Venus in Transit

Doug Daniels (Astronomical Secretary)

Because the orbit of Venus lies between the Earth and the Sun, every so often, as the planets rotate about the Sun, all three bodies will find themselves ‘in line’. However, the orbits of the Earth and Venus are not in the same plane, Venus’ orbit is tilted by about 3 degrees with respect to the plane of the Earth’s orbit. This slight tilt ensures that on most occasions when Venus passes between us and the Sun, it will transit either above or below the solar disk and it will be invisible, lost in the solar glare. However, if Venus happens to cross the Sun at the point where the orbits of the Earth and Venus intersect, Venus will be seen to transit the solar disk. On such occasions, Venus is seen as a jet black spot crossing the bright disk of the Sun. Due to the complexity of celestial mechanics, this condition occurs infrequently. Transits of Venus take place in ‘pairs’ separated by about 8 years at intervals of about 125 years. Just like total solar eclipses, not all transits will be seen from convenient locations on the Earth’s surface. On June 8th 2004 all these rare conditions were met and Venus transited the Sun and moreover, the whole event from ingress to egress was visible from Hampstead.

In the past, transits of Venus were important because if they were observed from widely separated locations on the Earths surface, and accurate timings were taken, they provided a convenient method of determining the Earth-Sun distance - the Astronomical Unit (AU). Knowing this measurement accurately is vital to determine the distances of all the other planets in the solar System. Today, of course, we have other methods at our disposal to determine this distance with considerable accuracy, but the transit of Venus was going to be of interest for other reasons. Since the last transit occurred in 1882, seventeen years before our Society was formed, no one living today has ever witnessed such an event.

The last transit occurred at a time when photography was in its infancy, so we really had no accurate record of what would be seen. For example, in 1882 it was reported that it was impossible to obtain accurate timings due to a phenomenon known as the ‘black drop’ effect. As the black disk of Venus touched the inner edge of the Sun’s bright limb, a distortion took place, drawing a column of black between the limb of the Sun and the disk of Venus. This effect was shown in many of the drawings made from visual observations at the time. In addition, no one has ever witnessed a transit of Venus using an Hydrogen Alpha filter. Could the silhouette of Venus actually be seen against the Solar Corona or perhaps against a Solar Prominence, just before it touched the solar disk? Would we be able to see Venus’ atmosphere illuminated by the Sun as it touched the solar limb? We were about to witness a very rare astronomical event, an event to which modern imaging technology had never before been applied, so we would just have to wait and see.

The astronomy section began to prepare early. After our experience with last year’s transit of Mercury and anticipating that the ‘mass media’ would soon become aware of the forthcoming event and whip up public interest, we anticipated that we would be in for a busy time! Simon Lang produced a useful information sheet and some advance publicity appeared in the Hampstead and Highgate Gazette two weeks before the event. Simon and John Hayden both volunteered to open the observatory shortly after sunrise as the transit was due to commence at about 05.20 hrs.UT. On the preceding the day, Simon, Ron Smith and Gordon Harding put in an enormous effort and a considerable number of man hours to prepare the Observatory for the anticipated hordes of visitors. Simon worked all through the night to erect a series of sun screens to provide shade for the visitors. We sincerely hoped that these would be required, for although the weather on the preceding day was fine and clear, there was no guarantee that the same conditions would prevail on the day of the transit. We still retained the vivid memories of the ill fated total eclipse expedition to Redruth/Cornwall in 1999!

The sunscreens were adorned with information sheets which Simon had produced on the subject of the Sun, Venus and transits in general and he was still working frantically when I arrived at 5.00 am, greatly relieved to see the sun rising in a clear blue sky. It looked as if we would by lucky and that we would see the transit under almost perfect conditions. My only concern was with the contrails from numerous passing aircraft which could impair the definition, as had happened to me last year during the transit of Mercury.

We set about preparing the 6-inch Cooke and fitting it with its projection screen, greatly relieved to find that at low declination the screen just cleared the dome rail. We were also pleased to discover that the trees to the east would not cause a problem at ingress. We then set up the Wildey 6-inch telescope outside the main dome together with my own 6-inch Helios refractor. This was to be the first ‘outing’ for the Wildey telescope, the last instrument made by the late Henry Wildey. This alt-azimuth mounted, short focus 6-inch refractor was acquired from Henry’s family after his death at the great age of 90 last October. Both the Wildey and the Helios instruments were equipped with safe objective solar filters made by Terry Pearce. These consisted of optical glass vacuum coated with layers of aluminium and titanium. Terry began experimenting with these filters well before the transit of Mercury last year, so we knew they would be safe and would allow members of the public to see the transit in all its glory with the jet black disk of Venus passing over the granulated solar photosphere. In addition to the Wildey and the Helios, Simon also bought along his 4-inch refractor, also fitted with a Wildey object glass and a Pearce filter, and Jacquey Oppenheimer came well equipped with her 8-inch Meade Schmidt Cassegrain, fitted with a ‘1000 Oaks’ solar filter. As it transpired, all these instruments were put to very good use as we were at times almost overwhelmed by the sheer number of visitors taking full advantage of the near perfect weather conditions to observe this historic transit.

Visitors began to arrive early to witness the ingress of the planet and observe the famous ‘black drop’ effect which to me did not seem at all pronounced. At high magnification the jet black silhouette of Venus could be seen clearly touching the inner edge of the sun’s limb. Every so often, as the atmosphere rippled the limb, the image of Venus distorted to produce a tiny dark connection to the limb. These effects were clearly due to atmospheric turbulence but at no time did they resemble the drawings made during the last transit of 1882. One can only conclude that the Earth’s atmosphere was more troubled during that event, or that telescope optics had vastly improved in the intervening period.

At first contact, just as Venus took a bite out of the Sun’s limb, we looked to see if any light was refracted by Venus’ dense atmosphere. Nothing of the sort was observed - just a jet black silhouette, but we were all surprised at the size of it compared to image remembered from last year’s transit of the diminutive Mercury.

Many parents brought their offspring along on their way to school and throughout the morning many classes of school children accompanied by their teachers came to observe this rare once in a lifetime event. We estimated that in all about 200 children observed it together with about 300 adults.

I had intended to make timings but the sheer volume of visitors kept me constantly occupied at the 6-inch Helios. I did at least manage to take a few digicam images around the times of ingress and egress and these were swiftly posted on the website on the following day thanks to Julie Atkinson. The 6-inch Cooke was used to project the image and John Tennant did sterling work within the dome demonstrating continuously to a packed and at times overheated audience. This event also gave us our first opportunity to put the Wildey telescope through its paces, ably manned throughout the session by Gordon Harding.

The large crowds were also very generous with donations to the Society encouraged by Michael Wynne and later Ron Smith, to part with a total exceeding £500 towards the upkeep of the observatory. When Michael asked me if we had a bucket, my first thought was that he was intending to rig up emergency toilet facilities, facilities sorely lacking in the vicinity of the observatory, but no, it was to be used to collect the money! It is entirely due to him that we collected so much!

The whole event, which lasted the best part of 7 hours was a total success and certainly raised the profile of the Society particularly to local schools and their science teachers. We thank all those members who came along to assist on a long, hot, exhausting but exciting day. Our special thanks must go to Simon Lang who cheerfully took on the ‘lions share’ of the hard physical work and organisation which made this historic astronomical event an unmitigated success and provived an exciting climax to a very eventful session at the observatory.

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Last updated   28-Jan-2018 contact