Words can be so confusing, well for some of us at least. I even confuse myself occasionally trying to explain certain things. You can therefore imagine my trepidation when it comes to interpreting what others are saying about a strange event like “crossing the centre of the galaxy in December 2012”. This one has come up a few times recently as we head towards the “END” of the world this December 21st, so I thought I’d have my say to try and explain what is going on.

Rest assured the world will not end this year or anytime soon. Nonetheless we appear to cross the plane of the galaxy this December, as we see it. In fact we do it twice a year at the winter and summer solstice because of the Earth’s tilt on its axis, which incidentally also causes the seasons. This is however a purely visual effect as our entire Solar System is presently something like 75-100 light years above the plane of the galaxy.

It gets worse. Yes we do pass through the plane in a complicated gravitational dance that takes about 33 million years to complete one cycle of above and below. We last crossed the plane roughly 3 million years ago and are not expected to do so again for another 30 million years, certainly not in 8 months’ time.

Will we ever really pass through the centre of the galaxy? No, and I can say with absolute confidence that is never going to happen. The Milky Way galaxy is very large at about 100,000 light years side to side. Our small solar system is about 28,000 light years from the massive black hole that lurks at the centre. We won’t ever go through the centre but we do “fall around” or orbit the centre of the galaxy about once every 250 million years.

In summary, this December like all Decembers we will appear to cross the plane of the galaxy as we see it, but we will really cross only in 30 million years’ time and we will never pass through its centre.

Venus passing close to the Pleiades star cluster with Jupiter nearby. The diagram is for 6:45 pm AEST on the evening of Tuesday 3 April 2012. Diagram Nick Lomb

The bright planet Venus often provides spectacular views as when it had a conjunction with Jupiter in March 2012. On the evening of Tuesday 3 April 2012 and on the following evening it provides the opportunity for another spectacular sight as it passes the famous star cluster Pleiades. Although, clouds permitting, the proximity of Venus to the star cluster will be noticeable to the unaided eye, binoculars are likely to provide a better view.

The Pleiades are known from Greek mythology as the Seven Sisters and are the most famous star cluster in the sky. They provide a good test of eyesight as most people can see six stars in the group while some sharp eyed people can see seven or more. Of course, through binoculars or a telescope many more can be seen. It is also advantageous to observe them from a dark spot instead of the light-polluted suburbs from where most of us experience the stars.

The stars of the cluster have been observed by different groups of people around the world for hundreds or thousands of years. Interestingly, many groups not just the Ancient Greeks refer to them as women. According to one group of Australian Aboriginal people they are the Kungkarungkara or Ancestral Women.

A colour image of the Pleiades put together from photographic images taken with the Palomar 48-inch (1.2-metre) Schmidt telescope between 1986 and 1996. Credit NASA, ESA and AURA/Caltech

Through a telescope hundreds of stars can be seen in the cluster together with blue dust clouds. At an age of about 100 million years the cluster is young in astronomical terms, so it would be easy to think that the dust is left over from the formation of the cluster. That is not the case, however, as astronomers have measured that the dust has a different motion through space to that of the stars. The blue colour of the dust is because it reflects and scatters the light from the stars and, as in the Earth’s atmosphere, blue light is preferentially scattered over other colours with longer wavelengths.

The brightest nine stars of the cluster all have proper names. They are the Seven Sisters, Alcyone, Asterope, Electra, Maia, Merope, Taygeta and Celaeno plus their mythological parents Atlas and Pleione. According to RA Allen in Star Names, their Lore and Meaning the last two were probably added relatively recently, possibly as late as the 17th century. There are a number of possible origins for the name of the cluster, but Allen suggests that Pleiades is probably derived from the name of the sisters’ supposed mother Pleione.

If clouds permit, dust off your binoculars and have a peek at Venus dropping in on the Seven Sisters.

