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, Astronomy Educator at Sydney Observatory (pictured at right).
The warm summer evenings in December provide an excellent opportunity to go outside and enjoy a comfortable view of the night sky. The clouds of Magellan, our closest visible galaxies, are easily seen in the south from dark locations. The famous ‘saucepan’ or Orion’s belt and sword dominate the eastern sky along with the dazzling Sirius the dog-star. The Geminid meteor shower peaks early mid month and is one of the best chances to see shooting stars all year. Geoffrey Wyatt from the Museum of Applied Arts and Sciences’ Sydney Observatory will help you navigate through the brighter stars, constellations, lore and December’s key events. For this and more, listen to the December 2015 night sky guide audio, or read the transcript below.
HEAR THE AUDIO
You can subscribe with iTunes or upload the audio to your iPod or mp3 player, or listen to it on your computer.
SEE THE SKY CHART
We provide an embedded sky map (below) and a December 2015 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.
BUY THE BOOK
Our annual book ‘The 2016 Australasian sky guide’ by Dr Nick Lomb has more information and star maps for months from December 2015 until December 2016 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 (additional packing/postage costs apply).
READ THE TRANSCRIPT (after the jump)
Transcript of the December 2015 monthly sky guide audio
Boodyeri kamaru. That’s “Hello there” in the language of the Gadigal people of the Eora nation, the traditional custodians of the land upon which Sydney Observatory was built in 1858.
My name’s Geoffrey Wyatt, I’m one of the Education team here at the Museum of Applied Arts Sciences’ Sydney Observatory. I’m going to talk to you about what’s visible in the sky for the month of December the 10th month of the old Roman calendar. No one is really sure when it became the twelfth month but it was sometime before the reform of Julius Caesar in 45 BC.
This audio guide, transcript and printable sky map are available free from our website at www.sydneyobservatory.com.au/monthlyskyguides.
To get the most from this podcast you’re going to need some resources. One of which will be a map from the website or from our book ‘The Australasian Sky Guide’ by Dr Nick Lomb. More details about that at the end of our podcast.
You’ll also need a clear view of the sky. Look, you can only get to what you can get to. The higher you are, the clearer the view you have of all four cardinal directions, north, east, south and west, away from lights, away from trees and buildings, obviously the better the view.
At this time of year it should be quite nice outside, so a nice comfortable blanket to sit on, and perhaps a drop of Chardonnay for those of us old enough to enjoy it will assist you in the enjoyment of the evening. What I want you to do is wait until about 30 minutes after sunset and look to a part of the sky that is 23 degrees right ascension, and 30 degrees declination.
Have you got it yet? This raises a problem. Most people don’t know what right ascension and declination are. They are the astronomer’s version of longitude and latitude. It’s a pretty hard way of finding your way around the sky, so we need some other way for you to find your direction. The easiest is a combination of azimuth and altitude or elevation.
Most of us can find the four cardinal directions north, east, south, and west. Depending on the time of year the Sun roughly sets in the west and rises in the east. We can then usually find north and south. If we consider this a little more systematically we can find our direction around the horizon starting from north and moving in a clockwise direction. East would therefore be 90 degrees azimuth, that is, 90 degrees east of north. 180 degrees azimuth is therefore south. 270 degrees is west, and so on. That part is relatively easy.
But now consider how high up from the horizon something might be. If I were to say to you an object is directly overhead, I think most people could figure out that that’s 90 degrees up. Halfway up would be 45 degrees. But when it comes to other angles, it is surprising how poorly we do at estimating them so we need a bit of help.
What I want you to do therefore is to hold out your arm and clench your fist, but then hold up your pinkie. For most people, regardless of your age and your size, because the proportions are all pretty much the same, your pinkie at arm’s length will cover roughly one degree of the sky or twice the size of the full moon.
Close your finger in now so you’ve got a clenched fist, and you’ve got a marker for roughly 10 degrees. Outstretch your fingers and your thumb, and from pinkie-tip to thumb-tip you have 20 degrees. With a little practice you’ll be able to do this with ease.
