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Wednesday, June 30, 2010

 

More Images of the Partial Lunar Eclipse

Australians wern't the only ones to enjoy the partial lunar eclipse of June 26, 2010. The delightful image sequence comes from long time correspondent Tony Travaglia of Otago, New Zealand. Tony struggled with perssitent cloud to get these images, and was snapping well into the wee hours. His persistence paid off with a lovely sequence for us to enjoy.

He used a Canon 40D with a Rubinar 1000mm mirror lens to take these images.

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Tuesday, June 29, 2010

 

The Sky This Week - Thursday July 1 to Thursday July 8

The Last Quarter Moon is Monday July 5. Venus is readily visible in the early evening, heading towards Regulus. Venus, Regulus, Mars and Saturn make an attractive line up. Mercury returns to the evening sky. Jupiter is prominent in the morning sky and is close to the Moon on Sunday July 4.

Morning sky looking east as seen from Adelaide at 4:30 am on Sunday July 4. Click to embiggen.

The Last Quarter Moon is Monday July 5.

Jupiter is clearly visible in the northern sky as the brightest object in the early morning. Jupiter is now high enough for telescopic observation to be rewarding. Jupiter looks a little different now that one of its bands has disappeared. Jupiter and Uranus are close together and can be seen near each other in a pair of binoculars (spotters map here). On Sunday July 4 the waning Moon is close to Jupiter.





Evening sky looking North-west showing Mercury,Venus, Mars and Regulus at 5:50 pm local time on Thursday July 8. Click to embiggen.

Mercury can be seen by the keen-eyed low above the western horizon half an hour after sunste by the end of the week.

Bright white Venus is readily visible above the western horizon from half an hour after Sunset, (even before) until past the end of twilight (about an hour and a half after sunset). Venus starts the week in Leo, forming a line with the Regulus, Mars and Saturn. During the week Venus moves closer towards Regulus as a prelude to some spectacular planetary alignments in July and August.

In the evening Mars can be seen low in the north-western sky. Mars is to the right of Regulus, the bright star in Leo the lion at the beginning of the week and will draw further away from it during the week, coming closer to Saturn. Mars is now only slightly brighter than Regulus, but is distinguishable by its reddish colouring.

Saturn is easily visible in the western evening sky as the bright yellow object between the bright stars Regulus and Spica, just up from Mars. Telescopic observation of the ringed world is now becoming more difficult. Saturn is high enough in the sky for the best telescopic views at around 7 pm. Saturn's' rings are opening, and look quite beautiful, even in a small telescope. On the 4th of July, Saturns' Moon Titan cruises just below the planets South pole.

If you don't have a telescope, now is a good time to visit one of your local astronomical societies open nights or the local planetariums.

Printable PDF maps of the Eastern sky at 10 pm ADST, Western sky at 10 pm ADST. For further details and more information on what's up in the sky, see Southern Skywatch. Cloud cover predictions can be found at SkippySky.

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Sunday, June 27, 2010

 

Music, Moons and a Partial Eclipse

The Moon floats over Adelaide City as the eclipse begins (June 26, 2010).

For the August 2007 Lunar eclipse we held an eclipse pizza party. For this June 2010 partial Lunar eclipse, the Bettdeckererschappneder Weisel's (BDW for short) choir had a concert on, and the elder kids were over friends places. I was being door man and general factoum for the concert. This just involved me looking clueless and taking peoples money, but they raised a decent contribution for the Hutt Street Centre on the night. I alerted people to the existence of the eclipse, and asked them to look up after the concert.

The concert was terrific (as NFBM concerts always are). And the BDW was pretty good in that Joe Jackson song about pretty women walking with gorillas, although my favourites were "Blue Moon" and "My Johnny".

After the concert we headed out for dinner with the choir, and a gorgeous full moon floated above us.

Partial Lunar eclipse as seen from Adelaide at 21:30 pm, 26 June 2010. 4" Newtonian Reflector, 20 mm Plossl eyepiece and Canon IXUS 100 IS (400 ASA, 1/15 exposure). Click to embiggen.

We had dinner at a marvellous Malaysian restaurant, where SmallestOne entertained us by demonstrating centrifugal motion with the table caddy (and crawled under the table tickling people), and I talked astronomy and eclipses.

Unfortunately, service was a little slow, so by the time we had dinner we had to rush out again to pick up MiddleOne from his friends place. As we left, I pointed out the chip in the side of the Moon to Smallest one.

We finally got home, unloaded sleepy children and various props from the singfest, and then I got ready to observe the eclipse.

Lunar Eclipse progression from 9:30 pm to 10:30 pm. Photo setup as for image above, click to embiggen.
An animated GIF file of the eclipse can be found here.

Eclipse imaged with a mobile phone held up to the lens of 10x50 binoculars.

By this time the eclipse was past maximum, so I used the 4" Newtonian, rather than Don, the guided 8" , as it is very quick to set up. I also used my lens-to lens camera adapter for the imaging and attached my Canon IXUS 100 IS.

This turned out to be harder than I imagined, as the auto exposure adjust kept overexposing the image, making it hard to focus on the already indistinct lunar markings. I had to use auto exposure, as the Canon IXUS 100 IS fixed exposures don't go below 1 minute, and I needed on the order of 1/10 to 1/60th of a second. For some reason, as the eclipse waned, the auto exposure made the image darker (which is why the animation looks weird).

Anyway, buy the time this was all set up and running, it was 9:30. The Eclipse was still spectacular though. Then I alternated between taking images and helping the BDW get ready for her work trip, as she was leaving 6:00 am Sunday (Yikes). I also took some shots with my mobile phone through 10x50 binoculars, but this wasn't as successful as using it through a telescope. Still, I was able to watch the sky grow lighter, stars dissapear, and see the shadowof earth slip slowly off the face of the moon. All in all a magical evening, rounded off with hotc chocolate in bed.

Today MiddleOne asked me "what's the point of an eclipse when not all the Moon goes dark?" It's still dashed pretty, that's what the point is.

I you want to hear (and see faintly) the BDW's choir in action, singing "Blue Moon" (definitly a song to watch eclipses by), click on the animation below.

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Saturday, June 26, 2010

 

That was a Nice Little Eclipse

Partial Lunar eclipse as seen from Adelaide at 21:30 pm, 26 June 2010. 4" Newtonian Reflector, 20 mm Plossl eyepiece and Canon IXUS 100 IS (400 ASA, 1/15 exposure).

That was a great partial eclipse, here's an image to whet you appetite, I'll do the rest tomorrow morning, I'm bushed and have to go to bed.

