Four views made over three consecutive days (6 to 8 August 2010) of a large prominence in the north-east section of the Sun, drawing and copyright Harry Roberts ©, all rights reserved

Prominence: any cloud of material visible above the solar surface in Hα may be called a prominence. They are the most beautiful of solar phenomena. How do they defy gravity?

In Astrophysics of the Sun Harold Zirin devotes a whole chapter (40pp) to answering this and other such questions –introducing us to various kinds of prominence. In this piece we look at a large quiescent prominence – the kind that forms in the sun’s “quiet regions” well away from any sunspot activity. And although named quiescent, they can erupt in the most amazing way – often triggering a coronal mass ejection in the process.

The Figure shows four views made over three consecutive days (August 6 to 8) of a large prominence in the sun’s NE that stretched around the limb for 15º. It was big and fairly bright – a few photographers captured it, see Spaceweather.com. Helio freeware was used to determine the solar coordinates of foot points on each day as well as heights at various places. The records are arranged from right to left in IAU orientation.

Prominence on north-east section of the Sun on 5 August 2010 at 22:35 UT, imaged in the light of hydrogen atoms. Compare with right hand drawing above made 85 minutes later. Photograph and copyright Monty Leventhal OAM ©, all rights reserved

On 6 August the prominence stretched from 27ºN to 42ºN (across 15º) with an obvious foot point at latitude 31º (and longitude 321º). Timings gave a height of 64,000 km while at the site 24h earlier Monty Leventhal had a prominence about half as high – rotation having carried it into full view. The log of the 6th showed it exceeded “Zirin’s Limit” (my term) i.e.

Virtually every prominence that rises above 50,000 km will erupt in 48 hours (p 267).

Clearly the prominence was living on borrowed time! It was surprising, then, to see it at the same site next day (7th 00:10 UT) – but part of the north end (~4º wide) had gone – the remainder however was little changed in 24 hours with the central foot point now sited at 34ºN (±1º).

The prominence was recorded again just 30 minutes later revealing slow changes underway –compare the two records in detail. A prominence so high has localised instabilities that eventually trigger the predicted eruption – that was to happen 17 hours later!

The big prominence lay only 30º to 40º of longitude east of a new sunspot group, AR11093, that at 18:00UT on the 7th hosted a GOES M1 flare (local night time), and I suspect the big prominence erupted at the same time, contributing to the CME from the flare site. As the Figure shows there was just a bit of the prominence at the original site next day (8th) now sited at 38ºN. While stated above that quiescent prominences avoid sunspot groups, in this case the prominence lay in a magnetic valley, part of a larger magnetic structure that also contained the spot group to the west, and the M1 flare probably triggered the prominence ejection.

Plotting the foot points locations onto current magnetograms and Hα disc images confirms the above scenario, as well as the NE to SW trend of the magnetic valley, revealing a long-lived magnetic structure that may persist for several rotations of the solar disc – perhaps with more prominences to be seen at the NW limb on August 26th. Keep watching!

Harry Roberts is a frequent contributor to this blog and a member of the Sydney City Skywatchers.

Photoshopped image of Sydney Observatory by work experience student Tauqir. Images from various sources

The Milky Way is just one of many of the countless number of galaxies in what we call the universe. A galaxy is a massive, gravitationally bound system that consists of such things as stars, stellar remnants and interstellar medium [which in turn is] consisting of cloud and gas. Galaxies range from dwarfs which contain as few as ten million stars up to galaxies which can contain a hundred trillion stars, which all orbit the galax[y’s] centre of mass. Galaxies can contain many multiple star systems, star clusters and interstellar clouds. There are probably more than 170 billion galaxies in the visible universe.

Many different cultures have their own perception on how the galaxy was formed. Some [Australian] Indigenous peoples believed that the Milky Way was called the “Sky River” where the Rainbow Serpent lived. They also have many different perceptions on how the stars were created. For example, one story was that 3 brothers were out fishing on a canoe for tea, but could only catch kingfish. They were not allowed to catch these fish though, because they are part of the kingfish clan. One of the boys got too hungry and decided that he would eat one of the kingfish. The sun woman saw this and punished them by sending them up into the sky. This story was how [some] Aboriginal people believed that the constellation Orion was created.

As a galaxy, the Milky Way is actually a giant, as its mass is probably between 750 billion and one trillion solar masses, and its diameter is about 100,000 light years. Astronomical investigations have revealed that the Milky Way is a spiral galaxy.

The Milky Way Galaxy belongs to the Local Group, a smaller group of 3 large and over 30 small galaxies, and is the second largest but perhaps the most massive member of this group. The two closest neighbours, both already mentioned, have only recently been discovered: The nearest of all, discovered in 2003, is an already almost disrupted dwarf galaxy, the Canis Major Dwarf.

Diagram of the Milky Way with features labelled, taken from Starts With a Bang!

The spiral arms of our Milky Way contain interstellar matter, diffuse nebulae, and young stars and open star clusters emerging from this matter. On the other hand, the bulge component consists of old stars and [the surrounding halo] contains the globular star clusters; our galaxy has probably about 200 globulars, of which we know about 150.

