The Transit of Venus –
by David Le Conte
The first transit of Venus across the disc of the Sun since 1882 was witnessed by a group of Astronomy Section members at the Astronomical Observatory. Preparations started at 04.30 Universal Time (UT), setting up the 6-inch coelostat about one metre from its normal position, in order to catch the early morning Sun, which had risen at 04.05 UT.
Our prime objective was to record the times of the four contacts, when the disc of Venus touched the limb of the disc of the Sun. We had pre-registered the details and locations of all instruments with the European Southern Observatory, which was coordinating observations world-wide, and calculating values of the astronomical unit from the data. The unit is, of course, very well known, and this was, therefore, an academic exercise designed to gain an insight into the methods used in the 18th and 19th centuries, when such transits were regarded as providing important opportunities for accurate determination of the scale of the solar system.
A further objective was to record the event photographically. I attempted digital photography of the first contact with the Takahashi, not very successfully, but did obtain good images of the coelostat image.
Conditions were perfect for the start of the transit, at We carefully observed and recorded the first
and second contacts, and then carried out general observations while waiting
the six hours until the last two contacts.
However, thick fog rolled in at about 07.45 UT, and lasted the rest of
the morning, blocking out the Sun. The
Meteorological Observatory at the Airport advised that this was general, not
localised, and that there was likely to be a cloud layer above the fog. We telephoned several locations around the
Despite the lack
of visibility we were able to show a constant stream of visitors our images,
and computer simulations followed the course of the transit in real time. The
The results of our observations were as follows. All times are in UT. No times were recorded for the Celestron, and I did not record the first contact time with the Takahashi, using it for photography instead.
The predicted contact times 2 were:
Contact h 20m 00s. Contact h 39m 38s.
Contact h 03m 40s. Contact h 23m 17s.
The accepted value 3 of the astronomical unit (AU) is 149,597,870 km.
Telescope Ceolostat Meade Takahashi
Observer Harris Spicer Le Conte
Contact h 21m 52s 05h 21m 30s - (photography)
AU (km) 150,458,302 150,281,206 -
Error (%) + 0.575 + 0.457 -
Contact h 39m 08s 05h 38m 29s 05h 39m 10s
AU (km) 149,260,158 148,945,078 149,276,294
Error (%) - 0.226 - 0.436 - 0.215
AU (km) 149,859,210 149,613,142 149,276,294
Error (%) + 0.175 + 0.010 - 0.215
Contact 3: - - -
Contact 4: - - -
Average of three independent observers: 149,582,882 km
Difference between observed value and accepted value: 14,988 km
Error - 0.01%
The final result was remarkably accurate, which I attributed to the relatively large positive and negative errors fortuitously cancelling each other out.
We noted the considerable differences in recorded times between observers using different instruments: 22 seconds for the first contact, and up to 41 seconds for the second contact. Even those observing the contact on the projected coelostat image interpreted what they saw differently. As expected, the first contact was particularly difficult to record, as, of course, Venus was not visible until its black disc started appearing against the bright solar limb. But even the second contact was problematic.
The differences were not, we believe, because of the infamous ‘black drop’ effect, which, in the 18th and 19th centuries, was blamed for observational inaccuracies. Indeed, there was very little evidence of a black drop (an observation confirmed by other reports published on the Internet). Rather, it was simply because it was found quite difficult to judge when the contact actually occurred. I found myself changing my mind during the event as to when to record it as having happened. It is not surprising, therefore, that the calculations of the astronomical unit showed considerable variations.
The second of the
current pair of transits, on
This report was originally published in the Transactions of La Société Guernesiaise (Vol. XXV, Part IV, pages 600-2).
visitors since 25 May 2009.