Tanya

DISPLAYING POSTS BY: Tanya (38)

Tanya

Dr Tanya Hill is the Planetarium’s astronomer. She loves exploring the Universe and it never fails to amaze her. It was the Orion Nebula that led her to astronomy and she’s reached for the stars ever since.

Total Lunar Eclipse

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by Tanya
Publish date
2 April 2015
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A total lunar eclipse will be visible from across all Australia this Saturday, April 4. But it will be a quick one. Rather than passing deep into the Earth’s shadow, the moon is skimming close to the shadow’s edge, as seen in this animation.

Total Lunar Eclipse - December 2011 The moon moves out of totality as seen from Sydney during the December 2011 eclipse.
Image: Neerav Bhatt
Source: Neerav Bhatt/flicr
 

The period of totality, when the moon is fully enclosed in the Earth’s umbral shadow, will last just five minutes or so. This makes it the shortest total lunar eclipse of the 21st century.

In fact, this eclipse has the shortest period of totality for almost 500 years. Back in 1529, on October 17, there was an eclipse where totality lasted for just 1 minute and 42 seconds.

The moon's passage through the Earth's shadow The moon skims the edge of the Earth’s umbral shadow. Note: this graphic is oriented for the southern hemisphere.
Source: Museum Victoria
 

How to see it?

The great thing about lunar eclipses is that no special equipment is required to view them. Just look for the moon, which will be visible in the north-eastern sky, weather permitting.

Eclipse timings Local circumstances for the eclipse.
Source: Museum Victoria
 

From Western Australia, the eclipse begins shortly after sunset with the moon low to the eastern horizon. However, by the time of totality the moon will have risen to a great vantage point.

For the rest of Australia, the eclipse occurs an hour or two after sunset.

The whole event will last for three-and-a-half hours – it’s only the moment of totality that will be very short.

Total Lunar Eclipse - April 2014 The moon disappears into shadow during the total lunar eclipse of April 2014.
Image: Phil Hart
Source: Phil Hart
 

Watching the moon slowly enter into the Earth’s shadow is always an amazing sight. And it’s during this partial phase of the eclipse that the shadow appears lovely and dark – seen in sharp contrast to the part of the moon which is still in sunlight.

Because totality is very brief and the moon sits so close to the shadow’s edge, this eclipse will not give us a deep red moon. Instead, we will likely see a variation in tone across the face of the moon.

The southern region of the moon – the topmost part as viewed from Australia – travels deeper through the shadow and should take on a reddish-orange glow. Whereas the northern part (or the bottom of the moon) that skims the shadow’s edge, will remain fairly bright.

Total Lunar Eclipse - October 2014 The moment the moon enters totality during the October 2014 lunar eclipse. A similar variation in colour across the moon is expected for this upcoming eclipse.
Image: Martin George
Source: Martin George
 

Where the shadows lie

What’s interesting to note about this eclipse, is that it’s a great test for eclipse modellers. It’s not surprising, that there is no sharp edge to the Earth’s shadow and therefore the timings of this eclipse are highly dependent on the model used to estimate the shadow’s size.

This was first recorded in the early 18th century when the French astronomer and mathematician Philippe de La Hire realised that to match the timings of a lunar eclipse, it was necessary to increase the predicted radius of the Earth’s shadow by 2.4%.

In fact, observations of lunar eclipses over the next 200 years showed that the size of the Earth’s shadow varied slightly from one eclipse to another.

By the late 19th century a standard value of 2% was adopted as the most effective increase to be made to the Earth’s shadow size to produce the best predictions for an eclipse.

The ‘fuzziness’ of the Earth’s shadow is due to a number of factors such as the sun’s apparent size and the transparency of the Earth’s atmosphere. Eclipse models also have to take into account the fact that the Earth isn’t completely round, but flattened slightly towards the poles. And even the Earth’s axial tilt can add a small variation in the size of the Earth’s shadow depending on what season it is on Earth.

But for most lunar eclipses, these variations have a small effect on the eclipse timings, amounting to differences of the order of 20 seconds or so. But because this eclipse occurs so close to the edge of the Earth’s shadow the model used is much more critical.