Sunset on 29 August 2005 from Sydney Observatory. At the equinox the Sun sets much further to the left or south. Photo Nick Lomb

There are four turning points in the year: the summer and winter solstices and the spring and autumn equinoxes. Tuesday 20 March 2012 is the day of the autumn equinox in the southern hemisphere. At 4:14 pm AEDT on that day the Sun crosses from the southern to the northern part of the sky.

At the equinox day and night are equal at 12 hours each or, more precisely, almost equal. In Sydney the Sun rises at 6:58 am AEDT and sets at 7:06 pm so that the day is 12 hours and eight minutes in length. Why not exactly 12 hours? There are two factors at work. One is that sunrise and sunset are calculated as the times when the top edge of the Sun touches the horizon while the expected equality of day and night refers to the centre of the Sun. In addition, the atmosphere bends the light from the Sun in such a way that we see it at sunrise and sunset even though geometrically it is below the horizon. These two factors come together so that daytime is always slightly longer than the night at the equinox.

Diagram of the seasons. In this diagram we are looking the Earth’s circular path around the Sun from side on and hence the path appears oval. The seasons take place because of the tilt of the Earth’s axis. If there was no tilt life would be boring with no seasons. Note that “NCP” on the diagram stands for north celestial pole, which is the point directly above you if you were standing at the north pole. Drawing Nick Lomb

As indicated on the above diagram the seasons occur due to the path of the tilted Earth around the Sun. At the equinoxes the relationship of the tilt to the Sun is that neither hemisphere is tilted towards or away from it. Hence on the day of the equinox day and night are approximately equal everywhere on Earth.

Another feature of the equinox is that the Sun rises due east and sets due west. Again this statement is a slight approximation and its accuracy depends on how close to sunrise or sunset the exact occurrence of the equinox takes place. On the autumn equinox on 20 March 2012 there is less than three hours between the time of the equinox and the time of sunset, hence sunset takes place on the day almost exactly due west.

‘The descent of the snake’ at Chichen Itza in Mexico during the March 2009 equinox. Courtesy Wikimedia Commons

The most famous equinox happening is ‘the descent of the snake’ at Chichen Itza in Mexico at sunset at the spring and autumn equinoxes. Chichen Itza is a pre-Columbian city built by the Maya civilization over a thousand years ago. One of its buildings is a tall stone pyramid, on part of which at the equinoxes the setting Sun creates a pattern of light and shadow that gives the impression of a descending snake.

If clouds permit have a look at the setting Sun on the day. You may not see a snake, but you will establish the exact position of due east and due west.

The constellation of Orion imaged on the evening of 12 March 2012 with the main stars labelled. Image and copyright Nick Lomb ©, all rights reserved

There is a lot to see in the evening sky this March 2012. The Moon is out of the way until near the end of the month and the two brightest planets Venus and Jupiter are close together in the north-west. On the other side of the sky Mars is still bright in the east after its recent opposition. The planets are augmented by the International Space Station making bright evening passes and there are occasional bright flashes from Iridium satellites.

Among all this activity it is also worth looking at some old favourites such as the constellation of Orion that is prominent high in the western sky. Orion is one of the easiest constellations to recognise with four bright stars in a rectangle and three stars in a row in the middle. In Australia many people refer to part of the constellation as the Saucepan – the three stars of the belt form the base and the dagger with the Great Nebula of Orion in the middle represent the handle.

The international GLOBE at night project wants people to observe one of two or three constellations in the evening sky to report on the brightness of their sky. Orion is one of the two southern hemisphere constellations available for this purpose. There are two opportunities left this year to contribute, until 22 March 2012 and 11 to 20 April 2012. Contributing an observation is easy to do and there is a cool webapp so that observations can be submitted in real-time.

The brightest star in the constellation is the blue supergiant Rigel that represents the left foot of the giant Orion according to Greek mythology. Strangely, the star is named Beta Orionis even though it is the brightest star. It is at a distance of 860 light years from us and radiates 85 000 times as much energy as our Sun. There is a faint companion that is itself double and is so far from the main star that it takes over 20 000 years to make one circuit.