An image of the Milky Way rising or setting over mountains, taken from Starts With a Bang!

Our solar system is thus situated within the outer regions of this galaxy, well within the disk and only about 20 light years “above” the equatorial symmetry plane (to the direction of the Galactic North Pole, see above), but about 28,000 light years from the Galactic Centre. Therefore, the Milky Way shows up as [a] luminous band spanning all around the sky along this symmetry plane, which is also called the “Galactic Equator”. The distance of 28,000 light years was confirmed in 1997 by the data of ESA’s astrometric satellite Hipparcos.

Jonathan and Tauqir, work experience students at Sydney Observatory, 29 August – 3 September 2010.

Wondering has powered scientific discoveries, inventions and technological developments. It has also inspired philosophies and social movements. It is what appears to distinguish us humans from our fellow creatures.

Wondering is native to us all: through time and across geography, oblivious to cultural differences; from our earliest ability to form sentences (and probably even before), we began to wonder.

The cover illustration of ‘You are a star’ (above) pictures that superbly: a young child with red bob has her (or maybe his – girls and boys will be able to identify with this character) hands pressed against the window of her house as she looks up, wondering about the stars, the sky, the vastness, and how we humans fit into it all.

These are questions I think we all wonder about when we are young. Before the complexities and mundanities of life that often afflict us, diverting us or narrowing our purview.

The storyteller encourages the child to look, to explore, to wonder, and to understand that we are all part of the grandness of the Universe.

The substance of this book is based in science – yet it rejoices in wonder and awe which are the foundation of myth as well as science. Mythology, after all, endeavoured to explain its time, just as science endeavours to explain ours.

Mythology is referenced in the splendid illustrations of the constellations, providing a link to the history of astronomy while the story weaves its way to describing, simply, the development of life on Earth – and that we are all made of stars.

I do take issue though with the part of the book that says: ‘You eat the fruit and then the tiny pieces of star are inside you.’ I think it’s a shame that it suggests that it is the act of eating the fruit that puts the stars into people. It seems to me misleading and doesn’t quite live up to the simple but logical clarity of the rest of the book.

But I don’t want to dwell on that because there is so much else in the book to recommend it: The idea that we are all made of stars is one I learned producing the Sydney Observatory monthly sky guide podcasts. I’d never made that clear connection before. It is a fantastic thought and a brilliant idea to carry through our lives. But more, the book is a call to exploration and discovery: ‘Come and see’. And it conveys the message that life is wonderful and miraculous. A fantastic message to give to children starting on their journey through life.

‘You are a star’ has just been published by ABC Books.

Total eclipse of the Sun as seen from Woomera, South Australia on 4 December 2002. Picture and copyright Nick Lomb ©, all rights reserved

Total eclipses of the Sun are probably the most spectacular events in Nature. During the day the sky becomes dark, birds and animals become confused and suddenly the faint outer atmosphere of the Sun, the corona, snaps into view. Many avid eclipse watchers travel great distances to have the opportunity of seeing such a wonderful event. Among these keen eclipse watchers is the Sydney Observatory group that travelled to Easter Island earlier in 2010.

There are occasionally opportunities to view total eclipses of the Sun closer to home, from the Australian continent. Here we mention some past and some forthcoming ones.

A total eclipse was to be seen from Sydney soon after the arrival of the first Government Astronomer for NSW, Rev William Scott. The eclipse took place on 25 March 1857 in the early morning at around 6:47 am in present day AEST and lasted just under two minutes. Sensibly Scott travelled out to South Head light house so as to observe the event over the ocean as the Sun was only about eight degrees above the horizon. Unfortunately, the sky was totally covered by cloud and poor Rev Scott saw nothing.

A much later eclipse that is still remembered by older astronomers is the one that passed over Melbourne on 23 October 1976. The eclipse took place in the late afternoon and lasted two and a half minutes. Many astronomers from Sydney travelled to the town of Bombala in the south of NSW and near the Victorian border to view the event. On the day there were patchy clouds and most observers missed the event though others nearby had a good view.

A later eclipse occurred on 4 December 2002 with the track of the eclipse passing through parts of South Australia. Many groups of amateur and professional astronomers travelled to Ceduna for the event. I led a Sydney Observatory group together with space expert Kerrie Dougherty from the Powerhouse Museum. We did not go Ceduna, but took the opportunity to visit the Woomera rocket range as that also on the track of the eclipse. The eclipse took place in the late afternoon with the Sun already near the horizon and lasted just under 30 seconds. From Woomera the sky was completely clear and we had a wonderful view as illustrated in the photograph above. Those who travelled to Ceduna also saw the eclipse in between clouds that cleared only at the last second.

The next eclipse to be seen from Australia will take place on 14 November 2012 and will be best seen from Cairns in North Queensland. It will be in the early morning with the Sun very low on the horizon so the possibility of clouds has to be considered. Once again there will be the opportunity to join a Sydney Observatory tour group and details will soon be available on the Observatory website. In the meantime I have prepared a factsheet with full information on the event for the Astronomical Society of Australia.