Now we have an easy way of finding out directions. I want you to go to an azimuth of 270 degrees, okay so that’s west, and I want you to look about 60 degrees up from the western horizon. That’s three outstretched hand spans. What you’re going to look for is what’s known as the fourth of the four Royal Stars.
People have been looking at stars for many thousands of years. Later on I’ll talk about some of the Indigenous stories of Australia, because I believe that the Indigenous people of this beautiful land have been looking at the stars and passing their stores from one generation to the next longer than any other community, and that’s something that we should all be proud of.
For the time being, we’re going to look for one of our four Royal Stars as was determined by the people living in Mesopotamia, that’s the region between the Tigris and the Euphrates Rivers, as many as 5,000 years ago.
You’re looking for a star that’s only 25 light years away, a light year? A light year is simply the distance that light travels in one year in the vacuum of space.
Light travels roughly 300,000 kilometres per second in the vacuum of space. Multiply that by 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and 365.25 days per year on average, and you’ll end up with something like 9,500 billion kilometres which is such a silly number we just don’t use it. It’s too complicated.
We simply say, a light year is the distance that light travels in one year in the vacuum of space.
The star that we’re trying to find, as I’ve said is 25 light years away, it is 1.8 times the diameter of the sun. It’s a young white star and one of the first stars to have had planets directly imaged in orbit around it. That was only done in 2008. This star is the brightest star in the constellation of Piscis Austrinus, the Southern Fish, and the star is called Fomalhaut.
The ancients that I mentioned a moment ago from Mesopotamia used stars like this as a form of calendrical marker. They used the stars Aldebaran in Taurus, Regulus in Leo, Antares in Scorpius and Fomalhaut, as we now see it in Piscis Austrinus, to mark the key points related to the solstice and the equinoxes.
Thousands of years ago but no longer, Fomalhaut was the brightest star near the point in the sky that marked the Winter solstice as seen from the Northern Hemisphere. When you look at Fomalhaut, there are not many bright stars nearby. What I want you to try and see is an image of a fish. Let your imagination go, particularly if you’ve had that glass of wine I mentioned a little earlier and just maybe you’ll be able to see a fish. If not, how about one of those fabulous paisley swirls that were so popular in the 1960s. If you can see anything that looks like a paisley swirl with a bright star, Fomalhaut, at the chunky end of the swirl, then you are looking at Piscis Austrinus, the Southern Fish. It is, incidentally, drinking water flowing from the jug of Aquarius the Water Carrier, but goodness me, that’s so very hard to see.
Now that you’ve seen, though with some difficulty I would imagine, Piscis Austrinus, look ever so slightly to your left, or to the Southwest. You will probably need a star map but try to look for a long necked bird with trailing legs in flight.
This particular group of stars is called Grus the Crane. It was created by Petrus Plancius, and I don’t know if I’ve pronounced his name correctly, in the late 1500s. He was a fairly famous Dutch astronomer.
I’ve challenged you with these two groups of stars because I want you to see straight away, they look nothing like the images you see in star atlases or on many star maps. You’ve got to use your imagination to change the stick figures into more elaborate drawings but don’t give up, it’s well worth a try and when you eventually do see some of these constellations, it’s one of those, “Ah ha, I can see it!” moments.
40 degrees, or two hand spans to the left of where we are, and about 60 degrees above the horizon, so that’s three hand spans up, you should be able to see the ninth brightest star in the night sky and the brightest star in the constellation of Eridanus the River. The star is called Achernar.
Achernar is a rather intriguing star, because it’s about 7 times the diameter of the sun, but it spins 15 times faster. The effect of that rapid rotation is that the star flattens at the top and bulges around the middle. In fact, its equatorial diameter is about 50 percent greater than its polar diameter.
You’re looking at this star as it really was 140 years ago. You’re looking back into time. That means that this star is at a distance of 140 light years.