The Alpha star in the constellation and the second brightest star is the huge red supergiant Betelgeuse. It is so huge that if it replaced the Sun it would engulf all the four inner planets, Mercury, Venus, Earth and Mars. It is at a distance of about 570 light years and radiates 85 000 times as much energy as our Sun.

Another one of the four stars forming the outer rectangle of Orion is Saiph. This star is a blue supergiant like Rigel, but even hotter. This high temperature means that more of its energy is radiated as ultraviolet and so it appears fainter than Rigel to our eyes.

The fourth of the stars forming Orion’s rectangle is Bellatrix. This is again a hot blue star that at a distance of 240 light year is closer to us than most of the other stars in Orion.

Finally, let’s mention Meissa that according to the old mythological drawings is the head of the giant Orion. This is a double star with one component being a rare O-class star with the extreme temperature of about 35 000 Kelvin while its companion is a little cooler 27 000 Kelvin. (The Kelvin temperature scale used by astronomers is the same as the ordinary Celsius scale, but with 273 added so that the freezing point of water is at 273 K.)

So on these dark autumn evenings once you had your fill of the bright planets and satellites, have a look at the giant Orion and then maybe report what you see to Globe at Night.

Reference: Stars by Jim Kaler

Diagram showing the position of Venus and Jupiter from 6 to 26 March 2012 at two day intervals as seen from Sydney at 8:15 pm AEDT. Jupiter’s position on 16 March is not shown. On 26 March a thin crescent Moon moves between the two planets. To give an indication of scale a bar of 10° length is shown. Note that a closed fist at arm’s length subtends approximately 10°. Diagram Nick Lomb

During this month March 2012 low in the north-west two bright objects can be seen in the sky. No, they are not aircraft landing lights, but the two brightest planets in the sky, Venus and Jupiter. Each evening they are in a slightly different position with respect to each other. The two planets are closest from 11 to 15 March with only three degrees or six moon-widths separating the planets at their closest.

If the weather is bad, as it has been in Sydney lately, do not worry. Arguably, the best sight will be on 26 March when the crescent Moon moves between the two bright planets. That will definitely be worth seeing and photographing.

It is relatively easy to photograph such bright planets, with or without the Moon. A tripod is needed to provide firm support for the camera, which needs to be able to make time exposures. Try a range of exposures from 1 second and longer. Also try different zoom settings. A flash should not be used unless you have a nearby tree or plant that you want to illuminate to provide a better composed image.

Conjunctions of Venus and Jupiter do happen every few years and are always worth observing. The most recent similar conjunction was the wonderful ‘smiley face’ one of 1 December 2008 when the Moon joined the two bright planets to form a smiling face. It was cloudy in Sydney on the night, but there was still a good view on the next evening.

Good luck with observing the two close planets this month!

Venus & Jupiter on 10 March 2012. Taken with a small camera on a tripod, two seconds exposure and 4x zoom. Photo Nick Lomb

Venus & Jupiter on 12 March 2012. Taken with a small camera on a tripod, two seconds exposure and 4x zoom. Photo Nick Lomb

The following email came in asking us to help identify a bright object in the night sky. It has a number of helpful observations and by Jove we love a challenge.

“Dear Sir/Madam, I recently sent an email to Kate **** from the CSIRO Astronomy regarding a bright round light that appears nearly each night from around 8pm and at this time is is low in the sky in the south east. As each hour passes it travels much higher in the sky towards the north east. On a clear night it is very bright white colour and does move reasonable quick. As nobody has been able to tell me what it is she suggested I contact you people and hopefully you can tell me what it is. We are watching this from our back yard in Burleigh Waters on the Gold Coast in Queensland. Thanking you in anticipation of your reply.
Regards Ray “

Let’s break it down and see what we can do?

“…nearly each night from around 8pm”. This tells us it was not a one off event. The “nearly” may simply be the result of cloud blocking the view.

