To help you learn about the southern night sky, Sydney Observatory provides an audio guide/podcast, transcript of that audio, and a sky map or chart each month. This month’s guide is presented by Geoffrey Wyatt, Sydney Observatory’s Senior Astronomy Educator.
Geoff provides fascinating insights into the night sky as we see it now and as we have learned from it through history. For example how Sirius, the brightest star in the constellation Canis Major (the Big Dog), and also the brightest star in the sky, enabled the ancient Egyptians to calculate the length of the year.
Among the night sky highlights Geoff tells us about for this month is the Geminid meteor shower which you’ll have a chance to see on 15 December. Listen to the audio, or read the transcript below for more details.
SEE THE SKY CHART
We provide an embedded sky map (below) and December 2011 night sky chart
(PDF) which shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.
[scribd id=69661744 key=key-27s6yn0qokvcfhsd6l8a mode=list]
BUY THE BOOK
Our annual book, ‘The 2012 Australasian sky guide’, by Dr Nick Lomb has more information and star maps for months from December 2011 until December 2012 inclusive, plus information about the Sun, twilight, the Moon and tides, and a host of other fascinating astronomical information. You can purchase it ($16.95) at Sydney Observatory and Powerhouse Museum shops or other good bookshops, or online through Powerhouse Publishing.
READ THE TRANSCRIPT (after the jump)
Hello. I’m Geoffrey Wyatt, Senior Astronomy Educator here at Sydney Observatory, and I’ll be talking to you about what’s visible in the sky for the month of December. Remember that December is the 10th month of the old Roman calendar, before it was reformed in about 700 BC, and then later on, of course, by Julius Caesar for the Julian calendar, and more recently the Gregorian calendar, making it the 12th month of the yearly calendar.
Now, you’re going to need a few bits and pieces to help you find your way around the night sky as we go through this podcast. You’ll need a printed star map, which you can download from www.sydneyobservatory.com.au, or you can have the map that’s in the ‘Australian sky guide’ book produced by Dr Nick Lomb.
What I want you to do is go outside after sunset and find yourself a fairly clear view of as much of the sky as possible. Now, obviously, if you’re at the bottom of a valley, that’s going to make things a little tricky. If you’re on top of a hill surrounded by buildings, that will also make things a little tricky.
Try and find somewhere where you’ve got as clear a view as you can of north, south, east, and west. We’re going to work our way around the sky, but it really is important that you do have that map with you because some of the star pictures that we talk about are somewhat challenging.
First of all, if you’ve got a comfortable blanket and a pillow to lie on, lay down, look up, and look at the stars. Eenie meenie minie mo, pick a star. It doesn’t matter to which one you go.
When you look at a star, I want you to realise that we are their children. Now, that’s quite a bizarre statement to start with. But we think that all the atoms that are inside our bodies, everything on this planet has been, if you like, recycled through the life and death of several stars before the Sun. So, you and I are recycled star material. Really quite a fascinating thought when you look up at a sky filled with beautiful stars.
Don’t forget, the best time to look at the sky is, of course, when there’s not too much light pollution, especially from that nearby, rascally Moon that tends to get rather bright once a ‘moonth’.
What we’re going to do is wait until the Sun has gone down in the west, so it’s a little bit dark. And I want you to start off and look to the position of 23 hours right ascension and 30 degrees declination to the south. Huh? How on Earth are you going to find that? Well, there are many different ways of navigating through the night sky. And don’t forget, one of the prime uses of the stars for thousands of years has been exactly that: to navigate. So, find your way through the night sky, then use the stars to navigate by.
Right ascension…. Declination…. You may or may not have heard of those things before. But what we’re going to do is use a much simpler way to navigate. You need to be able to find your cardinal directions, of course: north, east, south, and west. And you’re going to use your fist at arm’s length, your fingers spread from pinkie to thumbtip at arm’s length, and your pinkie held at arm’s length.
You see, for the average person, if you hold your hand at arm’s length and spread your fingers, from pinkie to thumbtip is roughly 15 degrees for the average adult. A clenched fist held at arm’s length is roughly 10 degrees for the average adult. A pinkie held at arm’s length is roughly one degree, or twice the size of the full Moon, at arm’s length.