Some indigenous communities across Australia use Achernar and the nearby bright star Canopus to represent the cooking fires of two celestial brothers represented by the Large and Small Magellanic Clouds at a distance of roughly 160,000 and 201,000 light years away, respectively. They are two of the closest galaxies to us.
Achernar is also intriguing because it’s on the opposite side of the celestial pole from the smallest of all 88 constellations, and some of us here in the South would argue the most important of all 88 constellations, that is the Southern Cross or as it is officially known, Crux. Unfortunately, at the moment, it’s not that easy to see unless you wait until about 3am, when it will be low in the Southeast.
If you were to draw a line through the long axis of the Southern Cross going all the way across the sky toward Achernar then go back halfway along that line that’s pretty close to the south celestial pole. It’s not dead accurate, but it’s close enough.
Now continue to your left and drop down to about 35degrees altitude looking for another bright star. It’s actually the second brightest star in the night sky but its low altitude at the moment will dim it by about 50%. It is the star Canopus at about 310 light years away. It’s pretty big. It’s 8.5 times the mass of the Sun and 70 times its diameter which makes it really big. At about 1,300 times the brightness of the Sun it is the brightest star within 700 light years of our star the Sun. Yet as we look at it, it’s only the second brightest star in the night sky. I wonder why?
So clearly not only are the star’s intrinsic properties important, but others such as distance. There is another star that we’ll come to shortly which is not as bright but because it’s closer, it looks brighter to us.
Canopus is a fairly famous star. It was listed by the incredible astronomer Claudius Ptolemy in his Almagest around 150 AD. This whole region of the sky used to be part of a big constellation called Argo, the ship that carried Jason and the Argonauts in search of the Golden Fleece but over the years, astronomers thought it was too big, so they broke it up into four smaller constellations that we have now, Carina the Keel, Vela the Sails, Pyxis the Compass and Puppis the Deck.
Canopus is now considered the brightest star in Carina the Keel. The name itself probably dates back to the time of the Trojan Wars, and according to the poems and the stories of the time, it was the name of the ship’s captain. Fair enough, we have the ship in the sky, why not have the captain as well.
What I like about this star is that the Boorong indigenous community, a clan of the Wergaia language group in Northwestern Victoria, see this star as a male crow by the name of Wah. Wah was the first entity to bring fire to the people and he was an elder of the Nurrumbunguttias, the old spirits in the sky.
If you have a telescope or a pair of binoculars, this part of the sky, although it’s a bit low at the moment, is actually a beautiful region to scan. Not far away from the star Canopus or Wah, we have the intriguing object called Eta Carinae which contains a cataclysmic variable star, though some more recent ideas suggest it is a violent binary star system.
The historian Stanbridge actually recorded that the indigenous people I’ve mentioned, the Boorong, referred to this star during its outburst in 1843. In the early 1800s, it went from a fairly inconspicuous third or fourth magnitude background star to being the second brightest star in the night sky and then slowly fade away from visibility.
The Boorong incorporated this star’s variability into their Dreamtime or oral traditions, which is really quite amazing. As a result, this star became known as Collowgullouric Wah, which means the wife of the star Wah or Canopus.
Continue to your left therefore the east and look about 20 degrees above the horizon. What you should see is a twinkling display of the brightest star in the night sky. It won’t appear to be as bright as Canopus, which is higher up at this point in time, because being lower your looking through so much more of our protective atmosphere.
It’s a lot closer, at only 8.6 light years away, making it the fifth closest star to us. It’s also quite young, at roughly 200 to 300 million years. Its size? It’s nearly twice as big as the Sun and 25 times brighter than the Sun. While I’ve just mentioned a few numbers the main thing to note is that it’s close.
It’s nowhere near as big or bright as Canopus, but because it’s relatively bright and very close, it becomes the brightest star in the night sky as seen from anywhere on the Earth, and it is Sirius the Dog Star.
It’s a beautiful object and historically incredibly important. Thousands of years ago the Egyptians watched it very carefully. They’d see it disappear into the glare of the setting Sun and for about 70 days it would be gone. Then they would then keep watch for it in the East in the early morning.