Timey wimey

For this article, I’ve sourced the eclipse timings from NASA’s eclipse website maintained by Fred Espanek, a retired astrophysicist from NASA’s Goddard Space Flight Center. His Earth shadow model is the smallest and gives the shortest period of totality at 4 minutes and 38 seconds.

But it’s not surprising that other reputable eclipse models are found to give different results. Australian David Herald is the author of the fantastic Occult program that predicts all types of occultation events (such as the Saturn occultations we saw last year), and it also predicts eclipses. The latest version of his program (Occult4), measures totality for this eclipse as lasting 7 minutes and 14 seconds.

While the longest measurement of totality is sourced from the U. S. Naval Observatory. Their lunar eclipse computer gives a prediction of 12 minutes and 18 seconds.

But what will we see?

What this highlights for me, is the beauty of nature. We can do our best to predict what will happen, but sometimes we just don’t know for certain until we see the event itself. And it’s one of the lovely things about lunar eclipses that each one is unique.

With this eclipse, it is the path through the Earth’s shadow which makes it particularly different. But what can also affect eclipses is how dusty the Earth’s atmosphere is at the time. The dustier the atmosphere the more light is blocked.

That’s when the moon can turn a really deep red, like it did 2011 when the atmosphere was full of ash from the eruption of Chile’s Puyehue-Cordon Cualle volcano.

Even nonlocal weather conditions can affect the appearance of a lunar eclipse. Storm systems and poor weather in the part of the world where the sun ‘disappears’ as seen from the moon, will make the atmosphere less transparent. In particular, if it’s a very deep eclipse and the moon passes right through the centre of the Earth’s shadow, such weather systems can help to deepen the Earth’s shadow so that the eclipsed moon becomes almost impossible to see.

So why not try your hand at timing this eclipse event and see which model gives the best prediction for you?

And while you are watching the eclipse, the star to the right of the moon is called Spica, the brightest star in the constellation of Virgo. While in the east Saturn will be visible at the head of Scorpius and over in the north-west Jupiter can be found shining brightly.

But if the weather doesn’t cooperate in your local area, you can also follow the eclipse via live streaming by Sydney Observatory, Slooh or the Virtual Telescope.

Most importantly, this lunar eclipse is well worth a look because it will be a while before we get the chance to see another. The next lunar eclipse to be seen from Australia is a partial one on August 7, 2017 and only 25% of the Moon’s diameter will be in shadow. The next total lunar eclipse won’t occur until January 31, 2018.

Many thanks to Martin George, Curator of Astronomy and Assistant Director, Queen Victoria Museum, Launceston, for very useful discussions that improved this article.

The Conversation

This article was originally published on The Conversation. Read the original article.

 

Two eclipses for April

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by Tanya
Publish date
11 April 2014
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Not one, but two eclipses will occur this month and both are partially visible from Melbourne.

Just before sunset on the 15th April, the Moon will rise already totally eclipsed. It should look quite eerie to see a red moon rising above the eastern horizon and it's always amazing how bright the Moon appears as it moves out of the Earth's shadow and returns to its usual splendour. While you are watching the eclipse, be sure to take a look at Mars, which will be just to the left of the Moon and the bright star Spica (in the constellation of Virgo) that will be found just above.

Lunar Eclipse The progression of a total lunar eclipse in August 2007.
Image: Phil Hart
Source: http://www.philhart.com/
 

Two weeks later on the 29th April, the Moon and Sun will come together in the sky and we'll see a partial solar eclipse. The eclipse will begin during the afternoon and reach its maximum point just before sunset. At the height of the eclipse 64% of the Sun's diameter will be covered by the Moon. The Sun will still be partially eclipsed as it sets below the western horizon.

Solar Eclipse The Moon takes a bite out of the Sun.
Image: Phil Hart
Source: http://www.philhart.com/
 

The timings for both the lunar and solar eclipse can be found from the Planetarium's monthly newsletter – Skynotes – which is a great guide for finding your way around the night sky.

Importantly, lunar eclipses are lovely to watch and you don't need any special equipment. Solar eclipses, on the other hand, require a bit of care and planning. Never look directly at the Sun.

