The Transit of Venus 6th June, 2012
Joe Cali & Geoff Sims
Nyngan, NSW   Latitude   31o33'28.00" S    Longitude   147o10'55.74"E

This report is composed of four sections
1.    Main report page for Joe & Geoff    
(this page)
2.    Gallery of our friends images
3.    Animation page (  big file  3.7MB)  & link to Geoff's video file
4.    Explanation of why the path Venus appeared to be a curve rather than a straight line as shown on TV

Joe Cali                                                                                                         Geoff Sims

Geoff and I made a 550km dash to Nyngan. Geoff drove from Sydney and I from Canberra respectively to get out from under threatening cloud.
Nyngan was the closest place that was predicted to receive mostly clear sunny skies. Luckily we both made the trip without any car problems or
parts issues. Being a parts geek I was ready for anything on the trip.

We arrived after sunset Tuesday night. Geoff encountered many traffic problems and road works so even though we both drove 550km and
through rain, Geoff's road trip took 10hrs while I had a clear run and mine took a little over six. I left many hours after Geoff and by chance
we both arrived one hour apart. Geoff arrived at sunset, I arrived an hour later. 

We had driven through thick cloudy skies all day but it was clearing rapidly as we arrived. It cleared early evening then clouded over again.

We prepared our gear but left it packed & ready for another dash in the morning if necessary.  Geoff went to bed early after his long drive, I
stayed up working on equipment and watching the weather.  By midnight, I observed that a predicted complete cloud clearance had come through
right on schedule and I thought to myself that the models are working and we were probably safe.  As I prepared to go to bed, calls from BBC
radio in London started coming in requesting interviews.  They asked at 2am if I could do an interview in the next hour for their drivetime
program. I agreed and stayed up only to be bumped off in favour of someone else.  I finally went to bed at 3am.  At 5:20 the alarm went off.  
Rise and shine!

The sky was still clear and it was crisp outside at perhaps 3oC with the Moon setting in the southwest.  

Getting our first look at the site in the light we realized that the tall river gums were going to be an issue.  

We elected to set up right next to our cabin as this gave us the best view of 1st/2nd contact then after an hour, move our gear once or maybe twice to dodge trees during the day as the Sun made it's low altitude semi-diurnal sweep across the sky.  

As it turned out, Canberra did clear up but Sydney saw only glimpses of the transit between clouds.  Even though Canberra cleared, I don't mind because Nyngan at only 120m altitude was pleasantly warm.  We were in tee shirts after the first hour.  My friend Bill Hall gave me a call from Canberra and was complaining about the cold weather.  

We set up two equatorial mounts holding 3 optical tube assemblies and one camcorder.

Mount 1
My lightweight eclipse single arm fork mount holding an Orion 4" Maksutov f12.5.  Geoff Sims attached his Canon 5D mkIII to this and used it for his imaging sequence.  

Mount 2
equatorial mount holding :
ED80 f7.5  EFL 600mm with switchable camera and eyepiece for white light observation & photography
Coronado PST for H alpha observations. (PST :  Personal Solar Telescope  a 40mm f10 refractor with integrated narrowband <0.5 Angstrom filter)

The mount also has a William Optics 70mm APO attached but on this occasion, it was just there for counterbalancing.
Ingress was successfully observed with the Sun at just 12o altitude.  The seeing wasn't at all stable. Even though the morning was cold, the Sun was heating things up fast.  You can see this in the images.  

Animation of the black drop at transit ingress.  Sun altitude, only 12o. The atmosphere was "boiling."

What is the Black Drop Effect?

A more detailed explanation of the black drop effect
by Jay Pasachoff can be found at   http://www.transitofvenus.org/faq/298-what-causes-the-black-drop-effect

I've summaraized Pasachoff's explanation here :

The black drop effect is the apparent smearing or extension of the edge of Venus against the edge of the sun during a transit of Venus.  It is caused by a combination of two effects, the blurriness of the optics used and the fact that the Sun has no defined edge. The Sun's gaseous atmosphere cools as you move away from the sun and hence gets darker but has no defined cutoff. At the extreme edge--and it turns out to be in that last little angle over which we see the black-drop effect--the Sun's gas is relatively dark.  Combined with the inherent blurriness of the telescope, that darkness seems to extend from Venus's dark silhouette out to past the solar edge.  And that is what we call the black-drop effect

The above animated sequence begins at 22:38:40 UT and runs at uneven intervals for 3m23s. the reason the intervals were uneven was that I also wanted to enjoy visual observations of the black drop and so I happily flicked back and forth between visual and photographic. Hence the uneven time intervals but one very satisfied observer.