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Observing Tonights Partial Lunar Eclipse (June 26, 2010)

The skies should be reasonably clear in Australia for tonight's partial Lunar eclipse. Midn you soon as I had extolled the virtues of the eclpise in this mornings radio interview, the sky came over all dark and rainy. The rainbow shot was taken this morning after the interview while we were making breakfast in bed for the Bettdeckererschnappender Weisle.

But it doesn't matter so much, even if it is cloudy, because the lead-up to maxi,um eclipse, and the maximum eclipse itself, last a reasonably long while. So you can pop out and watch for gaps in the cloud, or the eclipsed Moon shining through the cloud (which would be a cool effect).

Moon as seen from Adelaide, at 19:46 Saturday 26 June 2010.

So what is the best way to watch the eclipse? Well, with the unaided eye to start with. The sight of the shadow crawling over the Moon will be awesome, and you can watch the sky darken and the stars pop out as the eclipse progresses. Over in the west, Venus, Regulus, Mars and Saturn form a line, and are a nice accompaniment to the ongoing eclipse. You may even see a satellite going over.

You can use binoculars, the eclipse will liook quite nice in binoculars, and you will be able to see the darkened part of the Moon.



In a telescope, you will notice that the Earth's shadow is not sharp. You can use a low power lens to see the entire Moon to best effect, or with a high power scope you can time when various craters are covered by the shadow.

Photographing the eclipse can be done with simple digital cameras. You can just point the camera through your telescope lens and press the button, that works! For binoculars though, even at maximum eclipse the Moon is too bright and the bright part will be over exposed.

For photography without a telescope, you will need a tripod or something to keep the camera steady as you take the photo. You will need to take the photo on the fastest setting you can to avoid overexposure.

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Friday, June 25, 2010

 

Turn Your Radio On

On Saturday morning, June 26, 2010 at 7:30 am I'll be speaking briefly about the Lunar eclipse on ABC local Radio 891. If you are in SA have a listen (or outside SA it may be streamed live on the intertubes, at the ABC site above) if you want to hear me go ummm and ahhh a lot.

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In Which I Back Away Slowly

Not a Grand Cross, on the night of the lunar eclipse on June 26, Pluto, Mercury, Jupiter and Saturn do NOT align with the Earth and Moon. The X shows Earth's location and the red circle is Earth's orbit. You will definitely need to click on the image to embiggen.

Idly looking at eclipse information on the net, I stumbled upon this site, which is the 7th site linked to in Wikio’s eclipse listings (It used to be about 3rd, after two sites about about sparkly Vampires, but dropped over the past 24hrs, sadly, Sparkly Vampires are still the top two sites).

Now, I have no truck with astrology, which is a load of foetid dingos kidneys, but this site bemused me somewhat. To start with it says “Lunar Eclipse in Capricorn”. Umm, the Moon will actually be in Ophiuchus that night with the closest classical constellation Sagittarius; Capricorn is far away. Then I remembered that many astrological systems haven’t corrected for the precession of the equinoxes. (video explanation here), the slow wobble of Earth’s axis over the centuries. These astrologers still use constellation locations that were current when the Sumerians were looking at the sky, 4000 years ago (and these astrologers constellations are rectangular slabs of the sky that bear no relationship to modern constellation boundaries). Why the “mystical” properties of a slab of sky that hosted the constellation Capricorn 4000 years ago should remain the same when the actual stars are somewhere else is not clear.

A lot of people are afraid of this eclipse and while this is understandable, I don’t think there is real cause for concern.

Why one earth would people be afraid of this eclipse, it’s just the Moon going through Earths shadow! If the Moon and Earths orbit were perfectly aligned, we would have a lunar eclipse every full Moon, and a Solar eclipse every New moon. But because the Moons orbit is tilted with respect to Earth we have to wait until a Full (or New) Moon coincides with the Moon crossing Earths plane of the ecliptic. This happens about twice a year, but not everywhere on Earth can see them (we will have three lunar eclipses this year, there is a total Lunar eclipse later this year, but it happens near sunset for us Australians, so most people in Australia will not see it). So while lunar eclipses are not common, they are not rare and a trivial consequence of the Moons orbit.
It is dramatic with the Sun, Moon, Mercury, Jupiter, Saturn, Uranus and Pluto involved but we are talking about a hard core grand cross here.

Planetary positions on June 26 as seen with Earths orbit edge on, and Jupiter and Saturn aligned. Uranus is below and to the right of Jupiter and Saturn. Mercury can't be seen at this scale but is above the plane of Earths orbit. Click to embiggen.

Say what? How is Mercury, Jupiter etc. involved? The Moon crosses Earths shadow, that’s it. Mercury has nothing to do with making the Moon enter the shadow, it’s pure orbital mechanics*. Oh, wait a minute; there’s a “hard core” grand cross here is there. Hmmm, well, for some definition of “cross” that means “not very much like a cross at all” (see the diagrams above). You would expect that if Sun, Moon, Mercury, Jupiter, Saturn, Uranus and Pluto are “involved” then you might expect that if you drew a line through the Sun, Earth and the Moon it should intersect some of these objects. Well, if you draw a line through Earth/Sun/Moon it, ummm, goes nowhere near these planets (see the diagrams above). Mercury and Pluto do align in the sense that they will be almost directly opposite each other as seen from earth, but on the 27th. On the 26th Mercury is over 2 degrees from the Earth/Sun/Moon axis, and Pluto is over 5 degrees from the Earth/Sun/moon axis.

To put it in perspective; these planets are supposed to be “involved” in this eclipse but in terms of lunar diameters (which should be relevant here) Mercury is roughly 4 lunar diameters away from the Earth/Sun/Moon axis, and Pluto is over ten lunar diameters from the Earth/Sun/Moon axis. Another way to look at it is your hand, outstretched and held up like a “stop” sign, covers about the distance Pluto is off the Earth/Sun/Moon axis. That’s a use of the word “involved” with which I was previously unaccustomed**.

Jupiter and Saturn form a line as well, any two points will after all. Jupiter and Saturn did line up through Earth … on May 26, they don’t on June 26. A line dawn through Jupiter and Saturn not only doesn’t intersect the Earth (or the Moon, see the first diagram, earths orbit is a red circle, and X marks where Earth is on the 26th.), it doesn’t intersect the Pluto-Mercury line (it looks that way on the diagram, but that is because it is seen from above, the Pluto-Mercury line passes above the Saturn Jupiter line). You can continue the line from Jupiter to Uranus, but only if you jog the line down and to the side (it’s a bit hard to see due to the scale on the diagram, but in second diagram you can see Uranus is below and away from the Jupiter-Saturn axis). You can play with this in Celestia to get a feel for what is happening.

