The Night Sky in December, 2016 I am writing this newsletter on December 1 st in Barbados. Of course, it is not cold here like it is in England but it is hot and humid. So far we have only had one clear night when Venus looked magnificent high in the sky in the south-west after sunset. Venus will be the planet of the month in both the northern and southern hemispheres throughout December. It will be higher in the sky than it often is and will get bigger and brighter as the month progresses. The star chart below shows the sky in the south-west at 18:00 on December 31 st. Note that Mars will also be visible all month just above Venus in the constellation of Aquarius as can be seen in the chart. At the beginning of the month, the Sun will set at 15:54 GMT. There will be a Full Moon on December 14 th and a New Moon on December 29 th so the beginning and end of the month will be the best for stargazing. Several people have commented on how much they enjoyed the Greek legend I included in my last newsletter. So, this month I have decided to tell you the legend of the bears. First look in the north to find the Plough. The star chart below shows the sky in the north over Oxfordshire at 23:00 on December 1 st. The software I use to produce these star charts is called Starry Night Backyard and is produced by a Canadian company called Imaginova. Backyard is an old version of the software since they are now up to Starry Night Pro 7! However, I still like to use Backyard since it has a simple interface and serves my purpose. If you are interested in some free software which is similar, then try downloading Stellarium. Note that because the software I use is Canadian, it shows the North American name for the Plough which is the Big Dipper. The Plough is not a constellation. It is a bright pattern of stars called an asterism and forms part of the constellation of Ursa Major the Great Bear.
The star chart below shows the constellation of Ursa Major with an image of the bear superimposed on it. You can see that the Plough forms the tail and part of the body of the bear. The star chart below shows the constellation of Ursa Minor the Little Bear. To find it, look at the Plough which is in the north on the right of the star chart below. Find the stars Merak and Dubhe which you can see on the right-hand side of the bowl of the Plough. Follow the line of these stars until you reach Polaris this will be the first bright star you encounter. Polaris is also known as the Pole Star since it is situated near the north pole. It is the brightest star in that region but note that it is not as bright as many people expect it to be. You can see it in the middle at the top of the chart. Ursa Minor appears to hang off it. The stars in the tail of the little bear are not very bright and you may not be able to see them unless you have a very dark night and clear skies. But two stars that do stand out are Kochab and Phercad which are on the outside of the bowl. You can see these two stars better in the first star chart above. These two stars are called the guard stars since they guard the pole.
All these constellations close to Polaris are circumpolar. This means that they are in our sky all night and all year. Since we are spinning on our axis around the pole, these circumpolar constellations seem to us to be rotating around Polaris. So sometimes both Ursa major and Ursa minor will seem to be upside down. But how did these bears come to be in the night sky? The Greek legend says that Princess Callisto was the daughter of King Lycaon of Arcadia. She was very beautiful and Juno, the wife of Jupiter was very jealous of her. Jupiter desired Callisto and forced himself upon her. Together they produced a son called Arcas. This made Juno even more jealous and she obtained her revenge by turning Callisto into a bear which then ran off into the forest. Arcas grew up to be a great hunter. One day, he was hunting in the forest when he saw a bear in the trees. Arcas prepared to kill it not knowing that it was his mother! Fortunately, Jupiter saw what was about to happen and intervened. He turned Arcas into a little bear. Then he held both bears by the tail, swung them around over his head and hurled them into the sky where we can still see them to this day! So, now we know why they have long tails! This legend has been depicted in art many times and below you can see the painting by the 19 th Century French painter, Jean-Francois Millet.
This month you might like to have a closer look at some of the stars in the Great Bear and the Little Bear. If you do, I want to point out the second star in the handle of the Great Bear. This star is called Mizar. If you have good eyesight and clear, dark skies you might be able to see that Mizar is not alone. Don t forget to allow yourself about 30 minutes to get dark adapted. Getting properly dark adapted isn t just a question of dilating the pupils. Changes also take place in the cells of the retina to enable you to see better in the dark and this process takes 20-30 minutes. Mizar is in fact a binary star and has a companion called Alcor. Legend has it that this star was used as an eyesight test for young men wanting to enter the Greek navy. If they could see two stars, then they were accepted. The image below shows these two stars. If you can see something like this with the naked eye, then you stand a chance of getting into the Greek navy! They will probably be glad of you right now! If you can t see them with the naked eye, then try with binoculars. The Arabs used to call these two stars the horse and rider.
If you have a look at Mizar with even a small telescope (say 5inch diameter) then you should be able to see that it is a binary star. However, with the help of spectroscopy, it has been found that both of Mizar s stars are binaries that is it is a quadruplet. It is now known that Alcor is also a binary and is gravitationally bound with Mizar. So, this is a sextuplet system - that is 6 stars gravitationally bound together. There is only one other sextuplet in the whole of the night sky (including the southern hemisphere) and that is Castor in the constellation of Gemini! But don t forget that in December, all the great constellations of the winter are there to be enjoyed. The star chart below shows the sky in the east over Oxfordshire at 20:00 on December 1 st. Orion is just rising in the east at that time and is off the bottom left of this chart. Above it is Taurus the Bull. Here you can find the Hyades, a cluster of stars near the giant red star Aldebaran. Above this and to the right you can see the Pleiades cluster. These two clusters can be enjoyed at their best through binoculars. To the left of Taurus is the constellation of Auriga the Charioteer with its bright star Capella. To the right is Perseus and then Andromeda with M31, the Andromeda galaxy just above it. To the far right is the great square of Pegasus and below it Pisces the Fish. Lots to enjoy and photograph! Now to the Southern hemisphere!
What's Up in the Southern Hemisphere? The star chart below shows the sky in the south over Sydney at 20:00 on December 1 st. There is no star like Polaris close to the pole in the southern hemisphere. I have switched on a guide to show you where the south pole is in the constellation of Octans (the Octant). There are no distinctive stars in that constellation nor in the other constellations near the pole Pavo (the Peacock), Apus (the Bird of Paradise with no feet), Chamaeleon (the Chamaeleon), Volans (the Flying Fish) and Mensa (Table Mountain). However, you can see the distinctive star Canopus in the constellation of Carina. This is the second brightest star in the whole of the night sky, the brightest being Sirius the dog star in the constellation of Canis Major. Below Carina you can see the constellation of Vela. To the right of this you can see Crux, the southern cross, close to the horizon. To the right of this are the two pointer stars alpha and beta Centauri. Below is a chart of the sky in the west over Sydney also at 20:00 on December 1 st. Close to the horizon you can see Mercury about to set. Mercury is in the constellation of Sagittarius as is Venus above it. In the top right of the chart is the planet Mars in the constellation of Capricorn. Close to the south-western horizon is the top of the constellation of Scorpius. Above this is the arc-shaped group of stars called Corona Australis the southern crown. We have a similar group of stars in the northern hemisphere called the Corona Borealis - the northern crown.
The star chart below shows the sky in the north-east over Sydney at 20:30 on December 1 st. On the left you can see the Andromeda constellation with the Andromeda galaxy (Messier object M31) just below it. Above it is the Triangulum constellation showing the Triangulum galaxy (M33). In the middle of the chart you can see the Pleiades (M45) in Taurus the Bull. To the right of this is the constellation of Orion with Lepus the Hare above it. That is all until next year. Meanwhile have a good Christmas and all the best for 2017. Happy stargazing! Dark skies! Valerie Calderbank FRAS