When they saw it pop up in the East just ahead of the glare of the Sun, in something called heliacal rise they were able to work out the average length of the year to be 365 and a quarter days. Their error was just 11 minutes compared to the Tropical year we use now, and they did this thousands of years ago, which is truly an amazing achievement.
Sirius is beautiful, it’s also useful to work out the time of year. I should point out again that to the Boorong clans, it actually represents the star Warepil, and Warepil is a male eagle, and also an elder of the Nurrumbunguttias, the old spirits. To other indigenous communities across Australia, the appearance of Sirius marked the time of the year when it was time to go looking for tasty young dingo pups. Not so good if you’re a young dingo pup, but a use of the stars nonetheless.
Let’s continue towards our left, towards the east north-east and just 20 degrees above the horizon. Oh by the way, you may have noticed that we seem to be hugging fairly close to the horizon I’ll explain a bit more about that later on.
Look for an orange looking star. We call it a red super giant, but to most people, it’s not traffic light or laser red, it’s orangish. Anything that you see in this part of the sky that is not white or blue, and you’re probably looking at it. It’s the eighth brightest star in the night sky, 1,100 times the diameter of the Sun. Goodness gracious me, that’s a huge object.
Think about that for a moment. This little twinkling point of light that you’re looking at in the east north east, 20 degrees up, is 1,100 times the diameter of the Sun. Don’t forget, the Sun is 114 times the diameter of the Earth. You’re looking at something which is simply enormous.
Its distance? In the order of about 430 light years. It’s 100,000 times brighter than the Sun and its dying.
When you see a reddish looking star, it can be one of two things. It’s either an incredibly long lived, in fact, you could almost say, immortal star, or it’s a short lived star at the end of its life. The thing is, the very small, almost immortal stars, well, none of those are visible to the naked eye. So, when you look around the night sky, every single star that you see that is orange to red, is coming to the end of its life. They’re all dying.
We’re not exactly sure of the mass of this star, but we know it’s fairly big. As a result, when it does die, it’s going to explode as a Type II supernova. When? Next Tuesday at two o’clock. No, actually, we have no idea. It could be anytime within the next million years, a bit longer, a bit less. Who really knows? It would be really cool if it did explode during our lifetime because it’s relatively close by and would be spectacular to watch. However, let me assure you, it cannot possibly hurt us.
I haven’t told you its name yet, have I? This is one of the most unusual names in the night sky. A long, long time ago, its Arabic name was something like Ibt al Jauzah. Now, I’m sure I’ve mispronounced that but that’s okay. Most of us have pronounced it incorrectly and over the years its name has changed.
Its old name meant something like the ‘Hand of the Big Man’. It’s an unusual name and it’s difficult to pronounce. What happened is that over the years, its name has come to us from different Arabic communities in the Middle East. It’s gone from the ancient Egyptians, then to the Greeks, then of course, the rather unfortunate period called the Dark Ages, after which there was the Renaissance.
When people suddenly started to discover translations of a copy of the translation of a copy, we had all these strange star names.
As a result, many of them over the years have been mispronounced. You could perhaps say they have devolved. Therefore, Ibt al Jauzah is now commonly called Betelgeuse. Yes, I’m sure you’ve heard of it before. Some people call it ‘Betelgeeze’, ‘Betelguzz’, or even just ‘Betelgeeurse’. They’re all wrong. But they’ve become so common they’ve all been accepted.
Despite being the second brightest star in this particular group or constellation it is actually referred to as being the brightest which is just a tad odd. Which constellation or group is it? It is Orion the hunter, the might y hunter. Australians tend to get the name of Orion wrong, calling it the Saucepan, so that’s what we’re going to be looking for.
You should have a lovely clear view of it at the moment, looking towards the east north east. Find the orange glow of Betelgeuse, then go a little bit higher and you should see three stars in a row, close together of equal brightness. They make up the base of the Saucepan. You can go up one side, go back up the other side, and then off at an angle for the handle, and there you have it.