Now, I know that in astrology anything within 5 degrees of each other is considered “aligned”, but to call something a “hard core grand cross” when it doesn’t actually cross the Earth (or the Moon), the lines don’t intersect to form a cross anyway (they come within about 6 lunar diameters of each other at the “intersection point”), and the “Cross” is more like a crooked “T”; how this is “hard core”?

My recommendation is you look at the thing like you do the sky, lit up on the 4th of July as things are rarely this boldly drawn.

Okay. Well, the eclipse will be pretty. When the Moon enters the umbra, the dark inner shadow of Earth, Venus, Regulus, Mars and Saturn will be sinking in the west. At the deepest part of the eclipse Mars and Saturn gleam low in the west, but Jupiter (and Uranus) doesn’t rise until well after the eclipse has ended (and after Saturn has set). Uranus and Pluto are not visible anyway, and Mercury is lost in the glare of the Sun. Beautiful yes, boldly drawn? I think not.

I’ll back away slowly now, and enjoy the eclipse to the sound of the Bettdeckererschnapender Weisle’s accapella choir (weather permitting)

*The gravitational effect of Mercury could very, very slightly influence the Moons orbit by maybe a millionth of a second, but it is minuscule against the effect of Venus, which itself is minuscule against the effect of Earth and the Sun.

**Unless involved means “somewhere vaguely in the same region of the sky.” Not that this degree of “alignment” can’t be pretty though, on 8 August Mars, Venus and Saturn are all within 5 degrees of each other, and that will look very nice indeed. On 13 August the crescent Moon, Venus and Mars can just fit in a circle 6 degree across, that will be pretty too. But these distances, small enough when measured on the sky as seen from Earth, are actually huge and well beyond what one could say is “involved” with the eclipse.

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Thursday, June 24, 2010

 

A Storm World

HD209458b about to pass behind its sun, V376 Pegasi, as visualized in Celestia.

HD209458b is a bit of a superstar for a world whose name is a hodge-podge of letters and numbers. The first world to have been discovered by transit, dimming the light of its star as it crossed it, and the first planet to have its atmospheres composition probed, HD209458b is in the news again.

In a remarkable piece of research, astronomers have been able to precisely measure the mass of this world, and also to measure the speed of the winds in its upper atmosphere.

And what speeds they are, the winds whip across the world at a blistering 7,000 kilometres per hour, around 6 times the speed of sound at sea level on Earth. Blistering is the word in many ways. This world is a “Hot Jupiter”, orbiting its sun (which is bigger and hotter than ours) far closer than Mercury orbits our Sun. It orbits its sun in roughly 3.5 days (compared to 88 days for Mercury), its “surface” temperature is around 1000 deg C and it is in fact evaporating! HD209458b is tidally locked, which means that one side is always facing the glare of the Sun, and one side is facing darkness. The hot atmosphere on the day side hurtles over to the cooler night side, generating ferocious velocities that make the storms on Jupiter look sedate.

The astronomers measured the winds by looking at the minute shifts in the absorption spectrum of atmosphere as the planet underwent its transit. You have probably heard of the Doppler Shift, where the frequency of sound or light is changed depending on whether the source is coming towards you or moving away from you. The astronomers were able to detect the minute shifts in the absorption band of carbon monoxide in HD209458b’s atmosphere (which is mostly hydrogen, like Jupiter). They were also able to detect the slight shifts cause by HD209458b’s orbit around its sun. This allowed the astronomers to precisely calculate its orbital velocity (140 kilometres/second), and determine its mass accurately (0.69 Jupiter’s mass). This is another exoplanet first.

Amazingly, this was all done with earth based telescopes, when the new generation of super scopes comes online, what more can they accomplish?

HD209458b is already in the Celestia catalogue of extrasolar planets, but you will have too look under either V376 Peg or HIP 108859 in the “stars with planets” catalogue, if you use the “go to object” dialog box, type HD 209458 not HD209458. You way want to edit its entry with the new orbital parameters though (always back up your source file first).

The papers' abstract is here, ABC sciences take is here. Universe Today here.

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Don't Forget the Partial Lunar Eclipse this Saturday.

Don't Forget the Partial Lunar Eclipse this Saturday. Weather may be a bit dodgy in South Australia, and I'm playing doorman at the Bettdeckererscnappender Weisles concert, but I'm looking up anyway.

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Wednesday, June 23, 2010

 

Carnival of Space #159 is here.

Carnival of Space #159 is now up at the Next Big Future. Keplers big haul of exoplanets is a major theme, there's also astronomical art thieves, alien abductions, Lagrange points, the lost oceans of Mars and much, much more. Zip on over and have a read.

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Yet another Ring Around the Moon

While I was a little disappointed that cloud had stopped me seeing Venus near the Beehive cluster, when returning from bread shopping last night (3 boys can go through a LOT of bread), I looked up to see a prefect halo around the Moon. Absolutely brilliant and like the ISS pass I saw, makes my point about keeping on looking up).

The family, watching Master Chef in the warmth of the Living Room, were not excited, but MiddleOne did come out and express delight.

This is a great example of a 22 Degree halo. 22 Degree halos are the commonest kind of halo or arc seen in the sky, although they are far less obvious than rainbows, and may easily be missed.

22 Degree halos are formed by refraction of light through hexagonal columar ice crystals as they fall to the ground. The halo will form when there is a uniform distribution of randomly oriented crystals in the cirrostratus cloud through which the Moon (or Sun) is shining. Because random orientation is only possible if the crystals are tumbling as they fall, the crystals need to be quite small. This is different from what I've been calling "Moonbows", there are typically 8 degrees away from the Moon, and are caused by bullet shaped crystals.

The image above was made by stitching together two separate shots, as the halo was too bigh to fit into the camera field of view.

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Tuesday, June 22, 2010

 

The Sky This Week - Thursday June 24 to Thursday July 1

The Full Moon is Saturday June 26, and there will be a good partial eclipse of the Moon. Venus is readily visible in the early evening, heading towards Regulus. Venus, Regulus, Mars and Saturn make an attractive line up. Jupiter is prominent in the morning sky.

Evening sky looking east as seen from Adelaide at 9:08 pm on Saturday July 26, 2010. Click to embiggen.

The Full Moon is Saturday June 26. On the evening of Saturday, June 26 there will be a partial eclipse of the Moon. This will be seen through-out Australia, New Zealand, the pacific, south-east Asia and parts of the Americas.