What we’re going to do is start off and look towards the west for the brightest star that you can see, about 60 degrees – that’s a fair way up – above the horizon. Sixty degrees, from the method I just described to you, will be four handspans. So, four handspans above the western horizon is our starting point for our sky tour for this month of December.
The bright star that you find at this position is called Fomalhaut, the Solitary One. Fomalhaut is a relatively young star at 300 million years old. It’s about 25 light years away, making it relatively close, and the 18th brightest star in the night sky. Ooh, 25 light years away. Remember that a light year is the distance that light travels in one year. And it actually travels quite a long way.
At 300,000 kilometres per second in the vacuum of space, light travels approximately nine and half thousand billion kilometres per year. And we’re looking at a relatively young star in the western sky, 25 light years away.
The Hubble Space Telescope’s actually had a good look at this star, Fomalhaut, and found that it actually has the birth of some young planets around it. So when you look at this star with the naked eye, I want you to consider that you’re looking at what the Sun and the Solar System may have looked like four to five billion years ago. In effect, you’re looking at our younger sibling.
Fomalhaut is the brightest star in the constellation of Pisces Austrinus, the Southern Fish, not to be confused with the zodiac constellation of Pisces, the Fish. Pisces Austrinus, well, you’ve got the one bright star I’d have to tell you, and that is Fomalhaut. The rest of it is a bit challenging to see.
But if you’ve got one of those fabulous, old, paisley swirled ties from the 1960s that seem to become fashionable every 20 years or so, look for the paisley swirl in the sky, over in the western sky. And Fomalhaut is the brightest star in that group of stars, and that is the Southern Fish drinking the flow of water that flows from the constellation of Aquarius.
What I want you to do now is to go ever so slightly to your left from Fomalhaut, and you’re going to look for the long necked bird, Grus, the Crane. Grus is not the easiest constellation to see either. Intriguingly, it represents a crane that’s trying to eat the fish of Pisces Austrinus. To some people of the Pacific, Grus the Crane represents a fishing pole trying to catch a fish. So there’s an interesting crossover of, if you like, unrelated fishing stories there.
Continue to your left for about 40 degrees, so four clenched fists towards the south, and about 60 degrees, still, above the horizon, and you’ll be able to see the eighth brightest star in the night sky, Achernar, or the River’s End according to its Arabic name. What you’re looking at here is a star that’s about 3,000 times more luminous than the Sun, at a distance of about 144 light years away.
Rather intriguingly, Achernar, The End of the River, is quite unusual because, to some Indigenous cultures in Australia, this star is one of two campfire stars used by the Kungara Brothers, represented by the Large and Small Magellanic Clouds, where they take bad people at the end of their life to roast them.
So, in classical Greek stories from long ago, this star represents the end of the river. And, to some Indigenous cultures, it also represents, I suppose, the end of the river, in a different way.
Also, Achernar is intriguing because it spins about 15 times faster than the Sun. Now, since it’s about eight times the mass of the Sun; it’s a pretty big star. But spinning 15 times faster means it bulges around its equator nearly 50 percent greater than it does, if you like, in a polar circumference. That makes it one of the flattest stars that’s ever been observed. A rather intriguing star.
Also, Achernar is directly opposite the South Celestial Pole from a group of stars that people quite often wish to see at this time of year but simply just can’t see, and that group of stars is the Southern Cross. At this time of night, you’ve got absolutely no chance of being able to see it, unless you wake up until around 3am or later.
Unfortunately as a result of that, people quite often get confused by some false constellations or asterisms, such as the False Cross and the Diamond Cross.
What we’re going to do is continue around towards the south where we’ve just been looking at Achernar. And we’re going around towards the south-east to where we’ll see the second-brightest star in the night sky called the Canopus.
Canopus is a little bit lower than Achernar. It’s only about 40 degrees up from the horizon and it’s a star that’s about 15,000 times more luminous than the Sun and about 310 light years away. It’s one of the more famous stars in the sky because it is simply so bright.
It used to be part of a much larger, now defunct constellation called Argo, the Ship. It dates back to the time of the Trojan Wars and was a ship that carried Jason and the Argonauts in search of the Golden Fleece.