If you can find it, you’ve done well. I should point out that it’s not just Australians, in fact, our friends across the ditch in New Zealand often get it wrong as well, as do many people in Southern Africa.
There are many conflicting stories about some patterns in the night sky and the one I like about Orion is that he was the mightiest hunters. He used to hang around with the goddess of the hunt, her Greek name was Artemis.
But Orion made a mistake. He foolishly boasted that he could kill any animal on the planet. After all, he was a hunter. To teach him a lesson of humility Artemis created the giant scorpion, Scorpius (not ‘Scorpio’ – that’s its astrological name). The battle that followed was so incredible that it caught the attention of Zeus, king of the gods.
Ultimately, Orion was stung and killed by the scorpion. Zeus took the scorpion and placed him into the sky. Artemis, with a tinge of regret, took the body of Orion and also placed him into the sky. But as a reminder to teach mere mortals to curb their ambitions, she placed him on the opposite side of the sky from Scorpius so that the two could never come together again in battle.
With Orion coming up in the East now Scorpius has just disappeared from view over in the West.
It’s a fabulous constellation and at some stage or another, everyone has had a look at it. The problem is that here in the Southern Hemisphere, it’s upside down. We tend to look for and know the Saucepan, not so much the mighty hunter, Orion.
Before I move on, I should point out that some have suggested that Orion was, well, how can I put it? A little dim. You see the greatest hunter of all time is standing on a rabbit called Lepus the hare chasing a flock of doves in the form of the Pleiades nearby? Not exactly big game, are they? Again, there are many different stories.
Go back to the Saucepan as I’ve described it, and if you can, point your binoculars or your small telescope if you have one at the handle of the Saucepan. If you can do that, I want you to narrow in onto the middle star-like object of the handle.
You could, perhaps, call this a stellar maternity ward, for what you’re looking at is the birthplace of stars at the moment. You’re looking at is the beautiful object named M42. Oh, great, what a fabulous name. Remember that astronomers like many scientists love cataloging objects. M42 simply means that it’s the 42nd object in the catalog developed by a man whose name began with M, and it was Charles Messier.
He made up a list of well…red herrings, things not to look at if you were trying to find a comet. This particular object was simply the 42nd object in his catalog.
It is a nebula which is the Latin word for cloud. It’s a star forming cloud that’s roughly 1,300 light years away. It’s absolutely huge, 24 light years across. It’s part of a much larger cloud that you can’t see all of unless you do incredibly difficult astrophotography. The whole cloud is called the Orion Molecular Gas cloud.
The cloud of gas and dust that we can see is being lit up from within by at least 6 baby stars and we call them the Trapezium. If you have a look, you might just be able to see a few of them in there. We believe that there’s enough material to form around 700 stars at the moment. Six of them however, you can see relatively easily.
Leaving Orion, our next stop is a little bit further around towards the Northeast, and again, 25 degrees above the horizon. We’re looking for the star Aldebaran in the constellation of Taurus the Bull.
From here you’re going to see pretty much just another of these golden reddish orange stars. That tells us, as I mentioned before, the star is dying. But this is also the first of the four Royal Stars. Royal Stars? They were simply calendrical markers, the brighter stars next to a particular event in the sky.
Thousands of years ago, from Mesopotamia, Aldebaran in the constellation of Taurus was the brightest star near the Vernal Equinox. There are two points in the sky where the ecliptic, which is the path of the Sun the Moon and the planets, and celestial equator, the line directly above the Earth’s equator, cross and they’re known as the equinoxes. The Vernal equinox is where the Sun moves, as we see it from the Southern Hemisphere into the Northern Hemisphere marking the beginning of spring. This was also used to signal the start of the New Year in March.
The idea of starting the year on the first of January, was trialled a few thousand years ago but fell out of fashion especially during the middle ages. It’s only been since the Gregorian reform that it once again reverted to 1 January. England and its colonies, for example, only changed back to Jan 1 in 1752.