The timing of the eclipse is in the early evening on a Saturday, so this is a great time to get the family involved in watching. I've made a printable guide for kids with directions (Australia specific), and some activities they can do during the eclipse. Maximum eclipse depth is 9:38 pm eastern states, 9:08 pm central states and 7:38 pm WA, with over half the Moon covered. For more details see here.

The morning sky facing north-east in Australia on Sunday June 27 at 4:30 am local time showing Jupiter. Click to embiggen.

Jupiter is clearly visible in the northern sky as the brightest object in the early morning. Jupiter is now high enough for telescopic observation to be rewarding. Jupiter looks a little different now that one of its bands has disappeared. Jupiter and Uranus are close together and can be seen near each other in a pair of binoculars (spotters map here). There is no evidence of impact scars from the object that crashed into Jupiter on the 3rd of June.





Evening sky looking North-west showing Venus, the Moon, Mars and Regulus at 6:30 pm local time on Sunday June 27. Click to embiggen.

Bright white Venus is readily visible above the western horizon from half an hour after Sunset, (even before) until past the end of twilight (about an hour and a half after sunset). Venus starts the week in Cancer, forming a line with the Regulus, Mars and Saturn. During the week Venus moves towards Regulus as a prelude to some spectacular planetary alignments in July and August. By the end of the week Venus is in the constellation of Leo the Lion.

In the evening Mars can be seen low in the north-western sky. Mars is to the right of Regulus, the bright star in Leo the lion at the beginning of the week and will draw further away from it during the week, coming closer to Saturn. Mars is now only slightly brighter than Regulus, but is distinguishable by its reddish colouring.

Saturn is easily visible in the western evening sky as the bright yellow object between the bright stars Regulus and Spica, just up from Mars. Now is still a very good time for telescopic observation of the ringed world. Saturn is quite high in the sky for the best telescopic views at around 8 pm. Saturn's' rings are opening, and look quite beautiful, even in a small telescope. On the 26th of June, Saturns' Moon Titan cruises just above the planets north pole.

If you don't have a telescope, now is a good time to visit one of your local astronomical societies open nights or the local planetariums.

Printable PDF maps of the Eastern sky at 10 pm ADST, Western sky at 10 pm ADST. For further details and more information on what's up in the sky, see Southern Skywatch. Cloud cover predictions can be found at SkippySky.

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Monday, June 21, 2010

 

Flare Fun

This morning was a very nice Iridium flare low in the northern sky. Unfortunately, I placed the camera so that it was completely hidden by that tree in the lower right hand corner. Idiot.

It fitted the pattern of last night. Last night had some nice unexpected satellite passes. I was out checking the sky to see if it was going to clear up for some observations, and I saw a bright light flashing between the clouds as it passed below the Southern Cross. I had picked up an ISS pass I hadn't realised was happening. Later that night, while watching an amazing Iridium flare that I did know about, a second Iridium flare occurred just below it. Pretty good! Of course, I messed up photographing that too.

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Happy Winter Solstice!

Today, in Australia, it's the Winter Solstice, when the night is longest. Do something solsticy tonight.

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Sunday, June 20, 2010

 

Partial Lunar Eclipse, June 26, 2010

Evening sky looking east as seen from Adelaide at 9:08 pm on Saturday July 26, 2010.

On the evening of Saturday, June 26 there will be a partial eclipse of the Moon. This will be seen through-out Australia, New Zealand, the pacific, south-east Asia and parts of the Americas.

Lunar eclipses occur when the Moon passes through Earth’s shadow. Lunar eclipses don’t occur often, and for Australians this is the best eclipse since the total Lunar eclipse of August 2007.

The timing of the eclipse is in the early evening on a Saturday, so this is a great time to get the family involved in watching. With no school the next day, and the darkest part of the eclipse in early evening, this is a great time for the kids to watch. I've made a printable guide for kids with directions (Australia specific), and some activities they can do during the eclipse. Why not host a Moon-B-Q with friends?

If the sky is clear, this will be a beautiful sight, with the bottom half of the Moon going dark. You don’t need anything special to watch the eclipse, just your eyes.

The Moon rises in the east at roughly 4:45 pm on the east coast, 5:45 pm for the central states and 5:10 pm in Western Australia. It will enter the outer part of Earth’s shadow (the prenumbra) after twilight finishes in the eastern and central states. However, this shadow is faint and will not darken the Moon very much.

The Moon enters the darkest part of the Earth’s shadow (the Umbra on the diagram) at 8:16 pm on the east coast, 7:46 pm for the central states and 6:16 pm in Western Australia. For the eastern and central states the sky is fully dark, but in WA the sky is still in late twilight. Nonetheless everyone should be able to see a visible “chip” on the bottom of the Moon.

Over the next hour you will see the shadow slowly creep over the Moons face until more than half the Moon is covered by the shadow of the Earth (9:38 pm eastern states, 9:08 pm central states and 7:38 pm WA, see simulation above). Even the part of the Moon not covered by Earth’s shadow will be darker than normal.

After this the shadow will withdraw, and the eclipse will be finished by 11:00 pm in the eastern states, 10:30 pm in the central states and 9:00 pm in WA.

Cloud cover predictions can be found at SkippySky. Weather predictions form the BOM.

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Saturday, June 19, 2010

 

Carnival of Space #158 is here.

Carnival of Space #158 is now up at AARTSCOPE. An Australian edition, it features Hayabusa's return, Ikaros's successful deployment of its solar sail, space-based solar power, what to do if your warp drive core is damaged and much much more. Trim your sails and head on over,

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Friday, June 18, 2010

 

How many exoplanets!!!

The first 5 verified planets discovered by Kepler Image Credit NASA.

As everyone knows, I love exoplanets, I marvel at the weird and wonderful worlds our telescopes have revealed, and quite often make Celestia files to share them (or at least their simulations), with the world.

As of 15 June there were 461 extrasolar planets, after CoRoT had revealed a swag of 6 new exoplanets. How was I going to find the time to make Celestia files for all of THEM I mused.

Then on June 16 the Kepler mission announced 706 exoplanet candidates, nearly tripling the number of exoplanets we know about. Thanks Kepler! Well, okay they have only released 312 candidates around 306 stars, with 5 systems with multiple planets, and held back 400 for further follow-up, but nearly doubling our known exoplanet roster in one go doesn't help me much.

Transit light curves observed by Kepler. Image Credit NASA.