“…low in the sky in the south east”. A cardinal direction with a time is perhaps the second most useful observation, after all the sky is pretty big.

“As each hour passes it travels much higher in the sky..”. This tells us the motion is in accordance with the daily rotation of the earth and probably not due to the object itself. This hint points well towards an astronomical suspect.

“…it is very bright white colour’. Brightness and colour in this case will probably seal the deal. Now let’s look at a simulation of the night sky at 9pm instead of 8pm. This will give the object time to clear the horizon.

We will use the free program Stellarium available from www.stellarium.org to look in the area from the east to southeast from 8pm to 9pm.

There are two bright white objects in the area described by Ray. To the south east lies the second brightest star in the night sky, Canopus and the planet Jupiter to the east north east. The information is too hard to read from the small picture here so two larger inserts have been included as well. A larger version of the main image will be available on our Facebook page. Looking at the relative brightness of the two candidates, Jupiter at present is nearly 6.5 times brighter than Canopus.

On the balance of probabilities, Ray has been seeing the King of the Gods, Jupiter. It is perhaps not unsurprising that Ray, and we are sure many others, have noticed Jupiter at present as it will be in opposition and at its brightest on Oct 29th.

An earlier blog by Dr Nick has more details on this. http://www.sydneyobservatory.com.au/2011/watching-the-galilean-moons-of-a-bright-jupiter-through-binoculars/

With Jupiter so close it is an excellent time to come along for one of our regular night tours. If the weather cooperates and the time is just right you should be able to see some nice detail on this spectacular planet.

Mystery solved.

Created using Stellarium 0.11.0

On a dark clear night who hasn’t looked up to see the last hint of meteor fading from sight? “Damn it why couldn’t I have seen all of it?” And then perhaps minutes, hours or nights later you do get to see one while everyone else is looking the wrong way! Looking for meteors or “shooting stars” can be very frustrating.

Every now and then however the probability of seeing some increases as the Earth silently glides through the dust trail left behind by a comet. This causes a meteor shower.

Next month is one of the better chances or is it? The Orionid meteor shower this year should start around October 2nd and go through to November 11th but the peak is from October 19th to the 22nd.

The Orionids are the result of dust left behind by that most famous of comets, Halley that last swept by in 1986 and will return in 2061. As the Earth rotates on its axis while orbiting the Sun the relative velocity of up to 70km/s makes short work of the dust hitting the atmosphere between 80 and 120km up. The result is a short sharp streak of light in most cases though sometimes a more spectacular fiery event can be seen lasting for several seconds.

So what can we expect this year? A lot depends on the observer’s location. In a bright city or dark country side, clouds, fog and latitude. The closer to the horizon the key point known as the Radiant is the poorer the view and the number of visible meteors drops.

For the Orionids the Zenithal Hourly Rate (ZHR) and therefore maximum under PERFECT conditions is above 23 meteors per hour. For major locations like Sydney this drops but be how much?

The following formula helps.

Where HR is the hourly rate for an observer, r is the population index, in this case 2, which describes the numbers of fainter meteors of each magnitude, LM is the observer’s average limiting magnitude ie how bright the sky is. For cities like Sydney this would be about 4.5 until the time of Moonrise when the sky becomes much brighter. A is the altitude of the shower’s radiant above the horizon in degrees which sadly for the 22nd will only be about 17 degrees above the ENE horizon.

So for Sydney at around 2am on October 22nd (Moonrise is at 2:39am)

(HR) = 0.84 which is sadly well below the sporadic or background rate of 4.

This means this years Orionids are effectively a non-event for us on the East coast of Australia in 2011 as the radiant is so low before the rising waning Moon. Maybe we will have better luck next year!