Following the detachment of the black drop, things slowed right down.  If, to use olympic analogies, a solar eclipse is the 100m sprint, a transit of Venus must be the marathon.  It is a long time to maintain a continuous if intermittent level of activity aspecially when it is, to be honest, just a little black dot on the Sun.  Small black dot or no, it did keep us engaged for the full 6.5 hours.  

At one point a shearing gang came over to check out what we were doing.  They stayed talking to us and looking through the telescopes for a good hour.  

Geoff Sims snapped this picture of the transit in hydrogen alpha light through my PST with his point and shoot not long after second contact.

After hours of observing in the warm sunlight, egress finally approached.  So did some scattered cloud.  After observing the black drop in the morning with the Sun so low to the horizon,  I was looking forward to contrasting the observation with the black drop at egress with the sun at twice the altitude.

BBC in London called me at 4:20 UT for their live cross to run from 4:25-4:30 UT - black drop time.  Wired up with a hands free mobile I was ready for them and to give them a running commentary of the black drop.  However cloud intervened and we didn't see the black drop.

The cloud cleared just after the black drop and contact was established.  We then had a clear view through most of the egress until 2 mins before 4th contact when another small cloud blocked our view of the final moments.  If you look at Geoff's composite below you see egress black drop starting to form then the next image is many minutes later after the cloud cleared and the egress is well under way.

The single most interesting visual observation I made was that during egress the apparent progress of the transit looked different in H alpha and white light.  This is probably due to the visibility of the chromosphere, the egress looked less complete through H alpha than white light. I will verify this observation at the next Venus  transit.

Here is a s
tacked composite image.  Once again they are not at equal time intervals.  

Geoff took this image near maximum transit.

Geoff took this unfiltered image during one of the denser cloud passes.

As soon as the transit was finished, Geoff left for Sydney. As he drove east, he soon drove straight into the cloud mass that we'd driven to Nyngan to avoid.

I stayed behind for some touring in the region. Next day I travelled to Bourke to photograph birds on the Darling River.

I also tried out the astrotrace function on my DLSR camera.  The picture below is taken on a fixed tripod, the DSLR guides itself.  First you connect a GPS 
to the camera then calibrate the camera GPS so that the GPS knows where the camera is pointing.  Then the camera calculates what the sky motion will be and moves the camera sensor to cancel the motion of stars due to the Earth's rotation.  

This shot is a 5 minute exposure camera on tripod only.  17mm f4.5 ISO 800.

This image is 1100 pixels wide, approximately 20% of the original 4950 px image.  At 100% on the full size image, you can see some drift but it isn't objectionable and only just visible at this scale. Also this exposure was 5 minutes -  the maximum allowed length of exposure at 17mm.  I could do a 2.5 minute exposure at ISO 1600 and get a shorter trails with only a little more noise.

The Wandering path of Venus across the face of the Sun

More than a year ago I noticed that the locus of the transit across the Sun as seen from an alt-az frame of reference was interesting and variable depending on your observing location.  
Using Steven Van Roode's old software, I prepared the graphics below and many more and made this flash show I call the transit of Venus world tour.

Did you see Venus track make a strange curved path across the Sun?  

Following the transit, I had numerous enquiries from people who noticed this and could not understand it.
Of the many strange shapes, Cairns was located in such a position that the planet barely changed its PA relative to the terrestrial horizon.

 A teacher and his students from Smithfield High just north of Cairns were among the people who observed this and wrote to ask me about it.  I have prepared this explanation for them. The explanation I've written is specific to Cairns but the principle can be applied to resolve or explain any path.  

Steven Van Roode's old software tool that produces the above output is no longer available but he has a new interactive tool at

Here are a few examples from the tour :

s and e stand for start and end respectively.  If the dot is grey, it means the Sun is below the horizon when Venus is in that position.  
At the margins, places near the edge of visibility, Venus tracks more or less tangentially across the Sun maybe with a slight curve.  These plots show the path of Venus across the Sun when the terrestrial horizon .

Joe Cali