Note the "candidates" though. Kepler detects the faint dips in light from a star that occurs when a planet goes in from of it. However, there are other things that can cause transit-like dips (sunspots, other stars in a close binary system etc.).

While the transits have been vetted for consistency, planet-likeness and for most, observed for at least 3 transits, they haven't been followed up by other measurements (eg radial velocity measurements to eliminate double stars systems and so on). These 306 systems that have been released are "low priority" for follow up, and will contain an unknown number of false positives. The 400 that have been held back are "high priority" and are being subject to further study as I type. Scheduling radial velocity and other measurements for 400 stars is no trivial matter, with telescope time being precious, and the 400 held over stars will be released in February 2011.

Figure 2 of Borucki et al., 2010, comparing the sizes of 306 planets found with Kepler against the sizes of planets found by other surveys.

Well, what of the the planets they have found. When we first found exoplanets, back in 1995, they were jaw-dropping hot super-Jupiters screaming around their suns in a handful of days.

This was pretty amazing, as planets that big shouldn't form that close to their parent stars. We soon found lots of other "super Jupiters", because if you are looking at how a star wobbles, or the dip in a stars light, it's easier to find something big orbiting often then it is to find something small orbiting slowly.

For example, if you were looking at the Sun for Jupiter, you would have to watch for 12 years to detect it, and you would need to wait 24 years to confirm your detection. So it is no wonder that our explanet detections up until now have been dominated by massive planets orbiting closer to their suns then Mercury is to ours.

The Kepler candidates change all that. If you look at figure 2 above, you can see that there are relatively few Jupiters (11 times Earth radius [Re] and above), lots of "Neptune-sized" planets (5-10 Re, with 58% of all planets being between 4-5 Earth masses) and a goodly number of Super-Earths (1.5 -4 Re with 12% of all planets being between 1.5-2 Earth masses). It looks as if close "Super Jupiters" are a rarity, and smaller worlds, more like Earth, are to be expected.

Figure 8 of Borucki et al., 2010, comparing the frequency of various planet types against star type.

The vast majority of these worlds have very short orbital periods, the median being 9.7 days (remember, Mercury takes 88 days to go around the Sun). There's an interesting "mini-solar system" around star catalog number 5972334, with a 2 earth-mass planet screaming around its G class sun in 2.4 days, and a Jupiter mass planet moving more sedately around in 15 days.

Of course, given the length of time Kepler looked at any one spot (33.4 days), finding planets with longer orbital periods is hard. You can infer by the length of the transit what the orbital period is, even if the period is longer than the observation time, and there is one Jupiter mass planet with a period of 206 days, similar to Venus, and another Jupiter mass planet listed as having an orbit around 28 years. As it orbits a star 9 times the size of our Sun that makes it rather Jupiter like indeed.

There is another factor making it hard to detect planets with longer, more solar system-like orbits. The further away the planet is from its Sun, the less likely it is for its orbital plane to be sufficiently closely aligned with Earth for a transit to be detected.

Image Credit, NASA.

Kepler is looking back along the Orion spur in an area between Cygnus and Lyra. It's deliberately sampling more "sun-like" stars, focusing on K, G (our Sun is a G2 star) and F type stars. All stars have a fair distribution of planets, although K stars seem to have more super-Earths, this is probably due it being easier to detect small planets against faint stars.

So what does this mean for the detection of Earth-like planets in Earth-like orbits? It's hard to tell, as the issues of sampling mean that planets with orbital periods like Earth (365 days) will be under represented. And of course some of the detections will be false positives, but that shouldn't distort the statistics too much. Kepler sampled 156,000 stas and found planets around 706 of them, while only 0.5% of the stars observed by Kepler were found to have planets, remember that we can only see transits if the systems are nearly edge on to us, and orbits much longer that 50 days will be likely to be significantly underrepresented in systems that are edge on.

Over at Systemic Greg Laughlin did an interesting calculation, he assumed that half of the stars Kepler looked at had planets with orbital periods of 50 days or less, and that the solar systems orientations were random. He found that Kepler should have seen around 1100 stars with planets, not far from what was actually reported. The question of whether Solar System-like systems are rare or common depends on whether the majority of F, G and K stars have planets, so the incidence of Solar System like systems remains uncertain.

The 400 systems held over for further study will be interesting, the question of whether these are are Earth-like worlds will be answered early in 2011.

The paper discussing the Kepler planets is here, the paper discussing the multiple planet systems is here. You can search the Kepler archive and plot transits or download data here. Here is the Science Daily report, the (somewhat disappointing) Kepler Press release, the Dynamics of Cats report is here, the Systemic report is here and Starts with a Bang's musings are here.

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Moon, Mars, Regulus and Cloud

The Moon just before it is covered by cloud, Mars glows redly just to its right, Regulus is to the right and below on the edge of the cloud (click to embiggen to see Regulus clearly).

Well it was too much to hope for clear skies, but there was a brief interlude between cloud and rain where you could see all three objects together. It was pretty nice all in all.

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Thursday, June 17, 2010

 

Mars, Regulus and the Moon, 17 June 2010

Evening sky looking North-west showing Venus, the Moon, Mars and Regulus at 7:30 pm local time on Thursday June 17. Click to embiggen.

The waxing Moon, Mars and Regulus form a nice triangle on the evening of Thursday June 17. Regulus is the brightest star in the constellation of Leo the lion. The apposition should be visible from about an hour after sunset (look north-west of course). Hopefully the weather will be better so people can see it.

Two days later the First Quarter Moon will be near Saturn.

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We Climb the Hill

"We climb the hill, the lollipop rock where we have lunch is at the top. When we get down to the cars we have cakes I made from the big box"

MiddleOne is not the only artist in the family, but until recently he was the most prolific. Now SmallestOne is set to out do him (although MiddleOne and SmallestOne now engage inmutual strry drawing, where they both draw a long complicated story, usually involving robots, spaceships and explosions, on the same piece of paper).

This is SmallestOnes account of our trip to Montacute Conservation Park, as noted here. It is surprisingly accurate.

Today, instead of being in Melbourne for work, I'm at home with SmallestOne. After the medical hi-jinks on Tuesday, on Wednesday evening SmallestOne started an asthma attack. much administration of asthma medications and worried calls to the Bettdeckererschnappender weisle (who is in the back of beyoond for her work), SmallestOne asthma is controlled but I'm not going anywhere. At least he's doing lots of drawing while curled up in front of the heater.

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Wednesday, June 16, 2010

 

Venus and the Moon in Twilight

It should have been a simple check-up for MiddleOne, but they decided he need a lot of tests, so we went scurrying from Pulmonary Assessment to pathology for blood tests to the X-ray department for a chest X-ray. Not much fun for a young lad, but at least he thought seeing his chest bones was cool.