There are safe ways to watch a solar eclipse and the easiest is to purchase special eclipse glasses. They are available from the Scienceworks shop and will allow you to watch the event, while protecting your eyesight.

You can also create a simple "pinhole" projection. It's as easy as making a small pinhole in a piece of paper or cardboard. Do not look through the hole, but allow the Sun to shine through and project an image onto a second piece of cardboard. Even a blank wall or a clear patch of ground can make a good surface for projection.

And as I've mentioned previously on the Museum's blog, sometimes nature helps out too. If you can see sunlight travelling through the leaves of a tree, you’ve got yourself some ready-made pinhole projections. Check the ground and it might be covered with little crescent Sun images, just like this great example from the Astronomy magazine website.

Rings around an asteroid

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by Tanya
Publish date
31 March 2014
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In a surprise discovery, two rings have been found around the asteroid Chariklo, making it the first small Solar System body known to have rings.

Saturn is known for its magnificent rings and the other gas giants - Jupiter, Uranus and Neptune - have ring systems too, though not quite as impressive. Careful searches had not found any other ring systems within the Solar System and many astronomers were beginning to think that rings might only exist around large objects, until now.

Rings from Chariklo An artist's impression of the newly discovered rings around Chariklo.
Source: ESO/L. Calçada/Nick Risinger (skysurvey.org)
 

Chariklo is just 250km across and lies beyond Saturn, at a billion kilometres away. It is much too small and far away for the rings themselves to be seen, but amazing detail is now known about them. The rings are dense but narrow, just three and seven kilometres wide, and are separated by a clear gap of nine kilometres. If you were standing on the surface of Chariklo, the rings would appear as wide as our Full Moon and stretch from horizon to horizon.

The discovery was made possible because last June, Chariklo passed in front of an obscure star (UCAC4 248-108672). Not only did Chariklo block the star's light for 5 seconds, but two tiny dips in the starlight were seen, just before and after Chariklo moved by. This video from the European Southern Observatory (ESO) shows faint dimming caused by the rings, just before and after Chariklo blocks the star completely.

 

This event, known as an occultation, could only be seen from South America and an observing campaign was coordinated across seven observatories, including two telescopes operated by the ESO at La Silla, Chile. Having observations from all seven observatories, ruled out other possible explanations, except for a ring system.

What I really love is the data from the new high-resolution camera on ESO's 1.54m Danish telescope. (Anyone who has been to my Discover the Night Sky series knows that I am particularly fond of beautiful graphs!) This new camera was developed to search for exoplanets and can take up to 40 images per second. It was actually able to see the gap between the two rings – now that's beautiful science!

Chariklo Data The data captured by ESO's 1.54m Danish telescope, showing Chariklo blocking out the light of the star (the main dip). On either side are two small, double dips, as the rings also passed in front of the star.
Image: F. Braga-Ribas et al.
Source: Reprinted by permission from Macmillan Publishers Ltd: Nature (March, 2014)
 

The Planetarium's astronomer, Dr Tanya Hill, was recently appointed the Australian representative of the European Southern Observatory's Science Outreach Network.

Links:

Gravity waves, cosmic inflation and the multiverse

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by Tanya
Publish date
20 March 2014
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Scientists have been whipping up a frenzy this week, following the announcement of a truly mind-boggling discovery last Monday. Astronomers from the Harvard-Smithsonian Centre for Astrophysics (USA) have captured the signature of gravity waves from the early Universe. Now, just like the Higgs Boson announcement from 2012, this one takes a bit of time to wrap your head around, but the consequences are out of this world.

Gravity waves from after the Big Bang The twisting pattern shown here represents the first direct image of gravity waves from cosmic inflation, in the moments just after the Big Bang.
Source: EPA/Harvard University
 

Firstly, this discovery provides the first evidence for gravity waves, a phenomenon that scientists have been searching for over many decades. Gravity waves are tiny ripples in the fabric of spacetime and they were predicted by Einstein's General Theory of Relativity. This theory has now survived every test that could be thrown at it. Even the bizarre notion of time ticking at different rates, depending on gravity and speed, is put into practical use every time we use a GPS to find where we are.