But it was such a large constellation, it’s been broken up into four smaller constellations: Vela, the Sails; Carina, the Keel, of which Canopus is the brightest star; Pyxis, the Compass; and Puppis, the Deck.
If we continue around towards the east, we can see, just about 20 degrees above the horizon, so that’s two clenched fists, a star that is 25 times brighter than the Sun and just 8.6 light years away. Eight point six light years is actually not very far so it’s a relatively close star. It is the brightest star in the night sky, Sirius, the Dog Star.
If you look at the map and use a little imagination, you may just be able to see a fairly simple stick figure of a dog. Now remember, you have to use your imagination so don’t expect the elaborate diagrams that we so often see on antique star charts, but a simple stick figure of a dog.
You’re looking at the constellation of Canis Major, and the brightest star, Sirius, the Dog Star, which is an absolutely fascinating star. Not only is it the brightest star in the night sky, it’s a star that was used by Egyptians thousands of years ago to work out the length of the year to the 365 and a quarter days in something called heliacal rising.
What the Egyptian priests would do would be to measure the position of this star throughout the year and measure the angle between it and the rising Sun. As the angle got smaller and smaller, they’d keep track of it until the smallest angle they could get before Sirius was lost in the glare of the Sun.
And that’s our point of heliacal rising and from that, incredibly, they worked out the length of the year to be 365 and a quarter days. Which is not exactly right, but it’s within just about 11 minutes or so and that’s really quite an amazing feat.
Of course, Sirius is famous more recently because of its namesake of Sirius Black used in the ‘Harry Potter’ sequence of novels. And of course, HMS Sirius, one of the ships to come to Australia as part of the First Fleet.
By the way, some Indigenous communities of Australia used the view of Sirius low in the sky at this time of year as a calendar marker, if you like, to work out that this was the time of year that dingoes would have their pups. Because apparently, young dingoes made for fairly tasty eating.
So, Sirius can be used to work out the time of year. It can be used for navigation by the bright star and it can also be used as a time to look for dingo pups, which is not so good if you’re a young dingo pup, of course.
Continue ever so slightly towards the east-north-east and you’re going to look for a group of stars fairly close to the horizon. Look carefully for one star that is slightly orange reddish. Now typically, orange red means that a star is not very hot.
Now a star can be not very hot for one of two reasons. First, because it’s a very small dim, not-so-hot star in the first place, but we can’t see any of those with the naked eye. So, it means it’s orange reddish for the other reason and that is it’s dying. So typically, orange red marks a star that is coming towards the end of its life.
And the star that we’re looking for here is called Betelgeuse. Now, there are all sorts of different pronunciations for this, but none of them are correct according to the old Arabic pronunciation. Something along the lines of Ibt al Jauzah [with an accent like a dot under the ‘t’, and pronounced something like ‘Abtalyarzah’] which means ‘armpit of the big man’.
Not exactly the nicest name for a star, but according to one of the old star maps that’s been found, if you like, made its way through the Dark Ages when most of Europe went completely mental. It was Arabic astronomers that pursued knowledge and preserved it from the Old World, so many stars still do have Arabic names.
Unfortunately, since the Renaissance, the pronunciation of those names has been abused and changed dramatically. So we now, in fact, call this star Betelgeuse [pronounced ‘Beetle Juice’] and we’ve sanitised it a little bit to mean ‘shoulder of the giant’.
Now, Betelgeuse is an enormous star. It’s about 1,000 times the diameter of the Sun. That means if the hand of god were able to reach in and move it from where it is and plonk it into the middle of our solar system, it’s so big it would extend beyond the Sun, past the orbits of Mercury, Venus, Earth and Mars.
A truly huge star. It’s about a 135,000 times more luminous than the Sun, which is really quite amazing. Yet, it’s relatively young. It’s only about 10 million years old. Oh, it’s only a baby star, but that’s typical. You see, stars if they’re very large, consume their hydrogen fuel rapidly, evolve and die.
Will this star die in the most spectacular of ways and become a supernova? Well, probably not because it’s probably not quite large enough, but what it may do or what it will do is shed its outer layers in a very gentle death. And those outer layers will drift away to form a planetary nebula and expose the white heart furnace that was its nuclear heart to become a white dwarf.