(1) http://www.namnmeteors.org/

Sunset viewed from Sydney Observatory on 29 August 2005. Image and copyright Nick Lomb ©, all rights reserved

In 2011 spring equinox for the southern hemisphere is on Friday 23 September at 7:05 pm AEST. On that day the Sun crosses from the northern part of the sky to the southern and it is one of only two days in the year when the Sun rises due east and sets due west; the other being the autumn equinox in March. As on this occasion the exact time of the equinox is close to the time of sunset, the setting Sun will be almost exactly in alignment with azimuth 90° or west.

In many countries the change in the seasons is assumed to take place at the equinoxes and at the solstices in June and December. This is not the case in Australia where, for slightly obscure historical reasons, the seasons change at the beginning of the month so that spring has already began on 1 September. For whatever reasons we have that tradition, it does work well as the hottest days of the year do tend to occur in the middle of the December, January and February summer period. Similarly the coldest days tend to occur in the middle of the winter period of June, July and August.

The position of the Sun on the horizon when its centre is 90° from the zenith. Drawing and copyright Nick Lomb ©, all rights reserved

The most common question asked at the time of the equinoxes is why are there more than 12 hours of daylight on the day of the equinox? It is a good question for with the Sun rising due east and setting due west on the day, it would be sensible to assume that the length of daylight is halfway between its shortest in winter and its longest in summer.

In reality, there is always a little more than 12 hours of daylight on the day of the equinox. There are two factors involved. One is that all considerations involve the centre of the Sun. As indicated on the diagram above, when the geometric centre of the Sun is on the horizon, half of the disc is still visible. Sunset does not occur until that top half disappears below the horizon. As the angular size of the Sun is about 30′ of arc, an extra 15′ of arc has to be added for the calculation of sunset times.

The change in the apparent position of the Sun on the horizon due to refraction. Drawing and copyright Nick Lomb ©, all rights reserved

The second factor involved is the bending of light by the atmosphere or refraction. This means that near the time of sunset we see the Sun as above the horizon even though it is actually below. How far the sunlight is bent depends on both the temperature and the atmospheric pressure. An average value of 35′ of arc is normally assumed to be the extra motion the Sun has to undergo before it appears to set. This is, however, just an estimate and hence calculations of sunrise and set times are predictions only.

The two factors together add up to 50′ or arc so that the Sun has to be 90° 50′ from the zenith before it appears to set. This is the value assumed in calculations of sunrise or set times. To estimate how that extra 50′ of motion at both sunrise and set lengthens daylight note that to a rough approximation the Sun takes 4 minutes of time to move 1°. This suggests that at the equinox daylight will be 7 minutes longer than 12 hours. From the 2011 Australian Sky Guide the length of daylight on Friday 23 September can be worked out as 12 hours 8 minutes – close to our rough calculation.

If on the day of the equinox someone says that the times in the paper or in the sky guide are wrong, you can explain that they are exactly as expected after allowing tor the finite size of the Sun and for refraction.

The northern hemisphere constellation of Lyra the Lyre with its bright star Vega. Drawing Nick Lomb using Stellarium software

Did you know there is an upside down lyre hanging low in the northern sky each evening after dusk? This is the best time of the year to see the famous northern constellation of Lyra the Lyre from the southern hemisphere as it is at its highest (though still low in the northern sky) in the early evening.

The story of the Lyre from the ancient Greeks is that Apollo gave it to his son Orpheus, the musician. Orpheus played the lyre to accompany his mournful songs as he attempted to release his wife Eurydice from the underworld after her early death. The music was so moving that he almost succeeded except that he turned back to look at her earlier than he was allowed to and she vanished for ever.

The only bright star in the constellation is Vega. This is the fifth brightest star in the sky and one that astronomers use as a standard star with which to compare other stars. In recent times this role for the star has turned out to be somewhat awkward for it has been discovered that instead of being a slowly spinning star as had been thought, it is a fast spinning star viewed with its pole facing us. This spin distorts the star so that it is much broader at its equator than at its poles: through its poles its width is 2¼ times that of the Sun while at its equator it is 2¾ times wider than the Sun.