Finding gravity waves could open up a whole new window on the world around us. We are used to looking at the world using light, which are electromagnetic waves. But gravity waves hold just as great a potential. They could be used to see inside a neutron star, to watch a star collapse within a supernova or to examine two black holes colliding. They could even reveal new physics ideas and make it possible to pick up the vibrations of cosmic strings or determine the number of dimensions of the Universe.

Colliding black holes In this artist's impression, two black holes are entwined in a gravitational tango that will eventually collapse the pair into a single black hole.
Source: NASA
 

Secondly, the discovery provides the best proof of cosmic inflation – the idea that within a fraction of a second after the Big Bang, the Universe went through a period of rapid expansion. Picture a child blowing bubbles, but in this case, the Universe grew from absolutely miniscule to larger than the observable Universe in the blink of an eye.

Only cosmic inflation predicts the rise of gravity waves in the early Universe. And now that the theory has passed this test, other predictions of inflation may well hold true; such as spacetime being infinite and that the multiverse concept is real. We are one step closer to the possibility that there exists an infinite number of Universes, separate to ours, that are continually erupting, just like ours did, out of some sort of ever inflating "sea".

Links:

How the detection was made: First hints of gravitational waves in the Big Bang's afterglow by Krzysztof Bolejko (University of Sydney) and Alan Duffy (University of Melbourne), in The Conversation

Solar eclipse from space

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by Tanya
Publish date
31 January 2014
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During the early hours of this morning, from 12:30am to 3am, NASA's Solar Dynamics Observatory captured a stunning solar eclipse from space.  The Observatory sees a number of eclipses each year, but this was the longest one that's been recorded so far.

 

The eclipse was visible from the Observatory's vantage point, orbiting 36,000km above Earth. Since it could only be seen from space, the event is technically called a lunar transit. At its peak, the Moon covered up to 90% of the Sun.

Just as the Moon moves away, you can see a solar flare erupting from the left hand side of the Sun. This is just the kind of activity that the Observatory is helping scientists to better understand.

Solar Flare from the Solar Dynamics Observatory Perfect timing as the Sun releases a solar flare.
Source: NASA
 

Launched into space on 11 February 2010, the Observatory is on a 5 year mission to study the Sun as part of NASA's Living with a Star program. Our Sun is very active releasing flares and eruptions that can send energetic particles hurtling towards Earth. This can play havoc with our technological systems, bringing down power grids and causing blackouts. The ultimate aim is to better understand the cause of the Sun's activity so that one day we may be able to predict when such flares will occur to give us some prior warning.

The Observatory takes an image of the Sun every 0.75 seconds, and you can see all the beautiful images at the Observatory's Gallery. We have been loving the Gallery here at the Planetarium, and some of the footage will be featured in a new planetarium show to be released later this year.

A Christmas star for 2013

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by Tanya
Publish date
13 December 2013
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Just in time for Christmas, a new star has appeared in our southern sky!

Nova Centauri 2013 is the brightest nova to be seen since 1999. It is about as bright as the fifth star in the Southern Cross and is very easy to spot from Melbourne, and across Australia in general. It sits right next to Beta Centauri, one of the famous two pointer stars that leads the way to the Southern Cross.

Nova Cen 2013 from ESO This photograph was taken from La Silla Observatory in the Chilean Atacama Desert on the morning of 9 December 2013.
Image: Yuri Beletsky
Source: ESO
 

The nova appears just to the left of Beta Centauri, the bluer and higher of the two bright stars in the lower-right part of the image. The Southern Cross and the dark Coal Sack Nebula are also captured near the top of the image.

The nova was discovered on 2 December by John Seach from New South Wales. It is a 'classical nova' and is caused by a dead white dwarf star having a brief, but intense new-lease on life. White dwarfs are stellar embers, where nuclear fusion (the fire that keeps a star shining) has ended. However, this white dwarf has a close companion star. If enough gas from the companion falls onto the white dwarf it triggers a brief explosion on the star's surface.

The star undergoes an extreme burst of brightness. But unlike a supernova, the white dwarf remains intact and lives to tell the tale.

What better reason is there to slip away from the Christmas madness and spend a quiet moment or two, under the stars.

Links:

Ideas about the 'real' Christmas star from Phil Plait's Bad Astronomy blog

ESO Science Outreach Network - Australia

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