When will this happen? Soon, but we just don’t know when. It really is quite a beautiful star to look at when you consider what it is and what is will become. To most of us, however, as I’ve already indicated, Betelgeuse represents the armpit of a mighty hunter and that is, of course, the hunter Orion.
Orion was a fairly bold man. He boasted to the Goddess of the Hunt, Artemis, that he could kill any animal on the Earth. Now, not exactly a smart thing to say to the Goddess of the Hunt and to a goddess who was the protector of animals as well.
To teach Orion a lesson, she created the giant animal, Scorpius, to come to the Earth and attack him. Orion defended his life fairly well, but he ultimately did lose. But the battle was so epic and so spectacular that it even caught the attention of Zeus, King of the Gods, himself.
After the battle in which Orion died, Zeus placed the scorpion in the sky. Artemis, however, placed the body of Orion into the sky to teach mortals a lesson directly opposite the scorpion. So, if we see Orion rising in the east at the moment, then the scorpion will have just set or be just setting in the western sky.
Also, Orion sits very close to the celestial equator. And intriguingly, Australians, our cousins across the ditch in New Zealand and those in South Africa, tend to look at the main or the brightest stars in Orion the Hunter and see not a mighty hunter, but a saucepan. Yes, a typical kitchen saucepan used to boil your eggs or cook your pasta, whatever. This group of stars in Orion does look a lot like a saucepan.
What I want you to do is look for three bright stars, roughly the same brightness, equidistant that form a lovely straight line. These stars will form the base of the saucepan. From one side, you go up to the side of the saucepan. On the other end of the row of three, you go up, and then you see three fainter stars off at an angle. These stars make the handle of the saucepan.
If you have a pair of binoculars or a small telescope, please look at the middle star like object in the handle of Orion’s saucepan. What you’re looking at is a cloud of gas and dust that’s about 24 light years in diameter so it’s absolutely huge. And you’re looking at the birthplace of stars. You are seeing stars being formed at this very moment.
At about 1,300 light years away, there are about 700 baby stars being formed in this cloud of gas, which we give the truly unromantic name of M42. Yes, you’re looking at the 42nd object in the catalogue developed by a man whose name began with M, Charles Messier. Not terribly romantic. Sometimes also called the Great Nebula in Orion.
It is a spectacular object to look at, especially if you have a small telescope or if you can get to an observatory. Again, however, it’s best not seen from the city or near a bright light source or especially if the full Moon is up.
Only, as I’ve mentioned, Australians, Kiwis, and South Africans refer to it as the Saucepan, this part of Orion the Hunter, because of the way we see it. It was originally named from the Northern Hemisphere. So, to most people, it still represents, if you like, the belt and the sword of Orion the Hunter.
What we’ll do as we leave Orion’s belt, we’ll continue around towards the north-east, another 20 degrees or so and we’ll see, yet another orange reddish star. This is the base of an upside down V, which represents the head of Taurus the Bull.
The star is called Aldebaran. It’s about 150 times brighter than the Sun at a distance of 65 light years away and it’s the 13th brightest star in the night sky. Its Arabic name means follower’, but follower of what? Well, close by, if you look very carefully, you’ll see a small group of stars known as M45.
And M45 is a truly spectacular group of young stars. It’s about 25 degrees above the horizon and dominated by young, hot blue stars less than 150 million years old. At a distance of about 445 light years away, it’s actually fairly close and is quite beautiful to look at.
This young group of stars is typically called the Pleiades. Although I’ve also said it’s the 45th object in the catalog developed by a man whose name began with M, so it’s also called Messier or M45.
Astronomers love this group of stars. Astronomy students hate this group of stars. Why? Well, you see, M45 is an open cluster. It’s a young group of stars and it’s probably the nearest thing we have to a control situation in the evolution of stars. You see, these stars were all siblings in this little cluster that we’re looking at in Taurus the Bull.
These stars were born at the same time so they’re the same age. They have the same chemical composition because they’ve been made from the same cloud. They’re at the same distance of 445 light years away. The only variable is mass. And guess what? The most important variable it would seem to be for stars is indeed their mass.