For those from Sydney who do not want to travel and have the patience to wait there will be a total eclipse visible from Sydney on 22 July 2028. It should be an excellent eclipse occurring in the middle of the day with the Sun high in the sky and lasting for almost four minutes. Let us hope that the clouds will be kinder than they were for Rev Scott back in 1857!

The free monthly night sky map PDF (below) 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.

September 2010 night sky map

For a year’s night sky maps and much more information, you can buy ‘The Australian sky guide’ book by Dr Nick Lomb at Sydney Observatory, Powerhouse Museum, good bookshops or through Powerhouse Publishing.

Read the transcript of the audio podcast. Hear the audio podcast:

Hello. I’m Geoffrey Wyatt, Senior Astronomy Educator here at Sydney Observatory. We are talking to you about what is visible in the sky for the month of September. Don’t forget that September is the 7th month of the old Roman calendar but thanks to Julius Caesar and later the Gregorian reform, it is now the 9th month of the calendar.

You may need a few bits and pieces to help you with this tour of the sky. Of course, you’ll need a printed copy of our star map which is available from www.sydneyobservatory.com. Or of course, if you have your ‘Australian sky guide’ book, that’s always a bonus as well.

Now, you need to wait until it is nice and dark. You need to get yourself into a lovely position where you can see as much as the sky as possible. Obviously, if you are at the bottom of the hill or a valley that is going to restrict what you can see – so preferably up nice and high with a clear view of the cardinal directions that will allow you to see much more of the sky; make things easier to find.

We’re going to start off our tour for September by looking up quite high in the sky and then we are going to come back down towards the west. So, first of all, find your view toward the west where the sun has just disappeared. You might have a slight reddish glow of twilight over there.

I want you to look up about 45 degrees above the horizon almost due west. This of course raises a slight problem. Forty-five degrees, well, that’s not too bad – halfway up of course, overhead being directly 90 degrees up, and horizontal – zero so, 45 degrees – halfway up.

There is a fairly easy way for us to navigate around the sky once we do have our cardinal directions of north, south, east and west and that is to use your fist held at arm’s length and your fingers spread from pinkie to thumb tip. And of course, even the pinkie held at arm’s length. Now, it varies from person to person but roughly speaking, your clenched fist held at arm’s length is about ten degrees. If you are a big chap like me, well, it’s a bit more, but I’ve learned to accommodate that.

So, if you hold your outstretched hand at arm’s length from pinkie to thumb tip is about 15 degrees for the average size person. So, we start it off looking due west after sunset, when it is dark. Stretch out your pinkie tip to thumb tip and then go three of those hand-spans, if you like, up and you should be able to see the 16th brightest star in the night sky. It has a slightly orange reddish tint to it and it is called Antares.

It is the brightest star in the constellation of Scorpius although some people call it Scorpius. Scorpius is a very interesting constellation and I’ll talk more about that in just a moment but we’re using Antares, if you like, as our starting point.

This is a fairly interesting star because it is so big and because of its colour. Quite often people don’t realise just how much colour is visible in the night sky. Most of the stars do appear whitish but there are a few bluish and there are a few orange-reddish stars and this is one of those. Antares means ‘rival to Mars’ because every now and then the planet Mars appears to pass quite close by and the two of them look fairly similar.

Antares is about 800 times the diameter if the Sun which means it is a very, very large star. In fact its colour and its size tells us it is a dying star. It’s like about 600 light years away from us which means we see it tonight in September as it was 600 years ago – rather intriguing.

Within the group of stars of Scorpius, Antares represents the heart of the Scorpion. It is one of the easiest pictures in the sky to see. So with a little bit of patience and a lot of imagination, supplemented, if you are old enough, by a glass or two of red wine and you might just be able to pick it up in the giant constellation of Scorpius.

A rather intriguing story goes with this group of stars. You see there was a mighty hunter called Orion and according to one of the more common myths in the sky, Orion boasted to the Goddess of the Hunt, Artemis, that he could kill any animal on earth.

Now, although Artemis was a hunter herself, she also offered protection to all creatures on the Earth. So, she created the giant scorpion to deal with Orion and the two went into battle. Apparently, it was a fierce and mighty battle, one of such interest that it even caught the attention of the King of the Gods, Zeus, himself. Eventually, Orion was killed by the Scorpion and Zeus placed the Scorpion into the sky.

Artemis also placed the body of Orion into the sky as a reminder for mortals to curb their excessive pride. But she placed the two of the constellations as far apart as they could possibly be. So it is very unusual to see the two in the sky at the same time. These stars however happen from time to time but not this time of year.

So, what you are going to look for from the heart of Orion, Antares, is head down towards the west ever so slightly and you come to a T intersection of stars. The middle star quite close by represents the head of the Scorpion and the claws go out towards either side, past back up through the heart and there is the star on the side. And then you follow the long hook-like line of stars that go around and form the tail with the sting.

Scorpius is a large constellation that’s relatively easy to see and there is a starting point for our tour in September. In fact we are going to go backwards at this stage and just drop down below Scorpius inside between Scorpius and horizon. We are going to look for the three bright stars of Libra, the scales, that make up, well, a triangle. They are fairly easy to see and they have fabulous Arabic names – Zuben Elgenubi, Zuben Eschemali, and Zuben Elakrab. Now my pronunciation may not be right but effectively they mean the head in the northern and southern claws of Scorpius.

