What’s in the sky this December?
Constellations represent groups of stars that have been given a name and more recently a border. For millennia they have been used as a tool to share significant cultural stories. Today, the 88 western constellations used here help astronomers map the sky and search for astronomical objects. This December these constellations dominate the spring sky:
- Orion the hunter dominates the December north-eastern sky. One of the most famous non-zodiac constellations its famous three-star belt lies very close to the celestial equator. These stars also make the base of what many Australian’s refer to as the base of the saucepan. Within the handle of the saucepan, a nebula at around 1,350 light years away is one of the first things to look at through a telescope. Known as M42 or the Orion nebula, it contains enough gas and dust to make as many as 2,000 stars like the Sun.
- Taurus the bull is in the north and is possibly the oldest western constellation. It has the bright red dying star of Aldebaran and the stunning open cluster M45 or the Pleiades, a group of very young stars about 445 light years away. Below the horns is the remnant of a star that exploded as a supernova in 1054. It is now called M1 or the Crab nebula. A large telescope and clear northly view is needed for the best view.
- Eridanus – stretching high across the southern evening sky. Eridanus is one of Ptolemy’s original 48 constellations and represents a river with the beginning near the constellation of Orion and the end of the river is marked by the brilliant Achernar, a blue-white star shining at magnitude 0.5 that spins so quickly it is the least spherical star in the Milky Way galaxy.
- Crux or the Southern Cross and its pointers of Alpha and Beta Centauri are conspicuous by their absence in the evening sky as they are low in the south-east making them hard to see. Light pollution and the thicker atmosphere near the horizon dim the starlight.
- Geminid meteor shower – One of the best chances to see meteors in the southern hemisphere occurs in the early hours of 14 December, one day before the New Moon. Small particles ejected from the asteroid 3200 Phaeton collide with the Earth’s upper atmosphere causing bright short streaks of light. Look for Gemini’s bright star Caster as the general area from which the meteors will come.
Four planets: Venus, Mars, Jupiter and Saturn can all be seen at various times this month. Watch how their positions change throughout the month as they travel along the ecliptic, the path of the Sun and Moon.
- Mars – The red planet is in the north in the constellation of Pisces. On 23 December the waxing gibbous Moon is above or to the left of Mars, while the next night it will be above and to the right.
- Jupiter – The king of planets is low in the west in Sagittarius before moving into Capricornus mid-month. On the 17 December the waxing crescent Moon is above Jupiter.
- Saturn – The ringed gas-giant is also low in Sagittarius moving into Capricornus. At the start of the month Saturn will be approximately 2 degrees or four times the width of the Moon from Jupiter. On the evening of 21 December, they will be less than 1/10 of a degree apart in an event dubbed a ‘Great conjunction’. This is the closest they have been since 1623 and until 2080. To many people they will look like the one object. Optimal viewing will be from around civil twilight at 8:35pm for about one hour. A clear view to the western horizon is the main requirement to enjoy the view.
- Venus – In the morning sky, it begins the month in Libra before moving into Scorpius for 5 days and then Ophiuchus. On 13 December the young crescent Moon is below or to the left of Venus.
For the monthly movements of the moon, check out our Moon phase calendar.
Explore the universe through the lens of your telescope and take in some of the gems of the September sky:
- NGC 104 or 47 Tucanae a globular cluster – One of about 150 globular clusters within the Milky Way. Though it is second largest and second brightest globular cluster within the galaxy, its bright compact core makes it the best from urban skies using binoculars or a small telescope. It contains several million old stars and is 13,000 light-years away within the Tucan constellation.
- The Large and Small Magellanic Clouds are two of our Milky Way’s satellite galaxies. The Large Magellanic Cloud (LMC) is in the constellation of Dorado and is about 163,000 light years away, while the Small Magellanic Cloud (SMC) is in Tucana and is about 206,000 light years away. They are the brightest in the local group of about 30 nearby galaxies but are best seen away from cities and towns on moonless nights. They look like small parts of the Milky Way that have drifted away but are in fact approaching and are expected to merge with us in around 2.4 billion years.
- Tarantula Nebula (NGC 2070) – a huge starburst nebula (Latin: cloud) of hydrogen gas cloud approximately 1,000 light years in diameter on the leading edge of the LMC.
- The Orion nebula (M42) the finest and brightest nebula in the sky. Sitting close to the celestial equator it is visible to the unaided eye in Orion’s sword from both hemispheres making it one of the first targets for all telescope users. It is approximately 1344 light years away and 24 light years across. 700 young stars are in various stages of formation with 6 visible in the Trapezium in the heart of the nebula.
- The two brightest stars at night are now clearly visible in the east and south east. Sirius the dog star in Canis Major is a binary star just 8.6 light years away. It is twice the mass of the Sun and 25 times brighter. Canopus in the constellation of Carina is 8 times more massive than the Sun and 10,000 times brighter. Its distance of 310 light years drops it to the second brightest star as we see it.
- The dying variable red supergiant Betelgeuse in the constellation of Orion in the eastern sky is about 550 light years away and 750 times the size of the Sun. It will eventually explode as a supernova and leave a neutron star behind approximately 1.5 times the mass of the Sun. It is expected to explode sometime in the next 100,000 years…so, keep watching.
- Purchase the 2021 Australiasian Sky Guide by Dr Nick Lomb, featuring an annual report of what’s in the sky and the latest astronomical findings. Produced by MAAS Media.
- View the December sky chart, which shows the stars, constellations and planets visible in the night sky from anywhere in Australia
- Watch our 12 December Southern Sky Livestream with Sydney Observatory’s astronomy ambassador Karlie Noon on Sydney Observatory’s Facebook
- Check out these resources for getting started