The Night Sky This Month: December 2017

December Night Sky

With December comes colder temperatures and the shortest day of the year in which the North Pole is tilted furthest away from the Sun. At least after December 21st, the days will begin to lengthen, while the month’s night sky will present some of the best astronomical sights for stargazers to enjoy, including the prolific Geminids meteor shower.

Also in full display will be the winter constellations, headed by Orion and the nearby constellations which surround the “hunter”, including Gemini and Canis Major to its east, Auriga and Perseus to its north, and Taurus and Eridanus to its west. Of course, the five northern circumpolar constellations will also be visible as they appear to orbit around the North Star (Polaris) in Ursa Minor throughout the night, with the others being Ursa Major, Cassiopeia, Cepheus, and Draco.

 

Moon

The image below shows Plato, a large impact crater located close to the Moon’s northwestern limb, with the Appenine mountain chain that marks the boundary of the Mare Imbrium to its east, and the 79-mile-long Alpine Valley cutting through the mountain range. Lunar observers are able to see this striking feature with small telescopes on the night of December 9th, and then again on the night of December 26th, when favourable illumination highlights this conspicuous 101 km long and 1 km deep lava-filled crater.

Plato and Alpine Valley
Image Credit: Richard Bosman

Moon Phases

Full Moon: 3rd
Last Quarter: 10th
New Moon: 18th
First Quarter: 26th

Planets

This December is not a good time to observe the planets, although die-hard planetary observers may yet see something to enjoy during the month.

Mercury will not be visible for the first three weeks or so of December, since it will be passing between the Sun and Earth, to reach a position of inferior conjunction on the 13th of the month. However, from the 20th, the little planet begins to brighten rapidly in the pre-dawn sky, and by month’s end, it will shine at magnitude 0.3 from a position of about 23 degrees from the Sun. At this time, the ecliptic will be steeply inclined with respect to the horizon, giving Mercury a respectable elevation, and thus making its small disc easier to see.

Venus made a close pairing with Jupiter during the middle of November, but it is now sinking back towards the Sun. The planet will therefore rise only about 45 minutes or so before sunrise as December starts, and will be lost in the glare of the Sun from about the 12th of the month onwards, as it approaches a point of superior conjunction with the Sun. However, for the last week or so before it becomes invisible, Venus will shine at magnitude 3.9, and its disc will be about 9.9 seconds of arc across.

Mars starts the month of December about 3 degrees to the upper leftward of Spica, the brightest star in Virgo. The planet starts its new apparition as an early morning object, and rises about 4 hours or so before the Sun. Note that although Mars brightens slightly from magnitude 1.7 to magnitude 1,5 as the month wears on its angular diameter increases only slightly from 4.2, to 4.8 seconds of arc, meaning its surface detail will remain hidden from optical views. Mars will move eastwards into Libra on the 21st of the month, and pass close by Jupiter on the last day of December before forming a very close pairing with the giant planet on the 7th of January 2018.

Jupiter is now visible as an early morning object, and rises about 2 hours or so before the Sun at the beginning of the month. As December progresses, the planet’s angular diameter will increase to about 33 arc seconds, and its brightness will increase slightly to magnitude 1.8. However, Jupiter will remain fairly low on the horizon throughout the month, but given clear skies, it should be possible to observe the planet’s equatorial bands with small telescopes.

Saturn is now approaching a position of superior conjunction with the Sun, which it will reach on the 21st of the month. AS a result, the planet will only become visible early next year, when it reappears from behind the Sun as an early morning object.

Meteor Showers

December 2017 sees two meteor showers, the Geminids, and the Ursids. Below are some details:

The Geminid meteor shower derives from the debris of the asteroid 3200 Phaethon, and under clear skies can produce an impressive hundred or more slowly arcing fireballs per hour. Look for the showers’ radiant close to the star Castor in the constellation Gemini, with its peak expected on the night of 14th/15th December, staring around 10pm, with the best viewing time between 2.00am and 3.00am. Since there will only be a thin waning crescent Moon in the sky on the peak date, it may be worth rising early to observe the shower, as the Geminds is known as a particularly spectacular event.

– The Ursid meteor shower derives from debris shed by the comet 8P/Tuttle, and usually runs from 17th to 24th, with a peak on the night of 23rd/24th. As with the Geminds, there will be no bright moonlight on this date, and although this shower’s peak is usually only about 10 meteors per hour, the maximum count can occasionally increase dramatically, which might make braving the cold in the predawn hours worthwhile. Look for the shower’s radiant close to the star Kochab in the constellation Ursa Minor.

Deep-Sky Objects

The constellation Taurus is now prominent in the night sky, and given clear skies and good seeing conditions, it offers a wealth of spectacular deep sky objects (DSOs), such as the naked eye clusters, M45 (Pleiades), and the Hyades, which marks out the “face” of the Bull.

Hyades and Pleiades (M45) can both be seen in the image, with the Pleiades’ (upper right) eerie blue glow derived from a cloud of carbon-rich dust that the cluster is moving through, or more precisely, did a few hundred years ago. The Hyades can be seen towards the lower left, and at 153 light years distant, is the nearest open cluster to Earth.

Hyades and Pleiades

Note however that the bright red star shown here is Aldebaran (Alpha Tauri), which is not a member of the cluster, and at 65.23 light years away is less than half the distance from us as compared to the cluster, We only see Aldebaran as a part of the Hyades because it lays on the same line of sight as the cluster.

Orion Nebula, even without optical aid it can be seen that the central “star” in Orion’s sword is actually a non-stellar object. This structure is in fact the Orion Nebula, an enormous star-forming region that contains the famous Trapezium asterism, a grouping of four, very hot young stars. The red glow of the nebula shown here is caused by the excitation of hydrogen atoms by the UV light emitted by the Trapezium stars.

The Orion Nebula

However, when viewed through a telescope the nebula is seen as green-ish, since the human eye is more sensitive to the green light emitted by ionized oxygen, than it is to the reddish hue of hydrogen atoms. The color discrepancy is caused by the fact that cameras capture light that is not readily visible to human vision.

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