You see these stars used to be part of the giant Scorpion but they’ve been broken off to make up the Scales of Justice that the Goddess Virgo carries. So, Libra, the Scales of Justice, is setting quite low in the west. Not so easy to see – but if you can make out a big triangle between Scorpius and the horizon, you’ve done well.

At this point, we’re going to go back up the sky, if you like, passing through Libra, passing through Scorpius, along an imaginary line known as the ecliptic. The ecliptic is the line along which the Sun, the Moon, and the planets follow through the background patterns of stars.

The background patterns of stars that we’re seeing at the moment, Libra, Scorpius, and the next one is Sagittarius, make up the path of the animals, the zodiac as we more commonly call them.

So at the moment, we’re almost looking directly overhead and we’re looking for the mighty half man, half horse archer of Sagittarius. Look, I have to tell you. It doesn’t matter how good your red wine imagination supplement is, if you’re trying to see a half man, half horse overhead, good luck. You’re far more likely to be able to see, well, a teapot.

So look overhead. Let your imagination go wild. Join the dots and if you can make out an old fashioned teapot, then you’ve found Sagittarius. Those of you born under the star sign of Sagittarius may not like being demoted from a mighty Centaur to a Teapot. Oh well: them’s the breaks.

Intriguingly, however, the Teapot is very close to the centre of our galaxy as we see it. So, looking directly overhead this time of year, we hit a beautiful view, still of Via Lactea. Via Lactea, by milk – the Milky Way. And it will branch off to the left and to the right, going north and south as we look at it, if you like, perpendicular to the line of stars that we’ve been following so far.

The Milky Way is, of course, the brightest part of our galaxy. And we could see perhaps two or three thousand stars and you get this magnificent bright band of glowing stars and gas and dust as long as you’re away from the city, as long as there’s no bright Moon anywhere in the sky to ruin the view, and as long as it’s a lovely, clear night.

So, if you’re away from the city, there’s no Moon, and it’s clear, looking overhead going from south to north, you should be able to see the Milky Way passing through the constellation of Sagittarius.

As you’re looking overhead, by the way, the centre of our galaxy is about 26,000 light years away and at the heart of our galaxy lies a super massive black hole, one of the most bizarre objects in the universe. The one at the centre of our galaxy, we call SagA* (pronounced as ‘Sagittarius A [ay] Star’). It has a mass many millions of times that of the Sun and it’s so big, it can gobble up a star relatively easily.

So that is directly overhead at the moment. Oh, and there’s no need to worry. At 26,000 light years away roughly, who cares what it gobbles up? It’s not going to be affecting us.

As we pass from high overhead through Sagittarius/Teapot, we go along the ecliptic down ever so slightly towards the east. We’re coming to the next of the zodiac star signs and that is of the half-goat, half fish, Capricornus.

Half goat, half fish? How do people come up with such bizarre creatures as this? You have to remember that many of these constellations have been around for thousands upon thousands of years. They’ve been made up to entertain and educate people as they sit around the camp fire after a long day working out in the fields.

So people would use the stars, I suppose you would say, as a palette and they would make up pictures to accommodate stories – stories of heroes and villains, great deeds, great journeys.

This particular pattern in the sky is used to represent a story of Zeus, Jupiter, King of the Gods, who was out on a picnic with a whole bunch of other Gods when the Earth cracked open and a demon from hell, Typhon, arose and began to attack the God Jupiter himself.

Most of the other smaller deities did the obvious thing and panicked and started to run away. In fact, the word ‘panic’, comes from this exact story. You see, the God, Pan, the goat that played the pan pipes, panicked and thought, well, he’s a demon from hell. There’s only one thing to do and that is change into a fish and swim to safety.

Halfway through the transformation, he realised that Jupiter, Zeus, needed a bit of help. He played a note upon his panpipes to distract Typhon, which allowed Zeus to gain the upper hand and Zeus banished Typhon back to hell. As reward for his assistance, Zeus placed Pan, as he was, half-goat, half fish, into the sky as the sea goat, Capricornus.

But what we’re going to be looking for is basically a triangular group of stars. And if you’re real pedant with your Euclidean geometry, you’re going to look for a triangle that has, well, a slightly bent hypotenuse. If you can see anything that looks like a bent triangle, or if you’re a ‘Star Trek’ buff, it’s a little bit like the ‘Star Trek’ logo, then you’re probably looking at the zodiac constellation of Capricorn, half goat, half fish.

The next of the zodiac, so long as we head down toward the east, is Aquarius, but Aquarius is a fairly difficult constellation to see. It represents the youth Ganymede, supposedly the most handsome youth on the planet that was snatched by the bird, Aquila, and carried to Mount Olympus to serve the gods water and wine.

The only bright stars that we can see in Aquarius are the shoulders of the youth, himself, and the line of water as it flows from his jug across the sky that meanders towards the mouth of the Southern Fish, Piscis Austrinus.

