The nights are getting really nippy now, but don’t let that prevent you from observing some of the best sights this month’s night sky has to offer, including two meteor shower, a comet, and the impressive Apennine Mountain range on the surface of the Moon. So without further a do, here’s what to look out for in December’s night sky.
The Moon Phases
New Moon: December 11th
First Quarter: December 18th
Full Moon: December 25th
Last Quarter: December 3rd
The nights are getting really nippy now, but don’t let that prevent you from observing one of the most interesting regions of the Moon on the 13th, and again on the 19th of the month. On these days, you should aim your binoculars or telescope at the Lunar Apennine Mountains (Montes Apenninus), and observe this impressive 600km long curving mountain range on the mid-northern part of the Moon. They also form the boundary of a huge dark plain known as the Mare Imbrium, and if you look closely, you will spot the 7-mile wide, and 79 miles long Appenine valley, which is a cleft that runs across the range at right angles.
The Planets
Mercury
Mercury will reach its greatest elongation on the 29th of the month, which is when it will be visible low on the south-western horizon, with a magnitude of about -5. On this date the planet will about 20 degrees away from the Sun very low on the horizon, but do not try to observe it before the sun has dipped first!
Venus
Venus is now approaching the Sun again, but by the end of the month, it will still rise about three hours before dawn. Although its angular diameter will shrink from 17.5, to 14.5 seconds of arc (1/60 of one degree) during the month, its brightness will decrease only marginally from -4.2 to -4.1, since its illumination from the sun will increase from about 66%, to around 75%. At the start of the month, Venus will be about 5 degrees from the star Spica in Virgo. As the month progresses, the planet will pass close by the double star Alpha Libra on the 17th and 18th, and by month’s end it will be in close proximity to Beta Scorpii.
Mars
At the beginning of the month, Mars rises about 2 hours before dawn, but will rise progressively earlier as the month wears on, until it rises about 2.5 hours before dawn by month’s end. By the 23rd and 24th, of the month, Mars will shine at magnitude +1.3 as it passes the star Spica in Virgo, with a separation of about 40. During the month, Mars’ angular diameter will grow to just 5 arc seconds, rendering any detail on the surface invisible. However, the planet will be clearly visible by the end of the month when it will rise to an elevation of 28 degrees in the south-east. By May of 2016 however, Mars will be at its closes approach to earth since 2005, and by then its angular diameter will have grown to about 18 seconds of arc.
Jupiter
Jupiter will be visible during the next few months, which is great news for those who track the changes in its cloud tops. At the start of December, Jupiter will shine at magnitude -2 when it rises at about 00:30 UT, close to the boundary between Leo and Virgo. By the beginning of January, Jupiter will have moved into Virgo proper, but before then, its angular diameter will grow from 35.6, to 38.9 arc seconds. At the end of December, Jupiter will rise at about 22:30 UT, and shine at magnitude -2.2. At this time, Jupiter will be at about 44 degrees above the southern horizon in the pre-dawn sky. Earth is also approaching Jupiter during the month, which means that early risers should not experience any difficulty in observing the equatorial bands, and/or the four Galilean Moons.
Saturn
Saturn is now emerging from behind the Sun, and will become fully visible at about the middle of the month. However, by month’s end, Saturn will be shining at magnitude +0.4, and high enough above the south-eastern horizon for the ring system, which has opened up to about 25 degrees to be clearly visible from about 2 hours before dawn. By the end of the month Saturn will also be approached by Venus as it moves towards the Sun again.
Meteor Showers
The first of two meteor showers, The Geminids, is expected to peak on the 14th and 15th of the month, on which nights the Moon will only be a waxing crescent. One notable feature of the Geminids is the fact that the dust trail comes from the asteroid Phaeton 3200, and not from a comet as is the case with most other meteor showers. The radiant of the slow-moving Geminids is close to the luminous star Castor, in the constellation Gemini, hence the name, Geminids.