Taurus is perhaps the oldest of all the 88 constellations that we now officially recognise. It’s a very important creature. A bull is not only a food source for many of us, it’s a beast of burden and many people depended on it. It’s therefore not surprising that this animal worked its way into sky lore. But let me warn you. You should never actually approach Taurus the Bull, because in one of the stories, it’s actually the king of the gods, Zeus or Jupiter, carrying his lover, the beautiful young woman Europa, off to the island of Crete. This was such a famous story from long ago that the entire continent of Europe took her name.
Aldebaran, brightest star in Taurus is what we call a K5 orange giant. It’s the 14th brightest star in the night sky. It’s at a distance of about 65 light years and it’s coming to the end of its life. At the moment, it’s exhausted most of its hydrogen fuel in the core and has expanded to 44 times the size of the Sun, but a little under twice its mass. It will expand but it will die within a few million years at most.
If you scan the area around Aldebaran, it looks like itis part of a group of stars that form a large V, but it’s not. It’s actually between us and that V shape group of stars called the Hyades. The Hyades is an open cluster which means it’s a group of fairly young stars. They’re probably no more than 600 million years old. They’re about 153 light years away. They’re all, if you like, siblings, made at the same time from the same gas cloud, so they have the same chemical composition. The only variable is perhaps the most important one of all and that is mass.
Aldebaran sits between us and that group. With the two dimensional view of the sky that we have, they only look like they’re associated, but they’re not.
Go a little bit further toward the North and we’re still only 25 degrees above the horizon. You’re going to see a group of young stars that is yet another open cluster. But I think this one, and most people would agree, is the most spectacular of all. It’s called M45. Yes, it’s another one of those red herrings not to look at if you’re looking for a comet.
But it has another name, the Pleiades. To some indigenous communities, for example from the Maralinga area of South Australia, they have a story that relates to this group of stars. It’s one of my favourites. That’s the story of the Woode Gooth-Tha Rra and the Minma-Birnee.
The Pleiades are about 445 light years away. They’re not exactly close, but they’re very young, less than 150 million years old. They’re so cute. They’re baby stars that have just formed. In human terms, I suppose it’s a little bit like going to visit them in the maternity ward at the hospital.
When you look at pictures of M45 or the Pleiades online, you’ll actually notice that quite often it’s surrounded by a lovely bluish glow. That bluish glow is from dust that as it turns out is not part of the Pleiades itself. It’s between us and the stars. Again, the two-dimensional view that we have is a little confusing at times.
To most people around the world, the Pleiades are known as the Seven Sisters and what’s really intriguing is that this idea of the seven sisters seems to repeat itself. There are many different cultural stories that relate to them as being seven sisters including the one I’ve mentioned of the Woode Gooth-Tha Rra and the Minma-Birnee.
If you look at them however, you’ll probably be able to see six. If you’ve got really good eyesight you might see nine. Rarely have I ever met anyone who can say, “Well, do you know what? I can see seven.” Yet strangely, they’re called the Seven Sisters.
By the way, if you drive a Japanese car and it has an emblem on the bonnet of a group of stars joined by lines, you’re looking at a Subaru. Yes, that’s the Japanese name for this group of stars, Subaru.
To the ancient Greek, they represent the daughters of Atlas who carried the world upon his shoulders and his wife, Pleione, and their seven daughters. It’s well worth having a good look at.
The Pleiades used to be their own constellation, but for some time now, we consider them to be part of the larger Taurus the Bull. Continue now towards the north and look for another zodiac constellation with an enormous number of stars. Let’s count them together. In terms of bright stars, we have one, two, three… and that’s it. It’s fairly devoid of stars. What can you make out of a group of three stars? Well, I suppose pretty much anything but this particular one it is the horns of Aries the goat that produced the Golden Fleece that’s so famous in the story of ‘Jason and Argonauts’. There’s not a whole lot to see here unfortunately, but it is a very famous constellation in terms of sky lore and astronomy.