So Aquarius, I’m afraid, is a fairly difficult constellation to see, snuggled up to and wrapped around the constellation of Capricornus. Don’t worry too much because the next of these zodiacs rising in the east at the moment is Pisces, and Pisces is even fainter and more difficult to see. So these water zodiac signs, I’m afraid, are a bit too hard to see at this time of year.

But we’re now at the stage where we’re looking towards the east and we’re going to head around to our left as we look towards the north-east, and we’re going to look for a group of stars that looks like a giant square. You’ll find that astronomers, past and present, look for simple shapes and then give them rather curious and amazing stories.

So, we look at this giant square rising in the north-east. It’s not perhaps high enough for us to see fully yet, but you’re looking for the constellation, Pegasus, the Flying Horse. We do need to wait perhaps another month or so to get it to its best.

Continue towards your left and towards north, you’ll see a very large group of stars, if you have a clear view, that looks like a large cross. Much, much larger than the Southern Cross if you are familiar with it and once again, it will be quite low so you need a clear view.

What you’re looking at is Cygnus the Swan. And Cygnus the Swan is the home of the first suspected black hole, the first X ray source found in night sky called Cygnus X 1. And Cygnus is a very old constellation, one of the original 48 that were listed the by the second century astronomer, Claudius Ptolemy. And of course, it is one of the remaining 88 constellations that we have today.

If we continue past Cygnus, low in the north by north-east and around to almost due north, we’re going to look for the fifth brightest star in the night sky, Vega. It’s about 15 degrees above the northern horizon. Remember, 15 degrees for most people – a handspan from pinky to thumb tip, held at arm’s length.

Vega’s an intriguing star. It’s only 25 light years away. It’s relatively bright, as I said, being the fifth brightest star. And about 14,000 years ago, it was in fact, the North Polar star. If you’re all patient enough, it will be the North Polar star in another 11,000 years, although maybe that’s a tad too long for us to wait.

You see, everything changes position in the night sky. The stars may look fixed to us during our relatively short life spans, but the stars are all moving relative to each other. But of course, also, the Earth wobbles. It goes for a 26,000 year wobble and that’s why the star will become the North Polar star again many, many years in the future.

Vega is the brightest star in the constellation of Lyra the Harp. Lyra the Harp is intriguing because of many stories that relate to Vega and a nearby star on the other side of the Milky Way as see it, but not very far away at the moment. And that is Altair, Eye of the Eagle, which is about 47 degrees above the horizon at the moment.

So we go from 15 degrees from Vega up to three handspans, you’ll see another fairly bright star. Not as bright as Vega. This one’s the 12th brightest star in the night sky and it has an equidistant dimmer star on the other side. What you’re looking at there is the Eye of the Eagle, Aquila. And that’s the eagle that snatched Ganymede up into the night sky to become Aquarius, the Water Carrier.

The bright stars, Vega and Altair, represent a young boy or prince and a princess or a young girl in many Asian mythologies. To the Japanese, these two stars represent a princess and her prince in a festival known as Tanabata, celebrated on the 7th July each year. But very similar stories are told about these two stars in Korean and Chinese mythology as well.

So as you’re looking north at these two bright stars, this is an interesting point in time to think about it. I’ve mentioned so far, Greek constellations. I’ve mentioned some Arabic names when we talked about the stars of Libra the Scales, and now we have some Asian stories as well.

You see, just about every cultural group on this planet look up at the stars and use them for two different reasons. They use them to work out the time of year, therefore to keep track of the seasons and they use them to navigate, to work out directions.

What we’re going to do now is continue around towards our west back towards Libra, although Libra should be a little bit lower than where it was when we first started. Continue past the setting constellation of Libra in the west and we’re going to go around to the other centaur of the sky. Yes, there are two. Two half man half horses. One of them was really nice. One of them was a bit of a party animal.

We’re coming around to the nicer of the two and his name is Chiron and he is the constellation of Centaurus. Centaurus will be getting quite low in the south-west. Centaurus was a tutor, a teacher to heroes like Jason from Jason and the Argonauts, Hercules, and Achilles.

He’s also wrapped around the Southern Cross. The Southern Cross will be on its side at this time of year and only about 20 degrees up from the horizon. So, we’re pretty much at the end of the Southern Cross viewing season. It’ll be lost in the glow of the horizon, or more likely, trees and buildings.

As the Southern Cross disappears into the south-west, don’t forget that the Southern Cross doesn’t sit at the South Pole. We actually use it to point to the South Pole, but it really is a bit too low for us to do at the moment.

So what I want you to do is if you can see the mighty Centaur with its two bright Pointers, which represent the front feet of the horse. See if you can see, according to our map and it really is crucial that you have the star map there, the half man, half horse wrapped around the Southern Cross, setting in the south-west.

We’re going to continue through the deep south because there’s nothing terribly bright there at the moment and continue around, now, towards the south-east. I want you to look for the ninth brightest star in the night sky. It’s a rather intriguing star called Achernar. It’s also one of the flattest stars in the night sky.