The second shower, the Ursids, will peak on the night of the 22nd/23rd of the month, as the Earth passes through the trail of debris left by the comet 8P/Tuttle. Sadly though, the Moon will be full, or very nearly so, which means that only a few of the very brightest meteors may be seen. The radiant of the Ursids meteor shower is close to the star Kochab in Ursa Minor, and with some luck, the shower may be more productive than usual, so keep a lookout for bright meteors high in the northern sky.
Deep Sky Objects
Andromeda Galaxy- M31
Several constellations will be in the sky during the month, and among them is Andromeda, the constellation that contains our closest large galactic neighbor, M31, also known as the Andromeda Galaxy, which is about 2.5 million light years away. Andromeda is significantly larger than the Milky Way Galaxy, and it contains about twice as many stars. The Milky Way and Andromeda are the two largest, and most massive galaxies in the Local Group, a cluster of galaxies that comprises about 3 dozen galaxies of various types. Here a great little description I found on space.com describing how to find M31 using Cassiopeia:
“The first path to Andromeda starts from the constellation Cassiopeia. This is a circumpolar constellation, which means that it is always above the horizon at mid-northern latitudes, opposite the North Star, Polaris, from the Big Dipper. Its five second-magnitude stars form a distinctive “W” shape. We can use the right side of the “W” as an arrow head pointing towards the Andromeda Galaxy. Using the height of the W as a measure, go three W heights in the direction the arrow head points, and you will be close to the Galaxy. With a binocular and a dark sky, Andromeda will appear like a small pale puff of smoke, slightly elongated.”
The Pleiades- M45
As large star clusters go, the Pleiades, or The Seven Sisters, as it is also known, is among the most beautiful in the sky. More than 13 light years across, and containing more than 3,000 stars, the entire cluster consists of stars that are almost identical in age and composition, with the entire group moving towards the giant star Betelgeuse, in Orion. The main attraction of the cluster is without any doubt the blue haze that envelops it, and which hides the majority of the constituent stars from view. The haze is the result of scattered grains of carbon, that reflect more blue light from the powerful stars in the group than any other color, since blue light is scattered more efficiently by carbon than say, red or yellow light. The carbon grains are the remains of a giant cloud of dust and gas through which the Pleiades cluster is now passing.
Crab Nebula- M1
Near the tip of the left-hand horn of the bull, Taurus, but all of 6,500 light years away, is the Crab Nebula, a luminous cloud of dust and gas that was formed when a massive star exploded in the year 1056. The name, Crab Nebula, was thought up by the third Earl of Rosse, who observed, and sketched the nebula at his home in Ireland. According to the Earl, the nebula resembled a sand crab, a common insect in Ireland. On a dark night, and with good seeing conditions, the Crab Nebula may just be visible in binoculars, but a telescope of 6-inch or larger aperture will show it as a blur of light, but without any appreciable detail.
The Orion Nebula
There are many pretty nebulae, but the Orion Nebula, which hangs from Orion’s belt, is arguably the prettiest of all. The nebula contains huge star forming regions, but the central part of the nebula is being swept clear of dust and gas by the Trapezium stars, four young, but exceedingly hot and powerful stars arranged in the shape of a trapezium, hence the name, Trapezium stars.
Although some photographic images of the Orion nebula show a preponderance of red light, the red light is not visible to human vision through a telescope or pair of binoculars. Human vision is much more sensitive to the green light emitted by ionized oxygen, than it is to the reddish glow emitted by hydrogen atoms, which is why the Orion nebula appears to have a greenish tint to human eyes.
Comet 2013 US10 Catalina
At the beginning of the month, the bright comet 2013 US10 Catalina will appear in the pre-dawn sky close to the boundary between Virgo and Libra. As the month wears on, the comet will move northward at a rate of more than 0.5 degrees per day, and by the 10th, the comet will be an easy target about 20 degrees above the horizon in the south-east before dawn. At this point, the comet will be about 6 degrees above Venus, so if you have Venus in your field of view, simply move your sight up, and slightly to the left of Venus. The comet is expected to brighten to about magnitude 5.0, so it might not be visible to unaided view, but it should easily be visible even in modest binoculars.
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