The version of celestial longitude, right ascension that I mentioned at the start of this podcast, starts in this part of the sky and we call it the first point in Aries. Sadly, it gets rather complicated here because the Earth does a 26,000 wobble on its axis and everything changes position ever so slowly. The first point of Aries is no longer in fact in Aries but has now moved over into the next constellation of Pisces the Fish. In a few hundred years from now it will move over to the next constellation along, Aquarius. You may have heard someone sing the “Dawning of the Age of Aquarius” but technically that’s still a few hundred years away.
In my opinion, don’t waste too much time looking into Aries with only three bright stars. Continue past it and look towards the northwest for a group of stars that makes up a square. What you’re looking for is the flying horse, Pegasus.
If you’re away from the city lights and there’s no Moon and you have a good view towards the northwest, because it’s quite low, you should be able to see the body of the horse. Look carefully at one of our star maps and you should be able to pick out the long neck and the face of the horse. It’s got two cute little front legs, but sadly for a flying horse, what’s missing? The wings.
The main reason in spending so much time trying to find the square of Pegasus is that wrapped around it is a fairly faint dot to dot V-shape group of stars with a little bit of a loop at either end. Oh, goodness, that sounds a bit complicated and you will definitely need your star map to be able to find this.
The V-shape with the loop at either end represents Aphrodite and her son Eros, or if you like, Venus and Cupid. It is the constellation of Pisces, the Fish. Continue past Pisces and we’re going to finish off as we look towards the west for the constellation of Capricornus, the Sea goat. It’s just below Fomalhaut our starting point but being the second faintest of all the Zodiac the constellations it’s probably a bit too late for that.
I mentioned earlier that we’ve done a bit of a loop around the horizon, between no more than 30 and 50 degrees up. We haven’t looked directly overhead. Why not? Because at this time of year, the most interesting, the brightest part of our galaxy Via Lactea, the Milky Way is actually sitting on the horizon as we see it.
The stuff that’s directly overhead, such as Phoenix the Bird, which is one of the 12 constellations invented by Petrus Plancius in the 16th century, or Cetus the Sea Monster or some of the newer ones like Sculptor were introduced by Nicolas Louis de Lacaille in the 18th century. I’ve probably made a mess of their names, but that’s the best I can do.
These constellations are in effect astronomical fillers. There’s not a whole lot up there to have a look at with the naked eye. So they’re a way of breaking the sky up into more manageable regions, a bit like suburbs. Suburbs in the sky, they’re constellations. Again, all the action that we’ve just been talking about is that band that’s somewhere around 50 degrees altitude and below around the horizon in December at a nice time of night.
If you can get away from the bright glow of the cities or the towns and there’s no Moon in the sky, head back towards the south and you should be able to see the Large Cloud of Magellan and the Small Cloud of Magellan. These look like two fluffy bits of the Milky Way that have drifted off and broken away, they are faint, wispy glowing clouds of light.
The Large Magellanic Cloud is in fact an irregular galaxy, totally separate to our own. It’s the third closest galaxy to us and about a hundredth of the size of the Milky Way. There’s enough material in this galaxy to form about 10 billion stars the same as the Sun. At 160,000 light years away, astronomically, it’s very close. It’s so close, that the Milky Way is stripping stars away from the Large Magellanic Cloud in something called the Magellanic Stream.
If you’ve got a pair of binoculars or a small telescope, the Large Magellanic Cloud is actually a rather spectacular object to have a look at. You might be able to see one of the largest nebulae that we’ve ever seen, called the Tarantula Nebula. Once again, “nebula” is just Latin word for cloud. It’s a very rich star forming region and well worth a look.
Long, long ago, last century, in 1987 – goodness me, such a long time ago – this area of the sky was home to the first supernova visible to the naked eye since 1604. We’re desperate to see a star blow up in our galaxy. Well, not too close, of course because we haven’t seen one since the invention of the telescope more than 400 years ago.
