The Night Sky This Month: December 2018

Comet 46P/Wirtanen

From about the 13th of the month, comet hunters should be able to follow Comet 46P/Wirtanen without optical aid given clear skies and good seeing conditions. Late in the evening of the 15th, the comet should be visible at about magnitude +3 when it passes between the Pleiades and Hyades clusters in the constellation Taurus. By the 23rd of the month, the comet will pass close by the star Capella in the constellation Auriga, but since Full Moon occurs on the 21st, spotting the comet might be difficult.

Moon Phases

New Moon: 6th
First Quarter: 14th
Full Moon: 21st
Last Quarter: 28th


Mercury will appear as a pre-dawn object from the 6th of the month onwards, having passed between Earth and the Sun on the 27th of November, and by the 8th of the month, the little planet will shine at magnitude +0.0. Mercury is also moving away from the Sun, and it will reach its furthest western elongation on the 16th, when it will be 21 degrees west of the Sun. At this time, Mercury will rise about 90 minutes before sunrise, and it should therefore be easy to spot high in the sky, since it will be about 60% illuminated.

Venus starts the month shining at magnitude -4.9 about 32 degrees above the horizon. While its angular diameter reduces from 40.7 seconds of arc to only 26.6 seconds of arc as the month wears on, its illumination increases from 26% to 47%, meaning that its brightness will remain fairly constant at about magnitude -4.6 for much of December. Note that Venus will reach maximum elongation from the Sun on the 6th of January.

Mars will reduce in brightness from magnitude -0.0 to magnitude +0.4 as the month wears on, but it will nevertheless remain prominent at an elevation of 27 degrees as it moves eastward from Aquarius into Pisces, which it will reach on the 21st at an elevation of about 32 degrees. Note that the red planet’s angular diameter will decrease from 9.3 seconds of arc to only 7.5 seconds of arc during the month, which might make it difficult to discern any surface detail with small and medium telescopes.

Jupiter will appear as a predawn object in the east on about the 12th, having passed behind the Sun on the 26th of November. Although the planet will shine at about magnitude -1.8 and have an angular diameter of about 32 seconds of arc, it will be too low above the horizon to obtain clear views of it.

Saturn may just be visible very low above the south-western horizon during the first week or so of the month, after which time it will become lost in the glare of the Sun as it approaches a point of superior conjunction with the Sun, which it will reach on the 2nd of January.

Meteor Showers

December sees two meteor showers, these being-

The Geminids, that peak after astronomical midnight on the night of the 14th/15th of the month. This shower originates from the debris trail of the asteroid 3200 Pheaton, and although most meteors in the shower are relatively slow moving, many are big enough to produce near fireballs. Fortunately, the First Quarter Moon will set about two hours before the expected peak, and provided the sky is dark and clear, observers will see the meteors emanate from a point close to the bright star Castor in the constellation Gemini.

– The Ursid meteor shower peaks late in the evenings of the 22nd and 23rd of the month, with an expected maximum hourly rate of about 10 to 15 meteors, which is not very spectacular. Moreover, Full Moon occurs on the 21st, which means that only the brightest of meteors may be visible emanating from a point close to the star Kochab in the constellation Ursa Minor.

Deep-Sky Objects

Perhaps the most iconic constellation of all, Orion, the Hunter is now visible in the south in the late evening, and it contains two spectacular nebulae, one of which is clearly visible as a non-stellar object without optical aid. Below are some details:

Orion Nebula

Orion Nebula

Located just below the middle star in Orion’s Sword, this chaotic region is a vast star-forming region that contains the famous Trapezium asterism in its heart. Although the nebula can be viewed with binoculars, small telescopes offer better views, but great views can be had with a variety of filters.

While the nebula appears to have a greenish tint, telescopic views will be predominantly red. This is because telescopes gather more red light and so extinguishes the green light our eyes are most sensitive to. In this image, the red light is emitted by hydrogen that is being excited by highly energetic ultraviolet light emitted by the stars that make up the Trapezium.

The Horse Head Nebula

Just to the lower leftward of the leftmost star in Orion’s Belt is a dark dust lane that is pushing through the surrounding nebulosity. Powerful solar winds have sculpted that pillar of dust into a shape that resembles a horses’ head, hence the name Horse Head Nebula. However, since the nebula has a very low surface brightness that makes it very difficult to observe even with large telescopes. The best way to view the nebula is to take relatively long-exposure photographs of it through suitable filters.

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