The Night Sky This Month: December 2016

Pleiades and Taurus

As 2016 enters its final month, the Northern Hemisphere experiences its winter solstice on December 21st, which for those living in North America means roughly 14 and half hours of night time, after which the days will then grow lighter, albeit by just a few minutes each day. Here are some other astronomical events to look out for in the month ahead.

Moon

December 2016 offers another excellent opportunity to observe the Alpine Valley, the great slash across the Apennine Mountains on the Moon that marks out the edge of the Mare Imbrium. Small telescopes fitted with suitable filters will easily reveal the 7-mile-wide gash across the mountain range, although the rille that runs the length of the structure is a little more challenging to observe. Weather permitting; it might also be possible to see the waxing crescent Moon being lit up by light reflected off the Earth from the 1st to the 6th, and again on the 22nd to the 28th of the month when earthshine will light up the waning crescent Moon.

Phases

First Quarter: 7th
Full Moon: 14th
Third Quarter: 21st
New Moon: 29th

For observers in the UK, the Moon will be at its point of closest approach to Earth (perigee) on the 12th of December at a distance of about 222,737 miles (358,461 km), and its furthest point from Earth (apogee) on the 25th, when it will be about 252,196 miles (405,870 km) away.

The Planets

Mercury reaches maximum elongation from the Sun on the 10th of December, when it will be 21 degrees eastward of the Sun, but up to the 25th of the month, Mercury can be spotted with binoculars very low above the southwestern horizon. From mid-month onwards, Mercury approaches a position of inferior conjunction, and being between Earth and the Sun on the 28th, it becomes lost in the Sun’s glare. The best time to view Mercury (weather and seeing conditions permitting) during December is therefore around the middle of the month, when it sets around 90 minutes or so after the Sun. The bright object above Mercury at this time will be Venus.

Venus starts the month setting about 2 hours after the Sun, but by the month’s end, it will set about 4 hours or so after the Sun. Look for Venus and the nearby earth-lit waxing crescent Moon above it in the southwest on the 3rd of the month, both objects being in the constellation Sagittarius on that date.

Mars continues its eastward march across the sky, and steadily gains altitude as it moves out of the constellation Capricornus, and into Aquarius. The red planet dims somewhat to magnitude +0.7 as December wears on, but since it is now located in an area that does not contain any bright stars, it’s distinctly red hue will make it easy to spot above the southern horizon. Mars culminates in the south on the 5th of December, when it will be about 20 degrees above the southern horizon soon after the Sun has set. Also on the 5th, the crescent Moon will pass about 3 degrees to the upper left of Mars.

Jupiter is now in the constellation Virgo and still the brightest object in the hours before dawn, shining at magnitude -1.9. Jupiter starts the month rising just before 03h00, but rises progressively earlier as the month wears on to rise just after 01h00 at the end of the month. Look for the King of the planets about 7 degrees above the star Spica (Alpha Virginis) as the pair rises almost together in the early morning sky. On the 22nd of December, the waning crescent Moon will be about 5 degrees or so to the upper right of the planet, with the star Spica below the pair forming a pretty trio.

Saturn reaches conjunction with the Sun on the 10th, and it will therefore not be visible during December, with the possible exception of the last few days of the year, when it might be spotted very low above the south-eastern horizon for an hour or so before sunrise.

Uranus, now in the constellation Pisces, remains at the threshold of naked-eye visibility, at magnitude +5.78. However, around Christmas, a small telescope will reveal Uranus as one apex of an equilateral triangle, with the other two apices being the magnitude +5.24 star zeta Piscium, and the magnitude +6.03 star, 88 Piscium.

Neptune is also below naked-eye visibility, but a small telescope or even binoculars will reveal the planet about 2 degrees westward of the +3.74 magnitude star lambda Aquarii. Look for Neptune in the southwest during the early evening, when the much brighter planets Mars and Venus will be placed above it. On New Year’s Eve, Mars will be around 20 minutes of arc to the west of Neptune, (less than the Moon’s angular diameter), which makes it possible to observe both planets in the same binocular field of view.

On the following night, Mars will have overtaken Neptune on the inside track, so to speak, and will then be about 21 seconds of arc to the eastward of Neptune.

Meteor Showers

December sees two meteor showers:

The Geminids, which are associated with the asteroid 3200 Phaeton, usually peak on the night of the 13th at around 20h00, but sadly, the Moon will be full (or nearly so) on that date in the constellation Gemini, which will seriously limit the number of visible meteors. However, the Geminids shower is usually most active at about 02h00, when up to 100 meteors per hour is expected, among which are bound to be some bright ones, despite the effect of the Moon.

The Ursids, which is expected to peak on the night of 22/23 December, although sporadic meteors may be seen from the 17th onwards. Usual maximum hourly meteor counts average at about 10, but occasional “outbursts” may produce greater numbers. Since the Moon will be a waning crescent on the peak date this year, viewing conditions will be favourable.

Deep Sky Objects

Prominent constellations at around the middle of December are Lepus the Hare, Orion the Hunter, Taurus the Bull, and Auriga the Charioteer. Some easy binocular or small-telescope targets in these constellations are:

Orion – Messier 78 (M78, NGC 2068)

All amateur astronomers are familiar with the Great Orion Nebula, which is why the lesser-known M78 is featured here. This emission nebula completely surrounds two stars of 10th magnitude, and it also contains several dozen T Tauri type stars, which are young, variable stars that are still in the process of forming. M78 is about 1,600 light years away.

Taurus – The Pleiades (Seven Sisters) – Messier 45

M45 is arguably the best known, and certainly among the most beautiful of all the star clusters in the entire sky. Sadly, though, even though all the stars in the cluster are physically related, and formed at the same time, the cluster is only expected to survive for another 250 million years or so before gravitational interactions with nearby objects will cause the clusters’ constituent stars to disperse.

Auriga – Messier 36 (M36, NGC 1960)

Located about 4,100 light years away, this 14-light year- diameter open cluster contains about 60 stars, but due to the clusters’ relatively tender age of only 25 million years, it contains no evolved red giant stars. Note the more-than-passing resemblance of this cluster to the more famous M45, the Pleiades Cluster in Taurus.

Lepus – Spirograph Nebula – IC 418

Named after the intricate patterns that can be drawn with a Spirograph, the exact nature of this planetary nebula is still unknown, and research is continuing into the possible causes of the intricately interwoven shells of gas and dust that make up the structure. The Spirograph has an apparent magnitude of 9.6, and is located about 1,100 light years away.