The other small patch of light that you can see is the Small Magellanic Cloud. It has a mass of about seven billion times that of the Sun and is about 201,000 light years away. To some Indigenous communities in Central Australia, these two galaxies represent brothers who lived in the sky.
Key events for December 2015.
The month commences with the last quarter Moon on Thu 3rd at 6:40pm Australian Eastern Daylight Time
New Moon is on Fri 11th at 9:29pm
First quarter is on Sat 19th at 2:14am
Full Moon is on Fri 25th at 10:11pm
The solstice, the point at which the Sun reaches its most southerly point in the sky, is at 3:48pm, on Tuesday, the 22nd of December. This is also the longest day with the Sun above the horizon for 14 hours and 25 minutes as seen in Sydney.
Many of us assume that the longest day is the day on which we have the earliest sunrise and the latest sunset for the year but that is not the case. The earliest sunrise is in early December while the latest sunset doesn’t actually occur until early January and that is the result of our non-circular orbit around the Sun and a tilt of 23.5 degrees caused by an impact perhaps as longs as 4billion years ago, pretty cool huh?
Perhaps our best opportunity to see meteors for the year, is the Geminid meteor shower. It lasts for quite a while but perhaps the best day this year occurs on the morning of Tue 15th and with no Moon in the way it could be an excellent opportunity. Of course it is always best to be away from the city and have a clear view of the sky from around 2am. Oh, that’s rather unfortunate in terms of time but what can you do? It is possible to trace the path of any meteors that you might see back to a common point or radiant in Gemini but you don’t need to watch just that part of the sky around Gemini as they can appear all over the sky. What causes these spectacular meteors or shooting stars? Each year in December the Earth crosses the dusty orbit of the 5km asteroid known as 3200 Phaethon. Phaethon was the son of the Sun.
As the asteroid gets close to the Sun thermal shock causes small fractures and minute parts break off. The debris can then collide with the atmosphere to produce meteors and it really is one the best meteor showers of the year all around the planet. It can peak at up to 120 meteors per hour but as many of you have guessed from hearing this before, there are no guarantees. So what if there is no guarantee? Isn’t risk part of the reward? So go outside make yourself comfortable look up watch and wait…and wait. If you see a short sharp streak of light dart across the sky, you’ve quite probably seen a Geminid. The vast majority of these are caused by particles of dust, or rock, no bigger than a millimetre across. But they collide with the upper atmosphere at up to 71km/sec. That enormous velocity makes the atmosphere glow in pretty much the same way that a fluorescent tube glows. Occasionally there are bigger ones, a centimetre up to a few centimetres and occasionally metres across. They’re the ones that capture everyone’s attention especially if anyone’s luckiest enough to capture them on video such as a dash cam. The vast majority of these however do in fact appear over the ocean and therefore go unseen.
For those of you that like to get up early, the best time to have a look toward the East is on the 8th at about 4:00am. Look for the planets Jupiter, Mars, Venus and the waning crescent Moon. All of them will be visible, though somewhat spread out across the constellations of Leo and Virgo.
For the evenings in December we only have one planet visible and that is Mercury. By mid-month it will be low in the south west just after sunset. On the 13th the waxing crescent Moon will be close by.
If you want more detailed sky maps – sunrise, sunset, Moon, and tidal times and a whole lot more astronomical information – we highly recommend you buy the book ‘The Australasian Sky Guide’ by Dr Nick Lomb available from Sydney Observatory and Powerhouse Museum shops.
It’s only $16.95 if you come into our shops. There are additional postage charges if you order online. Our website has lots of up to date information in our astronomy blog and details about visiting Sydney Observatory to look through our telescopes, see a program in our 3D space theatre or visit the digital Sydney Planetarium.
We have programs for all ages. You can also engage with us on Facebook, Sydney Observatory, all in one word and Twitter @sydneyobs.
My name is Geoffrey Wyatt. I’m one of the education team from the MAAS’ Sydney Observatory and I hope you’ve enjoyed this tour of the December night sky.