Of course, EldestOne had not turned on his phone, so he didn't know his lift from soccer couldn't make it. In between shuttling between the various pathology tests I had to zip out to phone and try to get someone to pick up SmallestOne from after school care and try and find what the heck happened to EldestOne.

In the end I had to drive down after all the tests were done on MiddleOne and pick up EldestOne who was sitting forlorn near the soccer field. Of course there was a traffic jam as well.

At least the endless waiting in the stationary car allowed be to view Venus next to the crescent Moon, glowing through thin cloud. It was rather beautiful. I didn't get any photos naturally, my mind being otherwise occupied, so I've put up a photo from the day before (click to embiggen)

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Tuesday, June 15, 2010

 

Hayabusa Falls to Earth

Hayabusa falls to Earth, panels from probably the most poignant cartoon about a space-probe since XKCD's spirit cartoon.

Hayabusa, the little spacecraft that could, returned to earth on Sunday June 13. I've been following Hayabusa for some time, so it was a bit disappointing I forgot about its re-entry, but we were off bushwalking so I was distracted. The spacecraft itself was destroyed in the re-entry (and captured in the spectacular video below), but the asteroid sample return capsule was successfully retrieved and appears to be intact.

Whether there is any asteroidal material in the sampler is unknown, the projectile that was supposed to kick up material to be sampled didn't work, but asteroids are fairly dusty things, and the simple impact of the sampler tube make have captured some material. It will be a while before we know.



Emily Lakdawalla has a good rundown of the return (with fantastic images) and further updates. Hat tip to the Bad Astronomer.

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The Sky This Week - Thursday June 17 to Thursday June 24

The First Quarter Moon is Saturday June 19. Mercury is lost in the twilight glow. Venus is readily visible in the early evening. On June 17 the Moon and Mars are close. On June 19 Saturn and the Moon are close. On June 20 Venus is close to the Beehive cluster. Jupiter is prominent in the morning sky. On the 21st Earth is at Winter Solstice.

The morning sky facing north-east in Australia on Sunday June 20 at 4:30 am local time showing Jupiter.

The First Quarter Moon is Saturday June 19.

Mercury is low in the morning sky and by the end of the week it is lost in the twilight.

Jupiter is clearly visible in the north-eastern sky as the brightest object in the early morning. Jupiter is now high enough for telescopic observation to be rewarding. Jupiter looks a little different now that one of its bands has disappeared. Jupiter and Uranus are close together and can be seen near each other in a pair of binoculars (spotters map here).


Evening sky looking North-west showing Venus, the Moon, Mars and Regulus at 7:30 pm local time on Thursday June 17. Click to embiggen.

Bright white Venus is readily visible above the western horizon from half an hour after Sunset, (even before) until past the end of twilight (about an hour and a half after sunset). Venus starts the week in Cancer, forming a line with the bright stars Procyon an Sirius, and moves towards the Beehive cluster during the week. On the 20th and 21st Venus is close to the Beehive cluster, although you will need to observe this event in binoculars to see the Beehive without it being overwhelmed by Venus's light.

In the evening Mars can be seen low in the north-western sky. It has faded a lot, but is still the brightest (and clearly red) object in that part of the sky. Mars is to the right of Regulus, the bright star in Leo the lion at the beginning of the week and will draw away from it during the week. The waxing Moon, Mars and Regulus form a nice triangle on Thursday June 17.

Saturn is easily visible in the western evening sky as the bright yellow object between the bright stars Regulus and Spica, just up from Mars. Now is still a very good time for telescopic observation of the ringed world. Saturn is quite high in the sky for the best telescopic views at around 8 pm. Saturn's' rings are opening, and look quite beautiful, even in a small telescope. On the 18th of June, Saturns' Moon Titan cruises just below the planets south pole.

Earth is a Winter solstice, when the day is shortest, on June 21.

If you don't have a telescope, now is a good time to visit one of your local astronomical societies open nights or the local planetariums.

Printable PDF maps of the Eastern sky at 10 pm ADST, Western sky at 10 pm ADST. For further details and more information on what's up in the sky, see Southern Skywatch. Cloud cover predictions can be found at SkippySky.

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Monday, June 14, 2010

 

Venus near the Moon, June 15, 2010

Evening sky looking North-west showing Venus, the Moon, and bright stars at 6:30 pm local time on Tuesday June 15. Click to embiggen.

The evening of Tuesday 15th is enlivened by the thin crescent Moon being close to Venus. As the view develops as the twlight deepens, you will see the Moonshine begin to glow against the darkening night.

This is a good opportunity to see Venus in daylight. You must be very careful to make sure you don't look directly at the Sun though. Hide the Sun behind a large object such as a wall or building when looking for Venus. The Moon is around 6 handspans from the Sun, but it is rather thin, and will be hard to see. You way want to try about an hour before sunset to give it your best chance to see the Moon. When you find the Moon, Venus will be seen as a bright dot just below.

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The Biggest Obstacle to Astronomy after Clouds

Is Fondue. Chez Reynella headed up into the hills on the long weekend for dinner and bushwalking with friends. Clear skies beckoned, so I packed the portable telescope, the big binoculars and the binocular-tripod adapter.

And used none of them.

I had visions of showing the kids and sadults Saturn and some of the delights of clear skies. But we had a fondue night so we all sat around the fondue pot dipping bread into cheese, then dipping fruit into chocolate (at this stage the burner went berserk, as you can see in the image, made dipping a bit ... interesting). Enjoying food slowly while having good conversation.

In the end, after we had washed up and everyone else had gone to bed, I went out and had a look at sparking night skies. I got a shot of the star clouds of Sagittarius and Scorpius (3 x 10 sec images at 1600 ASA stacked in ImageJ, click to embiggen) and the clusters near the Southern Cross.

In the end, the biggest obstacle to astronomy is good food eaten with joy with good friends.


Souther Cross setting (1 x 10 sec exposure at 1600 ASA, click to embiggen)

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Saturday, June 12, 2010

 

And it Moves! Direct imaging of Beta Pictoris b as it orbits its Sun


Planet orbiting ß Pictoris imaged by the European Southern Observatory (image credit ESO).

Planet detection has come a long way in the past few years, we have detected planets by the minute wobbles their gravity produces in their parent stars, the dimming of their parents sunlight as they pass across them, by gravitational lensing and finally we have been able to directly image a handful of them.