Whenever you get a flat star, hopefully you know that stars are not just pinpoints of light. They are, in fact, typically spherical, although they do tend to flatten out around the top and the bottom because they’re spinning.

Well, this star, the brightest star in the constellation of Eridanus the River, called Achernar, is one of the fastest spinning stars we’ve ever seen. It bulges around the equator about 50 percent more than it does around the polar circumference because it spins roughly 15 times faster than the Sun. It’s about eight times the diameter of the Sun as well, and it’s relatively bright as I mentioned being the ninth brightest star in the constellation of Eridanus the River.

Interestingly, in classical earlier times, the name Achernar was given to a different star, a star that we now know as Theta Eridani or Acamar. Acamar, Achernar. They sound very similar. What happened was, Acamar used to be the brightest star at the end of the constellation of Eridanus the River as it was visible from Greece.

However, when people started to sail into the southern skies, they could see this nearby brighter star. They extended the river and changed Acamar to Achernar and gave the new brighter star the former name. Oh goodness me, confusing isn’t it? But according to its old Arabic name, Al Ahir al Nahr, it means the same thing, the end of the river.

As we continue past Achernar, we go towards the constellations which are extremely difficult to see of Phoenix and Sculptor as we continue back around towards the east to where we now see Pisces, a little bit higher in the sky, but no less difficult to see. We need to give Pisces a little bit longer to get up.

What we’ll do now is have a look at some special events for September 2010. The last quarter Moon will be on Thursday, 2nd September at 3.22 am. The new Moon will occur on Wednesday, 8th September at 8.30 pm. The first quarter Moon will be on Wednesday, 15th September at 3.50 pm. And the full Moon will be on the 23rd September at 7.17 pm.

Remember, the best time to look at the Moon is not when it’s full. People call us constantly and ask to come along to see the Moon at its best on the full Moon. No. You see, when the Moon is full, the sunlight is hitting it directly, if you like, square as we see it. There are no shadows. That means you can’t see any of the spectacular detail that you can see at first quarter or last quarter. So the best time of the month to view the Moon is the first quarter and that will be on or around the 15th.

We also have the Equinox occurring on 23rd September at 1.09 pm. Equinox simply means ‘equal night’. These are the two days each year when the Sun crosses the celestial equator from one hemisphere into the other.

This Equinox, for us of course, the Sun will cross from the Northern Hemisphere back into the Southern Hemisphere and will signify the beginning of spring and the coming of warmer months.

The stunningly bright Goddess of Love and Beauty, however – it should have been named the Goddess of Pain and Horrible Nasty Things. Venus will be seen low in the west shortly after sunset throughout the month. It’ll move higher from the constellation of Libra the Scales up toward the constellation of Scorpius. On 11th September, the young crescent Moon will be just below it.

But hang on a second. Goddess of Love and Beauty? Or more like the Goddess of Hell? You see, Venus is incredibly bright and we tend to associate bright things with being beautiful. I mean, after all, look at the sparkle of a diamond and I challenge anyone to say that they’re ugly.

Bu the thing is, Venus despite being bright and twinkling and to us looking rather pretty, is in fact, a nasty, nasty place to visit. The atmospheric clouds are so thick that the pressure they exert on the surface is 90 times what you experience at sea level here.

That means unprotected, if you are on the surface of Venus, you would be crushed and you’d end up thinner than a pancake. Oh dear. At 450 degrees Celsius, you’d also somewhat be toasty. The planet spins backwards compared to what we see other planets do. And the length of the day is roughly the length of a year of about 220 days.

It does rain at this incredible pressure and temperature, however, but mostly sulfuric acid. So why would it be called the Goddess of Love and Beauty? Well, of course, it was named long before we knew what the conditions were really like. Nonetheless, it is a spectacular object to look at either by eye, in binoculars, or through the small telescope.

The rather unspectacular planet, Mars, since it’s already well past its best viewing for the year, will also be in the west just after sunset. On 11th September at around 6.45 pm, Mars will be just below the Moon. Across towards the east, we’ll be able to see Jupiter, the King of Gods, rising from around 6.00 pm in the constellation of Pisces.

Jupiter is well worth a look. Now, you do need a small telescope. Binoculars, if you can mount them onto a tripod, you might just be able to see the four moons of Jupiter – or four of its moons, I should say, because it has quite a few of them, but the four larger moons named in the honor of Galileo.

The other thing is, the next few months is a really good opportunity, if you have a small telescope or a good pair of binoculars, but they must be mounted on a tripod, to be able to see in the same field of view at the same time, Georgium Sidus.

Ah hah, I hear you say. Georgium Sidus? George’s star? Have you heard of George’s star before? Yes, you have. You see, George’s star was discovered by Sir William Herschel on 13th March, 1781. And of course, it was named after King George. However, the naming of it after King George didn’t go down so well with lots of people and eventually, it was renamed as the planet Uranus.

So, we have the opportunity to see the planet Uranus fairly close to the planet Jupiter throughout this month and the next few months in the same field of view through a pair of binoculars.

Don’t forget. If you would like to get more information about what’s visible in the night sky, you can purchase your copy of ‘The Australian sky guide’ by Dr Nick Lomb or visit our website for more details at www.sydneyobservatory.com.

