.2s||-23d 23m 32.5s||-26.8||32.5'||100||0.984||Scorpius|
|Mercury||5th Dec 2012||15h 23m 28.7s||-16d 09m 48.4s||-0.5||06.7"||62||1.010||Libra|
|Mercury||15th Dec 2012||16h 13m 53.2s||-19d 56m 55.0s||-0.5||05.5"||84||1.213||Scorpius|
|Mercury||25th Dec 2012||17h 15m 34.7s||-23d 04m 21.8s||-0.5||05.0"||93||1.345||Ophiuchus|
|Venus||5th Dec 2012||14h 55m 31.5s||-15d 03m 42.5s||-3.9||11.6"||89||1.438||Libra|
|Venus||15th Dec 2012||15h 45m 35.2s||-18d 29m 33.8s||-3.9||11.3"||91||1.483||Libra|
|Venus||25th Dec 2012||16h 37m 33.5s||-21d 05m 54.4s||-3.9||10.9"||93||1.524||Ophiuchus|
|Mars||5th Dec 2012||18h 59m 02.0s||-23d 54m 28.9s||1.2||04.3"||97||2.157||Sagittarius|
|Mars||15th Dec 2012||19h 32m 29.3s||-22d 55m 00.2s||1.2||04.3"||97||2.183||Sagittarius|
|Mars||25th Dec 2012||20h 05m 37.4s||-21d 29m 29.5s||1.2||04.2"||98||2.208||Sagittarius|
|Jupiter||5th Dec 2012||04h 37m 31.9s||21d 17m 18.4s||-2.8||48.4"||100||4.070||Taurus|
|Jupiter||15th Dec 2012||04h 31m 51.2s||21d 07m 17.5s||-2.8||48.1"||100||4.097||Taurus|
|Jupiter||25th Dec 2012||04h 26m 43.2s||20d 58m 02.9s||-2.8||47.5"||100||4.154||Taurus|
|Saturn||5th Dec 2012||14h 21m 09.2s||-11d 39m 36.2s||0.8||15.7"||100||10.562||Virgo|
|Saturn||15th Dec 2012||14h 25m 05.3s||-11d 57m 55.5s||0.9||15.9"||100||10.454||Libra|
|Saturn||25th Dec 2012||14h 28m 39.6s||-12d 13m 53.2s||0.9||16.1"||100||10.326||Libra|
|Uranus||5th Dec 2012||00h 17m 31.0s||01d 06m 27.5s||5.8||03.6"||100||19.679||Pisces|
|Uranus||15th Dec 2012||00h 17m 23.8s||01d 06m 06.7s||5.8||03.6"||100||19.844||Pisces|
|Uranus||25th Dec 2012||00h 17m 35.6s||01d 07m 49.5s||5.8||03.5"||100||20.016||Pisces|
|Neptune||5th Dec 2012||22h 10m 31.6s||-11d 55m 35.8s||7.9||02.3"||100||30.193||Aquarius|
|Neptune||15th Dec 2012||22h 11m 08.6s||-11d 52m 04.0s||7.9||02.2"||100||30.358||Aquarius|
|Neptune||25th Dec 2012||22h 11m 57.4s||-11d 47m 27.4s||7.9||02.2"||100||30.511||Aquarius|