So you have a situation where there’s only one major variable and as a result, this group of stars, because it’s so easy to see and so famous, is quite often in astronomy exams. So it is a beautiful group of stars to look at, but it’s just a little bit testing if you’ve got an astronomy paper coming up quite shortly.
What these stars represent, according to one legend, are the gods, Atlas, who carried the world upon his shoulders, his wife, Pleione, and their seven daughters. And that gives you a bit of a hint to their common name. You see, for reasons which we just don’t understand, universally these stars seem to be referred to as the Seven Sisters.
It’s strange for two reasons. First of all, most people who look at them can see only six. If you’ve got really good eyesight and the sky is really clear, you can typically see nine. Rarely do you meet anyone who can see seven. So, why they’re called the Seven Sisters, we just don’t know.
Secondly, why are they Seven Sisters? Why not Seven Hills of Rome? Why not Seven Dolphins Swimming or Seven Birds Flying? Universally again, however, these stars are typically referred to as Seven Sisters. Really quite strange.
And I should point out that even in some indigenous communities across Australia, there’s one fabulous story of the Woode Gooth-Tha Rra and Minma-Birnee, which refers to the Seven Sisters who descended from the heavens to form all the living creatures on the Earth. But one of the sisters left the group with the Spirit Man and as a result, she was never allowed to never return to her place in the sky.
So, we also have this idea of one of the Seven Sisters having gone missing. Again, most people see only six. It really is a strange thing to consider for this beautiful group of stars.
As we leave Taurus the Bull which, by the way, is one of the oldest of all constellations in the sky and we continue around towards almost due north, we can see another one of the zodiac star signs. But unfortunately, it doesn’t have much to it – and that is Aries the Ram. Look, it’s only noteworthy because of its brightest star, which is called Hamal. And it has only about two other bright stars in it.
Now, these stars represent Aries the Ram that produced the Golden Fleece that was so important in the story of Jason and his Argonauts. And that is a story that crops up time and time again as we look across the night sky.
Continue past Aries the Ram towards the north-west and quite low, you’ll be able to see four stars that make up a fairly large square. Well, you might think, well, big deal. It’s a square. So what? Ah, but this is no ordinary square. You are looking at the constellation of Pegasus the Flying Horse.
If you use your imagination and consider the square of the horse to be the body, you may be able to see from one corner, the long neck going up to the head and down to the nose of the horse. But for a Flying Horse, what’s missing?
How about legs, especially rear legs, and more importantly, wings? Not easy to see, but if you can make up a square with two short front stumpy legs and the long neck, low in the north-west, you are looking at the constellation of Pegasus.
A little bit higher than Pegasus and wrapped around it somewhat is a large group of stars marked on the star map that looks a bit like a large V. In fact, a very large V that has a bit of a knobby circle at either end. What you’re looking at here is one of the zodiac constellations of Pisces.
Pisces the Fish represents the Goddess Venus and her son, Cupid. You see, in one particular adventure, they turn themselves into fish to swim along a river. But so they wouldn’t get lost, they tied a thread to each other. So what you see, although it’s fairly difficult, is a little group of stars representing Venus and Cupid. And the big V shape between them, wrapping around Pegasus is the thread that keeps them together.
Continue back around towards the west where we started and you’ll be able to see…Well, you may just catch a hint setting in the west of Capricornus the Sea Goat. I think it’s a little bit too low for us to worry about at it this time, but we’ll look at that another month.
So what we’ve done, we started looking west fairly high into the sky to the star Fomalhaut and the constellation of Piscis Austrinus, and we’ve worked our way right around the horizon. But we haven’t looked so much directly overhead. Why not? Well, quite simple really.
At this time of year, there’s not a whole lot overhead. Sure, there are some stars up there, but you’re only looking at constellations such as Fornax the Furnace or Phoenix the Bird, Cetus the Sea Monster (or Whale depending on some stories) and Sculptor. None of these constellations is easy to see or particularly famous or noteworthy.
You see, to some extent, they’ve been manufactured. Most of those constellations were produced in the 18th century by Nicholas Louis de Lacaille. And they’ve been put there just, if you like, to fill out missing gaps in our constellations.
I didn’t mention it earlier on, but the constellations that we’ve been talking about are simply suburbs in the sky. There have been patterns that have been made there to help us find and navigate our way around the night sky.