My name is Geoffrey Wyatt, the Senior Astronomy Educator at Sydney Observatory, and I hope you’ve enjoyed your tour of the September night sky.

Calendar wheel from the Hayden Planetarium at the American Museum of Natural History in New York. Placing the year under the correct month indicates the calendar for that month. As can be seen the calendar for August 1993 starts with a Sunday. Picture and copyright Nick Lomb ©, all rights reserved

Z asks: Is this true? This month of August 2010 has 5 Sundays, 5 Mondays and 5 Tuesdays, which hasn’t happened for 800 years?

Answer: Any month of 31 days must have five of three consecutive days of the week. Which three depends on the day of the week with which the month begins. For example, if the month starts with a Wednesday then it has five Wednesdays, Thursdays and Fridays. Now as there are seven days of the week with which the month could begin, we would expect that every seven years August will begin with a Sunday as in 2010. However, the leap years every four years provide some complications.

The last time August had five Sundays, Mondays and Tuesdays was in 2004, six years ago as there was a leap year in between.

The previous August with five Sundays, Mondays and Tuesdays was in 1999, five years earlier as there were two leap years – 2000 and 2004 itself – in between. Then 1993 and then, surprisingly there was no such August until 1982. And 1982 is, of course, 28 years prior to 2010.

We have now established the patterns in the calendar. The calendar can repeat at five or six year intervals, but does not always do so. The only simple pattern is that it repeats every 28 years. It repeats every 28 years as 28 is the lowest common multiple of the 4 year cycle of leap years and the 7 weekdays.

The 28-year calendar cycle works well as long as the 4-year cycle of leap years is maintained. However, in the Gregorian Calendar that we use century years are not leap years unless they are divisible by 400. So 1900 was not a leap year, 2000 was a leap year while 2100 will not be a leap year. We are now in the middle of a long unbroken sequence of 4-yearly leap years from 1904 to 2096, but then in 2100 the 28-year calendar cycle will be broken.

That gives us many years until 2100 to recycle yearly calendars. You could store your 2010 calendar for use in 2038 and, as we saw above, you may even get a chance to use it earlier!

The Hubble Space Telescope watched a rare triple eclipse on Jupiter on 28 March 2004. Courtesy NASA, ESA, and E. Karkoschka (University of Arizona). Original video is here.

David asks: The kids and I very much enjoyed viewing the transit of the shadows of Io and Ganymede across Jupiter on 20/8/10 through our 200mm Celestron. However, now we have a few questions, please. 1) How often does this happen? 2) Do you ever get all four Galilean moon shadows visible at once? 3) What would a solar eclipse on earth look like from Mars with a telecope? Would Martian astronomers see a shadow transiting the Earth’s surface? Thanking you

Answer 1: How often do multiple shadow events occur on Jupiter? This is not immediately obvious, but an astronomer has taken the trouble to sort out all the shadow transits during one Jupiter year, which is 12 Earth years. The results were:

271 double shadow transits and 3 triple shadow transits. (Details here.)

The last triple shadow transit I am aware of was on 28 March 2004 as illustrated by the video from Hubble Space Telescope images above. These events are rare because the large distance of Callisto from Jupiter means that its shadow often falls below or above the planet.

Answer 2: There can never be a quadruple shadow event as the orbital relationships are such that if Io and Ganymede are transiting Jupiter then Europa must be behind the planet. This is because the periods of the three inner Galilean moons are in an exact 1:2:4 resonance in the sense that in the time Ganymede circles Jupiter once, Europa circles twice and Io four times.

The motions of the three inner Galilean moons in steps of Io’s period. Io is yellow, Europa grey and Ganymede dark. Jupiter is not shown, but is in the centre. As can be seen, no more than two of these moons can be in front of Jupiter at a time. Diagram, with a slight modification, from Wikipedia.

Answer 3: Yes, a solar eclipse on Earth would look similar as viewed from Mars or elsewhere in space. The dark spot would be much larger though compared to the disc of the planet since the width of the Earth is 11 times less than that of Jupiter. Moreover, the spot would be somewhat fuzzier as the penumbra of the Moon’s shadow is more prominent than that of any of Jupiter’s moons.

Diagram showing the two types of shadows cast on Earth by the Moon during an eclipse of the Sun: umbra and penumbra. Diagram Nick Lomb

The penumbra is the Moon’s shadow in places from where the Sun is only partially covered, so some sunlight falls in those places but less than on places totally free from shadow. From Earth the Moon and the Sun subtend the same angle in the sky so that the penumbra is prominent. Jupiter is about five times further from the Sun than the Earth and hence from there the Sun subtends an angle one fifth of that from Earth. That means places on Jupiter are generally in or out of the shadow of Io or the othe Galilean moons leading to less prominent penumbras.