The problem with directly imaging planets is that their reflected light is feeble compared to that of their parent star, and we need to block out the light of the star in some way, either by placing an obstacle in the telescopes light path or using interferometry to cancel out the light of the star. Usually this means we can only see big planets far out from their parent star.

Dust disk around Beta Pictoris, with wobble. Image Credit Hubble.

We have long suspected that ß Pictoris had at least one planet around it. The star is surrounded by a dusty disk, and there were wobbles in the disk that was consistent with a planet plowing through the dust.

Then in 2008 the first image of what appeared to be a planet around ß Pictoris was reported. Now that report has been followed up with a second image that shows the dot that was reported back in 2008 really was a planet.

As seen in the top image (which is a composite of images taken in 2003 and 2009 laid over an image of the dust disk, the image of the star is blanked out) the dot had moved to the other side of the star, if it was just a back ground star that had moved due to the relative motion of ß Pictoris the dot would have been in a completely different location. So the dot is a planet.

And a very interesting planet, roughly 8 times more massive than Jupiter, it orbits roughly where Saturn would be in our solar system. This is important for two reasons. One is that it is at the so called "snow loine" where water is stable as ice. The snowline is thought to separate disk regions where rocky or gaseous/icy planets form. ß Pictoris b is at the right place to have formed by the process called core accretion (how Jupiter and Saturn are thought to have fomed. The other massive planets that have been directly imaged are too far from their parent star to have formed in situ and must have been ejected to their current orbits (such as Fomalhaut B, 115 AU away from its sun compared to the 12 AU of ß Pictoris b).

This also means it is the first time we have been able to see a directly observed planet complete such a large part of its orbit, all the others are too far away to mocve far in the time we we have observed them but in 15-20 years we could see a full orbit of ß Pictoris b (okay, so it's not like the super fats "hot Jupiters" that scream around their suns in days, but this is the most "normal", from a solar system perspective, plant that we have directly imaged).

As well, ß Pictoris b looks to be responsible for the warp in the inner dust disk, but can't be responsible for the warps in the outer disk, there must be more planets still undetected in the system.

Finally, the system is young, only 8–20 million years old, so giant planet formation must be very fast (in geological time that is). Observing ß Pictoris b and the ß Pictoris system will allow us to understand the early evolution of solar systems much better than before. A preprint of the journal article reporting this find is here.

Image of ß Pictoris b generated in Celestia, I have included the orbital elements for ß Pictoris b below. Copy the script and save it to a file called beta-pictoris-b.ssc in the extras folder.

The Bad Astronomer reports other details of ß Pictoris b and tells of his personal involment with the story.




=======8<===========beta-pictoris-b.ssc=============8<====================
"b" "HD 39060" # beta pictoris's HD number

# beta Pictoris b, long suspected to exist now directly imaged
# orbital movement detected 2003-2009


{
Texture "exo-class5.*"
NightTexture "exo-class5night.*"

Color [0.49 0.89 1]
Albedo 0.5

Mass 2542.9 # M.sin(i) = 8 Jupiters
Radius 81009.7 # rough guess
Oblateness 0.01 # guess

#InfoURL "http://exoplanet.eu/planet.php?p1=beta+Pic&p2=b"

EllipticalOrbit {
Period 6000
SemiMajorAxis 12.0
Eccentricity 0.05
Inclination 80 # rough estimate from image, doesn't transit
ArgOfPericenter 37
#MeanAnomaly 166
}

Obliquity 82 # guess, to match inclination
#EquatorAscendingNode 96 # guess, to match ascending node

# likely to be in captured synchronous rotation
}

AltSurface "limit of knowledge" "HD 39060/b"
{
Texture "extrasolar-lok.*"
}


=======8<=====================================8<=====================

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Friday, June 11, 2010

 

Carnival of Space #157 is here.

Carnival of Space #157 is now up at Out of the Cradle. There is lots of news from the 29th annual NSS International Space Development Conference, new on the deployment of the IKAROS space sail, Hyabusa returns, the legacy of Atlantis and much, much more. Buzz on over and have a look.

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Update on the ISS Transit and Occultation

Why yes, they were both clouded out. Did you need to ask?

Actually, for the ISS transit there was a small window of opportunity between the rain and the cloud. I rushed outside to set up, and got involved in an amusing tangle of gear. The transit probably happened while I was trying to unjam the mounting I had jammed in my haste to set up. The the clouds came over again.

And this morning the rain was kind enough to wake me in time for the occultation, should it have been visible behind the clouds.

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Thursday, June 10, 2010

 

Occultation of the Pleiades, Friday June 11, 2010

The morning sky facing north-east in Australia on Friday June 11 at 6:30 am local time showing the Moon and Mercury. the Moon is covering some of the beautiful Pleiades Cluster.

On the morning of Friday June 11 the thin crescent Moon is close to Mercury. At this time the Moon also occults the Pleiades Cluster. This will be difficult to see if you don't have a fairly level, clear eastern horizon.

You will need a pair of binoculars to see this event at its best (telescopes might be a bit hard to bring to bear as the event is close to the horizon).

Times of prominent star disappearances and appearances for major cities are listed here.
However, if you start looking from around 6:00 am local time you should see the crescent Moon starkly outlined against the bulk of the Pleiades and various faint and bright stars passing behind the Moon then being revealed.


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Wednesday, June 09, 2010

 

The ISS passes in front of the Sun in Adelaide.

On Thursday June 10 at 14:51:46 the International Space Station will cross the Sun as seen from Adelaide.

To see this event you will
a) need to practise safe solar projection techniques (see also here and here). NEVER look directly at the sun, or use so called solar filters. Especially with binoculars or telescopes. For a dramatic example of what happens when you look at the sun directly through a telescope; look at this video...nasty isn’t it.

b) be quick, the ISS crosses the Sun in 2 seconds.

So you will need an accurate clock or timer and to be staring intently at the image (or have a fast camera).

Of course, this is in the afternoon during a work day, so watching may not be feasible, but if you can organise to watch please do so.

ISS path at14:50- 14:52 Thursday June 10 as seen from Adelaide.

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Tuesday, June 08, 2010

 

Comet C/2009 R1 McNaught is becoming bright(ish)

C/2009 R1 (McNaught) is becoming brighter, and may soon be (just) visible to the unaided eye ... in the northern hemisphere. It's completely unobservable for us Southern hemisphere types, although we may see it late July. There is a good image at Astronomy Post of the Day, and some lovely images of the comet near NGC 891 by Francois Kugel and Michale Jaeger.

