For 122 years, every time anyone mentioned a transit of Venus. they alwasy added, “last seen in 1882”. That ended on June 8, 2004. On htat day at 7:20 a.m. Local Summer Time (LST), Earth’s closest planetary neighbour began to cut a diagonal path across the face of the Sun. Appearing as a black disk 32 times smaller than the Sun, the transit lasted just over six hours. It ended at 1:24 p.m. LST.
The Astronomical Society obtained special permission to hold its public activity at the Saluting Battery in the Upper Barrakka Gardens, Valletta. By 7:00 a.m., 20 minutes before first contact – the initial meeting of Venus and the Sun – all equipment was poised and readyL the Society’s 17.8cm Maksutov-Caassegrain telescope; Alex Pace’s homemade 25.4 cm Newtonian reflector and 50-mm refractor; Tony Tanti’s 80-mm refractor; Umberto Mule Stagno’s 15 X 80 binoculars; cameras with telephoto lenses. Vincent Zammit’s 80-mm refractor was connected to di-ve.com’s webiste via a webcam and laptop computer. The image was also projected onto a large monitor for the convenience of visitors. Most instruments had visual solar filters secuely mounted to their front lens cells. Others were used to project the Sun’s image onto a white screen.
As the sun rose beyond Ricasoli point, the sky conditions were not good. Low misty clouds were moving inland form the southeast and, as first contact approaced, a veil of high and middle clouds covered the Sun. The ingress was observed with difficulty through clouds, but as the transit proceeded, the sky conditions improved and the day turned into a sunny one.
Visually, Venus resembled a large. perfectly round sunspot. It was seen easily through the telescopes at all magnifications. Venus also was easy to spot with the naked eye through the special solar glasses that were available from the Society.
The burning question of the transit observation surround the enigmatic ‘black drop effect’ – which plagued astornomers’ efforts to time the transit in centures past. Som epeople reported seeing it, while others did not. Did it happen at all?
Joseph Caruana of Ghajnsielem, Gozo, failed to detect the black drop at both ingress and egress. he observed with a NexStar 8i (20.3 cm f/10 Schmidt Cassegrain) telesopce. Unfortunately, Caruana experienced equipment problems during the beginning and end of the transit.
Images taken by Martin Galea De Giovanni form the Upper Barrakka with a digital camera held at Umberto Mule’ Stagno’s 15 X 80 binoculars show distinct black drops at ingress and egress.
Tony Tanti watched the transit with an 80-mm refractor and reported seeing the black drop at egress but not at ingress. At ingress. under poor seeing conditions, the black drop was evident. At egress the seeing was much better, but he did not see the black drop. The exit of the disk was very sharp.
A report by David Shiga posted in Sky and Telescope’s website corroborates our observations. According to Shiga, most observers didn’t report seeing a black drop and of those who did, most saw something much less pronounced than the the effect observed in the past – so much less pronounced that they hesitated to call it a black drop at all. Opinions ranged as to why there is a discrepancy. The leading theory cited today’s better telescope quality. Unusually good atmospheric seeing wsa also suggested.
Perhaps the most interesting observation was reported by Joseph Caruana. Durin gthe moments when Venus was partly inside and partly outside of the solar limb, the planet’s disk could be made out all around its circumference showing a “discontinuous thin white border around the circumference which gave that black dot a somewhat three dimensional aspect…” This phenomenon is known as the ‘ring of light’ and was observed by Henry Chamberlain Russell during the transit of 1874.
The four contacts ov Venus with the Sun’s limb were timed by Alexei Pace and Tony Tanti. The measurements were submitted of he European Southenr Observatory’s Venus Transit 2004 Project which tried to repeat the classical transit-based experiments in order to calculate the value of the Astronomical Unit (AU) from ingress and egress timings at different latitudes. Our timings resulted in an average AU of 149,672,076 km, corresponding to a solar parallax of 8.7898″. The average error was 0.050%.
The global results of the Venus Transit 2004 Project (as of June 28) are as follows:
The calculated AU turned out 149.631,155 km, compared to the official value of 149,597,870 km. The results showed a dispersion of +1-1,698,611 km and an absolute difference of 33,285 km (0.022%). 1,365 observers (mostly from Europe submitted a total of 3.932 contact timings.
The last time a Venus transit would have been observable in its entirety from the Maltese Islands was in 1040! On that occasion, long before the invention of the telescope, the phenomenon went unnoticed. The next transit, on June 6, 2012 will be partly visible in Malta at sunrise.