Vega is 25 light years from us and is surrounded by a disc of dust that glows in infrared. Possibly the disc is a system of planets that is forming or that will form from the disc. Another distinction for Vega is that due to the wobbling of the Earth’s axis (precession) in 11 000 years or so it will become the northern hemisphere pole star, a role presently filled by Polaris.

Another famous object in Lyra is the Double Double star. In good conditions from the northern hemisphere where the star can be high in the sky, some people claim to actually make out the two components of the star with their unaided eyes. For us, with the star low in the northern sky, binoculars are necessary to do the same. Still, this possibly is the only double star that can be so easily seen.

Interestingly, each of the components of the Double Double star are themselves double. The close pairs circle each other roughly once a thousand years while the two visible in binoculars are believed to take about 500 000 years. Don’t expect any change as you watch!

A wide-angle view of the Ring Nebula in Lyra. Image taken at Sydney Observatory on 29 September 2004 using a telescope in New Mexico

The most famous object in the constellation is the Ring Nebula in Lyra. A telescope at a dark sky site is needed to see this planetary nebula or dying star. Planetary nebulae are stars that throw off their outer envelopes at the end of their lifetimes and leave behind a very hot remnant . This hot remnant excites the thrown off gas to shine in different colours that depend on the intensity of the radiation, which drops off with increasing distance from the central star.

The age of the nebula can be estimated from its rate of expansion and turns out to be somewhere between 6000 and 8000 years. Like the case with all planetary nebulae, the distance of the Ring Nebula is hard to estimate, but one suggestion is that it is about 2300 light years from us.

The stars of the constellation of Sagittarius the Archer. As indicated on the drawing, the constellation has an informal alternative name of the Teapot. The positions of the two best known nebulae in the constellation, Messier 8 and Messier 20, are also shown. Image and copyright Nick Lomb ©, all rights reserved with the drawing of the teapot from Microsoft clip art

In August/September the constellation of Sagittarius the Archer is almost overhead in the early evening as seen from Australia. It is one of the 13 constellations of the Zodiac, that is it is one of the 13 constellations in front of which the Sun moves during the year, and it is the constellation that follows Scorpius the Scorpion. It is normally drawn as a half–man half–horse creature, known as a centaur, with a bow and arrow in its hands. The lower two stars Rukbat and Arkab represent the creature’s left foreleg.

As usual with ancient constellations, a lot of imagination is neeeded to see this group of stars as a centaur. Instead, to modern eyes some of the brighter stars of the constellation clearly outline a teapot. The stars Kaus Media, Kaus Australis and Alnasi represent the spout, the stars Kaus Media, Kaus Borealis and the star to the right of Nunki represent the lid while Nunki, the star to its right, Ascella and the star to its left represent the handle. Once seen in this way the constellation becomes quite unmistakeable.

The brightest star of the constellation is Epsilon Sagittarii or Kaus Australis, meaning the Southern Bow. It is a hot white star that is 140 light years from us. Delta Sagittarii is known as Kaus Media, meaning the Middle of the Bow. This is an orange coloured star at a distance of 300 light years.

A few other stars of Sagittarius:

Gamma Sagittarii or Alnasi, meaning the point. It is an orange coloured star at a distance of 100 light years.
Zeta Sagittarii or Ascella, meaning the Armpit. It is a double star in which the components take 21 years to circle each other. The stars are 90 light years away.
Sigma Sagittarii or Nunki, meaning the Star of the Proclamation of the Sea. This wonderful name seems to refer to the fact that the following zodiac constellations, Capricornus, Aquarius and Pisces, are all associated with to water. It is a hot blue-white star that is 224 light years away.

The constellation contains a number of nebulae. One of these is the famous M8 or the Lagoon Nebula that is a huge cloud of gas and dust illuminated by a cluster of newly formed stars. Another famous object in Sagittarius is M20 or the Trifid Nebula that has three clearly visible dark lanes of dust in front of the bright part of the nebula.

Sagittarius is an excellent target through binoculars or through a small telescope. On a clear evening go outside and have a look!