Looking to the Skies December 2020 By Brian Davidson

We have been struggling a bit recently with observations  but the close approach of the Moon and Mars produced the goods on the 25th November.  It was good to see. Let’s hope ‘the great conjunction’ lives up to expectations but more on that later.
 
Observing
            December is the last month in the year and the winter solstice takes place on the 21st so I thought it was time to take a look at the bigger picture rather than focussing on the detail and where better to start than the winter night sky.The charts below represent the whole night sky at 10.00pm on the 8th December and at 9.00pm on the 23rd December. The first chart is the night sky facing north.

Most people believe that it is a good idea that we all know some basic first aid. How to deal with cuts, bruises, stings and nose bleeds. More serious problems can be left to the professionals. By the same token I think everyone should know something about observing the night sky and I’ve come up with my list of five objects in the winter night sky which I think everyone should recognise. (Yes, I did have a job limiting it to five!). Obviously this is my personal list and some of you may come up with your own different selection. These are the things which I think you should be pointing out to your children and grandchildren on a crisp clear winter’s evening. It might just get them hooked on astronomy.
 

1. The Plough    Everyone in the northern hemisphere should be able to recognise The Plough. As an asterism it has a large clear distinctive shape- no need to worry about the fact that it is part of a constellation much of which is indistinct. Of course it is even more interesting when you point out that the ‘pointer stars’ of the plough lead you to the pole star, Polaris, and most children know that it gives the direction of north. The distance to Polaris is about four times the distance between the ‘pointer stars’.
2. Cassiopeia      I’ve chosen Cassiopeia both because it’s worthwhile in its own right, easily recognisable for its ‘W’ or ‘M’ shape and because of its link to Polaris and The Plough. A line from the third star in from the end of the plough handle through Polaris and extended about the same distance beyond leads to Cassiopeia. No need to get too technical but it is worth saying that The Plough and Cassiopeia are always in the night sky and rotate about Polaris during the year as if connected by a pole with Polaris at the centre. Of course the other stars do likewise but many dip below the horizon at certain times of the year and cannot be seen. This explanation might just help people understand why the night sky changes from month to month as it does.
   

This second chart is the night sky facing south.
 
3. Orion   As I said last month, Orion has everything and I would defy anyone not to have it on their list of things to see in the winter’s evening sky. It is a brilliant object in the sky and attracts everyone’s attention. Its bright stars and distinctive belt are likely to produce questions so you might want to be able to name Betelgeuse and Rigel if asked. Like The Plough, Orion also leads you on to other objects like my next choice.
4. The Pleiades ( or Seven Sisters)    The Pleiades doesn’t form a large object in the sky but it does catch the eye when looking at the sky hence on the list. Found by extending a line from Orion’s belt through the red star Aldebaran. Use a pair of binoculars to enhance the view.
5. Sirius Simply a star but it does happen to be the brightest star in the night sky when it is present so it should be up there with the tallest mountain and longest river as an item of general knowledge. It is also of historical interest because its appearance was used by the early Egyptians to tell that the river Nile was about to flood. Again easily located to the bottom left hand side of Orion.
 
I’m sure many of you will have your own favourites and might be wondering why I have not included them but I wanted to restrict the list to five. Remember this is just a starting point.            
 
Something to look out for
            Mars continues to be easily observed during the evening but this month it will have to take second place to ‘the great conjunction’. First though, there is a close approach of a three day old Moon with Jupiter and Saturn on the 17th December. Visible above the south-west horizon from around 4.30pm as darkness falls, this will provide an opportunity to get your bearings right for the ‘great conjunction’ four days later.   On the evening of the winter solstice, 21st December, the planets Jupiter and Saturn will be within about a tenth of a degree of each other. Remember that the Moon has an angular width of about half a degree so these two planets will be separated by about one fifth of the Moon’s diameter. Jupiter is much brighter than Saturn so they might be difficult to resolve. Of course these planets are widely separated in space and this optical effect comes about because Jupiter revolves around the Sun in just under 12 years whereas the orbital period of Saturn is over 29 years and so Jupiter laps Saturn every twenty years. This is the closest they have appeared since 1623 and won’t appear as close again until 2083 so definitely a once in a lifetime event.

I’ve included the chart just to show that Jupiter and Saturn will be close to the horizon and will set at 6.30pm. The Sun will set at 4.03pm so you will need to be outside during twilight to be ready to observe either side of 5.30pm. and you will need an unobstructed view to the south-west/west horizon. Let’s hope the skies are clear. The planets will still be within a degree of each other the weeks either side of the winter solstice so try to catch them whenever you can. The diagram below shows their separation over a two week period.