Observation of Venus Transit

Report by Phil Beastall.

One of the good things about teaching is that you can get young people involved in interesting events. So it was that somewhat before 6 a.m. on Tuesday 8th June 2004, I arrived at Guildford County School's sports field at Urnfield near One Tree Hill. Getting students to get up early is a slightly different matter and it was my wife and two sons who joined me at 6 o'clock ready for the start of the transit. This was ten minutes after my fancy clock that receives a signal from the atomic clock transmitter decided to pack up - the transit timings were all done with a ten pound wrist watch from a market stall. Fortunately I had calibrated this - I had noted the difference between it and the correct time the night before, simple as that. This was a theme for the observation, some systems failing, highlighting the prudence of backup - belt, braces and a piece of string. The next thing to play up was the VCR. I missed the first few minutes due to condensation on the heads - the high tech cleaning method consisted of my younger son blowing gently through the front slot, appropriate technology to the rescue. In fact the previous week I had been testing the drive on my 100mm Maksutov and had used a 6v power supply. There was some odd behaviour and no driving. It turned out that the regulator was faulty and the power supply was giving too high a voltage. Unfortunately the PICs used in these controllers are very sensitive, so if anyone has a spare RA controller for an EQ2 mount that they are willing to part with (or loan so we can read the code).... let me know.

The external ingress timing was difficult to the point that I discarded the result, I realised that it must have happened five to ten seconds beforehand. To obtain a decent estimate of the AU would require much greater precision than this. Due to the previous transit of Venus being 122 years previously, I now had as much experience of this as anyone else in the world! The internal ingress was difficult but this was done with a much greater confidence, looking for a circle mentally extended through the stalk of darkness of the black drop to be touching the inner edge of the Sun's circumference. There are two approaches to this.

The black drop

The folks at Royal Holloway College have written a very sophisticated piece of software to analyse their video frames - it is very impressive. I took the simple approach - processing by natural computer - and the result was almost to the same accuracy. Mind you, their software approach is very impressive. Those who did this exercise themselves will share my admiration for those who obtained accurate results by eye and clock - armed with a quartz movement and having the benefit of knowing roughly when to expect the event, it was difficult enough. I was satisfied that the internal ingress timing was as good as it could be. I compared this with Fred Espenak's prediction for London and found that we were a couple of seconds after that - a very good sign.

Just as the black drop effect finished, the first of my students arrived (sorry, but not wise to mention names on the web - a sad reflection of the times that we live in). He set up my 76mm refractor with projection screen, and took charge of it for the duration. He plotted timed points of the track of Venus across the face of the Sun.

By the time that my colleague Barry Hobbs arrived at 7 a.m. it felt as if it had been a long day already, and by now we were video-recording the slow progress of Venus' silhouette across the Sun. He had to get to school and so did my family, but more students were beginning to arrive and took over control of the telescope with the video camera attached. As I had burned out the controller, this was now student powered!

A second 100mm Maksutov, albeit on a standard camera tripod, had a 35mm SLR attached (Pentax Spotmatic - a wonderful old mechanic camera, ideal for the job) was used for conventional photographs. Both this and the video telescope were protected by solar filters, one mylar from Beacon Hill and the other Baader film. The open apertures were around a centimetre or so as I found that the full aperture Baader filter admitted far too much light. Exposures were bracketed, but all may be incorporated in an animated sequence at some stage in the near future.

The next glitch occurred after three hours, yes the length of a video tape. I had two three hour tapes, but the second one was recognised as a two hour tape by the machine. This promptly rewound after two hours. One of my students made the very sensible suggestion of going over the "boring hour in the middle" with the last hour of the transit.

The "boring two hours in the middle"

After several hours of watching the black pea cross the luminous football, things became hectic at about noon. Naturally we were very keen to get good footage and stills of the black drop effect as well as recording the times of internal and external egress. We had a lot to do.

As the telescope with which I had times internal ingress was now being used for the SLR the timings for egress were done by team members clustered around the TV monitor and projection screen. We ensured as accurate a result as possible by taking a consensus of when we had reached a timed event, although I did have to make sure that one maverick opinion did not lead us astray. We did our best and were quite confident in the results. When I compared the timings with the London predictions I knew that we must be very close.

Dashing back to school via home to stow equipment, the next stop was teaching a year nine class. They were a little surprised to see their exhausted and very hot teacher rush in. I have to admit that with the early start and excitement I had forgotten to apply sun cream and had gone rather pink. They were much more interested in what I had been up to than in the planned lesson.

The next dash was down to Jessops to have the film processed. The young lady who dealt with it had been involved in a time-lapse movie of the transit herself, and so I felt very confident in handing my film to her. I paced around for an hour and have to say that I was very pleased with the result. Then back to school to submit the data.

The University of Central Lancashire were coordinating the effort, and I duly submitted our latitude and longitude plus the timings to their web calculator. Yes, I know it sounds like cheating but the mathematics are a bit hairy if you want to calculate the AU based on timings from one place. This works because during the transit, the earth both moves around its orbit a little and rotates on its axis. This gives you a base line from which to triangulate. I was duly informed that our timings yielded a value for the AU of 151,000,000 km which is 0.9% away from the accepted value. I knew that we must have done quite well on the timings.

The following day I was given a message to contact one of the people in the Physics Department at the University of Central Lancashire because we and another school down in Dorchester had produced the most accurate timings of the 500 or so that had submitted data. Their value was 148,000,000 km which is only 1.1% from the accepted value. So if we take an average, the two school teams between them were pretty well on target for accuracy. As you can imagine, we were well pleased with our result and how it compared with those of others.

The experience of observing the transit was a very positive one. It had elements of scientific measurement, hard work and preparation, excitement and elation. It was a shared endeavour that resulted in success.