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Monday, June 07, 2010

 

The Sky This Week - Thursday June 10 to Thursday June 17

The New Moon is Saturday June 12. On the morning of June 11 Mercury is near the Moon which occults the Pleiades cluster. Venus is readily visible in the twilight. On June 15 the Moon and Venus are close. Saturn is seen in the evening above the northern horizon near the bright stars Regulus and Spica. Mars is very close to Regulus. Jupiter is prominent in the morning sky.

The morning sky facing north-east in Australia on Friday June 15 at 6:30 am local time showing the Moon and Mercury. the Moon is covering some of the beautiful Pleiades Cluster.

The New Moon is Saturday June 12.

Mercury is in the morning sky and is visible low to the horizon below Jupiter. On Friday June 11 the thin crescent Moon is close to Mercury. At this time the Moon also occults the Pleiades Cluster. This will be difficult to see if you don't have a fairly level, clear eastern horizon, Times of prominent star disappearances and appearances for major cities are listed here.

Jupiter is clearly visible in the north-eastern sky as the brightest object in the early morning. Jupiter is now high enough for telescopic observation to be rewarding. Jupiter looks a little different now that one of its bands has disappeared. Jupiter and Uranus are close together and can be seen near each other in a pair of binoculars (spotters map here).

Evening sky looking North-west showing Venus, the Moon, and bright stars at 6:30 pm local time on Tuesday June 11. Click to embiggen.

Bright white Venus is readily visible above the western horizon from half an hour after Sunset, (even before) until past the end of twilight (about an hour and a half after sunset). Venus starts the week close to the bright star Pollux in Gemini, and moves away towards the Beehive cluster during the week. On the 15th the thin crescent Moon is close to Venus. This is a good opportunity to see Venus in daylight. You must be very careful to make sure you don't look directly at the Sun though.

In the evening Mars can be seen low in the north-western sky. It has faded a lot, but is still the brightest (and clearly red) object in that part of the sky. Mars will be just to the right of Regulus, the bright star in Leo the lion at the beginning of the week and will draw away from it during the week. The waxing Moon, Mars and Regulus form a nice triangle on Thursday June 17.

Saturn is rising before Sunset and is easily visible in the evening sky as the bright yellow object between the bright stars Regulus and Spica, just up from Mars. Now is still a very good time for telescopic observation of the ringed world. Saturn is quite high in the sky for the best telescopic views at around 8 pm. Saturn's' rings are opening, and look quite beautiful, even in a small telescope. On the 10th of June, Saturns' Moon Titan cruises just above the planets north pole.

If you don't have a telescope, now is a good time to visit one of your local astronomical societies open nights or the local planetariums.

Printable PDF maps of the Eastern sky at 10 pm ADST, Western sky at 10 pm ADST. For further details and more information on what's up in the sky, see Southern Skywatch. Cloud cover predictions can be found at SkippySky.

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Animated Southern Cross

Inspired by this animation of the Milky Way over Cotopaxi, I've tried my hand at a short experimental timelapse video. Now I've done lots of animations before but all short ones.I wanted to see if , in principle, I could do something like Cotopaxi.

The video below is assembled from from 12 images, taken every 10 minutes over two hours. Because its from my back yard it's not really spectacular, in particular, there's no interesting foreground like Cotopaxi to make it stunning, but it odes show that in principle I can assmbel such animations. Next time I'm out camping I'll see hat I can make.

video

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Sunday, June 06, 2010

 

Mars and Regulus are close!

If you have been watching the evening skies you will have seen Mars and Regulus drawing closer together. They look quite a sight now close together in the evening (the two brightest objects in the lower left hand side of the image, Saturn is top right, click to embiggen).

Mars is at its closest to Regulus on Monday June 7. The Morning of Monday June 7 is when Jupiter is close to the waning crescent Moon, and Jupiter and Uranus are at their closest on the 8th.

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International Space Station, Canopus and Crux



Seeing as the ISS has been a bit of theme here recently (see here and here), thought I'd post more ISS pictures. The left hand image is an overlay of two separate images showing the ISS near Sirius (right hand bright star) and Canopus (left hand bright star). The right hand image shows the ISS below the Southern Cross and the Pointers at the other end of its pass. Click on any image to embiggen. The ISS will soon pass from the eveing skies for a while, so these are probably the last images I will get for a couple of weeks.

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Saturday, June 05, 2010

 

Using The Gimp for Astrophotography (Part 3)

In previous posts (part 1, part 2) we have covered using the free image manipulation program GIMP for stacking images to improve brightness and resolution and creating animations. This installment handles how to create mosaics. We create mosaics for many reasons, making wide field views of the sky for example. I mostly use it to make mosaics of the Moon from images shot through my webcam on one or other of my telescopes.

The process is similar to that before, using layers and the “difference” function in the layer pane. First of course you create the images that you want to combine into a mosaic. For lunar mosaics I take images with about a 1/3rd overlap to ensure adequate registration without distortion near the edges of the image. Then in the GIMP, choose File | New to create a canvas to assemble your images on. The size of the blank file will depend on the size of your images (eg since my individual images from my webcam are often 640x 480, I often create a new file 3000x2000 to assemble the mosaic.




Left image: two images stacked but not registered, with the layer mode set to "difference". Right Image: When the images are aligned the overlap area is dark (click to embiggen for more detail).

Once you have a new file as your bank canvas, open the first image as a layer (File | Open as layers in the image window menu), where you place that image depends on where the other images fit. For lunar mosaics of the waning Moon I often place it in the top left hand corner. The open your second frame as a layer. Roughly align the overlapping sections, then in the Layers panel (File | Dialogs | Layers if it is not coming up automatically), in the Mode drop down box choose “Difference”. The overlapping sections now go dark and light. Use the arrow keys to move the sections over each other until you get maximum darkness – this gives you maximum alignment. Now go back to the mode box an choose “Normal” and your first part of the aligned mosaic is finished.

Overlaid images in Normal mode after alignment, click to embiggen.

Or not quite. My webcam images have a light border around them which messes up the smoothness of the mosaic. So I use the selection tool to select the part of the image that doesn’t have the lightness, then the menu item Layer | Crop to selection (important! use the Layer NOT the image menu item, if you use the image you will crop away everything) to get rid of the border.

Repeat the process until you have added all your images, and you are almost finished. When saving, if you save in JPEG, GIF or PNG format you will be asked to merge of flatten images, chose Merge (if you save as a GIMP XCF file you don’t need to worry about it, but if you want to show the image on a web page you need to save as one of the standard formats).

Now, rejoice in you completed Mosaic (click to embiggen).

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