Nick Lomb with the surviving one of the two original metal (speculum) 1.2 metre diameter mirrors of the Great Melbourne Telescope. The mirror, as well as the other parts of the telescope, are in the collection of the Museum of Victoria. Picture by Barry Clark, copyright Nick Lomb ©, all rights reserved

From many years from 1869 onwards, the City of Melbourne could boast of having the largest steerable telescope in the world. Known as the Great Melbourne Telescope, it was truly large with a metal mirror 1.2 m across and weighing about 8 tons; it truly merited the “great” epithet. By the time Melbourne Observatory was closed down in the 1940s the telescope was regarded as an out-of-date pile of junk and transferred to Mt Stromlo in Canberra. There parts of the telescope were incorporated into a more modern telescope with a slightly larger glass mirror. This was destroyed in the Stromlo fire of 18 January 2003. As discussed in a previous post, the remains were eventually returned to Melbourne and to the care of the Museum of Victoria.

Parts of the tube of the Great Melbourne Telescope. Picture and copyright Nick Lomb ©, all rights reserved

In mid 2009 a group of volunteers from the Astronomical Society of Victoria began Project Phoenix to restore the telescope. They are working on the newly returned components of the GMT plus other pieces either returned in the 1980s or that had remained in the care of the Museum of Victoria. Fortunately, there is an astonishing wealth of engineering expertise in the project team enabling sophisticated work to be done.

The Great Melbourne Telescope at Melbourne Observatory in its roll-off roof building. Picture copyright Powerhouse Museum ©, all rights reserved

During a visit to see the Phoenix project team in action at a store belonging to the Museum of Victoria, I was shown around by Dr Barry Clark, who is an expert on the history of Melbourne Observatory. What struck most was that every part has been identified. Barry pointed to one small non-descript piece of metal after another and could indicate for each what it did and whereabouts in the telescope it had come from. Identifying the pieces must have been a complex task as there are no extant plans for the telescope and there was the added complication that mixed in with them there are also parts fabricated at Mt Stromlo when the telescope was installed there. Not only have the parts been identified, they have been cleaned, catalogued, described, photographed and CAD drawings prepared.

The GMT house in about the year 2000. Picture and copyright Nick Lomb ©, all rights reserved

The team consider that about 90% of the original parts of the telescope have been found. Their next major task is to identify and describe or design the missing pieces.

Dr Barry Clark of the Astronomical Society of Victoria in the GMT house standing on the original footings of the telescope. Picture and copyright Jenny Andropoulos ©, all rights reserved

What will happen to the restored telescope? No final decision has yet been made and funding still needs to be obtained. Ideally, the telescope would be placed back in its original roll-off building in the grounds of Melbourne Observatory. As shown in the pictures above, the building still stands, but it is in need of much tender loving care. As well the roof is now fixed into place. There in its original position, the Great Melbourne Telescope would be likely to be both a major tourist drawcard and an important public education facility.

Two views of sunspot AR11092, drawing and copyright Harry Roberts ©, all rights reserved

“Sunspots like to be round and are most stable in that form. But as (the) illustrations show, they are often distorted from that shape, primarily by the proximity of other strong flux” (Zirin, “Astrophysics of the Sun” p323). And recently we have seen quite a few “contented” round sunspots – very stable, very quiet and slow to decay: AR11092 was one good example.

Sunspot umbral fields are almost vertical, but penumbral fields are horizontal, and so when polarity is simple a symmetrical round sunspot results, with a grey “skirt” of fibrils (a penumbra) surrounding the dark umbra. In AR11092 (Fig 1) with arc second seeing single fibrils showed well. Yet when first recorded this spot had a small companion within its penumbra (PU) and that extra magnetic complexity was enough to cause the PU to be somewhat pear-shaped and to disappear where the two umbrae were closest (Jul 31, 23:45UT). As well the large simple preceding (p) spot had a few followers (f) 8º behind it, and complex fields connected the (p) and (f) components (Fig).

When next logged (Aug 3, 23:40) the (f) spots had gone and the small spot within the (p) spot’s PU had shrunk – accordingly the (p) spot had arranged itself into a nearly perfect circle (Fig). A very faint light bridge seemed to be forming across the dark umbra. Solar Dynamics Observatory (SDO) 171Å images showed a nice spiral pattern of field arches emerging from the main spot (Fig 2)

A Solar Dynamics Observatory image of sunspot AR11092

Very little is known of flux tubes where they lie well below the solar surface. Some speak of them as twisted by Coriolis forces into rope-like bundles (Fig 3)– while others prefer a non-twisted “bunch” model (Ibid p322). Helio-seismology cannot resolve this question, and it’s suggested that sunspots behave as if no longer connected to the solar core once they break through the photosphere.

One model of twisted flux tubes of magnetic energy, drawing and copyright Harry Roberts ©, all rights reserved

As Fig 2 shows, at least in the case of this very symmetrical (p) spot of AR11092, the emerging field transition arches (FTA) did take on a spiral form – suggesting that perhaps for spots of low magnetic complexity the flux ropes are twisted as they emerge through the photosphere – but the question remains: are they twisted all the way down to the solar core?

One arc second is 700 km at the sun’s distance, and recent clear, cold, days have provided some very good white light views – revealing seldom seen details of sunspot structure, making for some breathtaking solar astronomy!

Harry Roberts is a frequent contributor to this blog and is a member of the Sydney City Skywatchers