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December astronomy column highlights the changing shaps

Grab your telescope and look up to the night sky this June.
Grab your telescope and look up to the night sky. - 123RF Stock Photo

By Rolly Chaisson 

Hello again fellow Sky Watchers
We are approaching the end of another year in the sky and this month I’d like to show you how the shape of the sky can change with the time of year.
Let’s go back to late June at 10 p.m. (in our imagination) and we would see a shape which we called the Summer Triangle. It is made up of three stars, each in a different constellation – Deneb in Cygnus, the Swan, Vega in Lyra, the Lyre, and Altair in Aquila, the Eagle. We discussed this in a previous article in this column. It was seen well up from the Eastern horizon, and in particular, Vega was at the top of the triangle, Deneb lower and to the left, and Altair at the bottom. Now, indeed, the Summer Triangle first rises in April and only disappears from the sky as December passes. So, it’s up in the sky for ¾ of the year. But, do we see it with the stars in the same place as the year passes? Not at all. As the Triangle passes lower into the West-Northwest, we find the star Deneb in Cygnus, now sitting at the top of the Triangle with the other two stars, Vega and Altair forming a line making up its base. Indeed, as we arrive in January, Deneb is the last part of the Triangle to disappear.
Is this the only shape change that occurs in the sky? Not at all, at certain times of the year, Cassiopeia, the Queen, looks like a large 'W' and some months later, it has flipped and looks like a large 'M'. Changes, such as these are typical of our sky throughout the year, and this is only one of the many things that make our sky so wonderful and entrancing.
So – what is in the sky this month?
As always, let’s start with the naked eye planets first.
So, it’s night, after the sun has gone down. Would you like to see a partial bracelet or necklace? Look somewhat low in the Southwest on Dec. 1 and you will see Saturn, Venus, and Jupiter in a line, evenly spaced and looking like jewels on a band. But I have to warn you, over a few nights, it changes quickly, as Jupiter sinks even lower, slowly followed by Saturn. But at the same time, Venus races ever higher, and towards Saturn each night.
I should interject here that this is a little like a month of losses. Jupiter will disappear into the sunshine at dusk by the end of the second week in December. It will then pass behind the sun and later, re-appear in the morning sky.
Saturn is following this same path and it will be lost from sight by Christmas.
Venus and Saturn pass closes to each other on the night of Dec. 10.
Now for the morning sky. Here we have Mars and Mercury.
Mars is getting a little brighter and a little further before sunrise. It is up in the predawn sky a good 3 hours before sunrise in the Southeast.
As the month begins, Mercury is also present, but again, it’s the month of losses as Mercury sinks into
The predawn twilight and by Dec. 17 will be lost to our sight.
But – yes – a month of losses, but as always, all will return!
Now December is the month of the Geminid Meteor shower, but as has been the case for so much of the year, the best peaks of the Shower are the Evening and morning of Jan. 14. However, that is within 2 days of the full moon and therefore, many of the fainter meteors will be lost from view. That is not to say we shouldn’t watch, but it won’t be the great shower that it is in some years.
Don’t forget that this is the month when Orion – “the Great Hunter” will begin to dominate the southern sky along with the Winter Circle of stars around it. Don’t miss it.
This month:  
Full Moon – Dec. 11
New Moon – Dec. 26
Good hunting. Merry Sky Christmas.
See you next month.
Your Night Sky Guy,
Dr. Rolly Chiasson

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