You see, if you can see 2,000 3,000 stars, which you could see from a dark location if you’ve got good eyesight, it’s nearly impossible to remember the position of all those stars.
But break the sky up into manageable chunks, such as the zodiac constellations, and suddenly it’s much easier and more convenient to find your way around. So these newer constellations have never had any historic significance. They’ve just been added, if you like, to fill in some of the empty areas.
So all our action this month has been within, if you like, the band 40 60 degrees above the horizon. That also explains why you can’t see any of the Milky Way if you look outside.
The Milky Way, the river in the sky, Via Lactea, is simply not visible at this time of night, at this time of year. You’ll need to wait much later, early in the morning, and you’ll be able to see some of the Milky Way, rising in the east.
Don’t forget that absolutely everything in the sky that you see –despite the fact that you can’t see the bright part of the Milky Way – is part of the Milky Way, except for three things.
And those three things are the great galaxy in Andromeda, which is too low for us to see in the north by north west, at the rear end of Pegasus, the Flying Horse. But we should be able to see high in the southern sky, the two Clouds of Magellan.
So, look for the Clouds of Magellan, and what you are looking at, they look like pieces of Milky Way that have broken off and drifted away. What you’re looking at are two irregular galaxies that are approaching us at phenomenal speed.
They’re about 160,000 and 200,000 light years away, respectively. We don’t have to worry about them, even though they are coming toward us because our Milky Way, the bully on the block, is tidally pulling them apart.
In fact, we can already detect a stream of stars going from the Large Magellanic Cloud towards the Milky Way. So our galaxy will disrupt and, if you like, assimilate – if you’re a science fiction fan – the Large and Small Magellanic Clouds, but not in the immediate future.
Highlights for the month of December 2011. The first quarter Moon will be on Friday 2nd at 8.52pm. Full Moon on Sunday 11th at 1.36am. Last quarter on Sunday 18th at 11.48am and new Moon on Sunday 25th at 5.06am.
The summer solstice will occur on Thursday 22nd at 4.30pm.
The night sky is dominated by two very bright planets, and that will be Venus quite low in the west after sunset in the constellation of Capricorn. And higher up you’ll be able to see the second brightest planet in the night sky at the moment, Jupiter in the constellation of Pisces.
On 27th December the crescent Moon will be slightly below and to the right of Venus. On 6th December the gibbous Moon is below and to the left of Jupiter in the constellation of Pisces.
In the morning sky the Moon scoots past the planets on a few different days: on the 17th, the gibbous moon is above and to the left of Mars.
On the 20th it will be to the left or the north of Saturn. And on the 23rd the crescent moon will be to the north and to the left of Mercury although that will always be a hard one to spot as Mercury is so close to the Sun.
There will be a total lunar eclipse this month. It will commence on the 10th at 10.45pm, and totality will go through until 3.17am on the morning of 11th December. So it’s a very good opportunity – although it’s a tad late – to get up and enjoy the Moon passing through the Earth’s shadow. However it’s a fairly brief one and it will only be in totality for about 51 minutes as opposed to 1 hour and 40 minutes as it was earlier in the year for the June eclipse. Although this time we will see all of it.
December is also the best time to spot some meteors in the form of the Geminid meteor shower. Sadly this year, however, early on the morning of the 15th, the gibbous Moon will be in the way and that will certainly blot out the fainter ones, so you’ll only get an opportunity to see the very bright ones – if they do occur. And don’t forget that the Geminid meteor shower, low in the north, is caused by an asteroid called 3200 Phaeton. And it’s an asteroid that travels closer to the Sun than any of the other large discovered numbered asteroids. So it is a good opportunity, however, the Moon will be in the way.
Don’t forget, if you’d like to print of a sky chart or map, you can do so from our website at www.sydneyobservatory.com.au.
And you can purchase your ‘Australian sky guide’ by Dr Nick Lomb at Sydney Observatory or at the Powerhouse Museum. You can also purchase it from good book sellers, and from the Powerhouse Museum’s website.
My name’s Geoffrey Wyatt, and I’m the Senior Astronomy Educator here at Sydney Observatory, and I hope you’ve enjoyed your sky guide tour for the month of December 2011.