The Transit of Venus    8 June 2004

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.


Peter Langford manned the 6-inch coelostat, with Jessica Harris recording the observations with it, and Frank Dowding taking videos of the projected image.  Colin Spicer observed with the 16-inch Meade, Geoff Falla with the 11-inch Celestron, and I used the 5-inch Takahashi refractor.  All telescopes were, of course, fitted with proper solar filters, and all gave good images.  A visitor, Sylvie Browne, who had travelled from Albany, New York, having read about the transit on the Section’s web site 1, took on the task of calling out the time, second by second, at the crucial contact moments, using a radio-controlled clock.


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 05:20 UT.  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 Island, but were unable to find anywhere where the Sun could be seen, so we decided to stay at the Observatory, in the hope that the situation would improve, and to provide hospitality and information to the curious.


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 Guernsey Press, Channel Television and BBC Radio Guernsey conducted interviews with us, and we were joined by some 50 invited children from La Houguette Primary School, who were rewarded with certificates which had thoughtfully been prepared by Jessica and Debby.


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 1:  05h 20m 00s.  Contact 2:  05h 39m 38s.

Contact 3:  11h 03m 40s.  Contact 4:  11h 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 1:            05h 21m 52s             05h 21m 30s                - (photography)         

  AU (km)            150,458,302             150,281,206                -

  Error (%)           + 0.575                     + 0.457                        -

Contact 2:            05h 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 6 June 2012, will be visible from Guernsey for the last 50 minutes only, as most of it will take place during our night time.  The next pair of transits will be on 11 December 2117 (not visible from Guernsey) and 8 December 2125 (visible for the first 2½ hours only) 4.



© David Le Conte






2.             Computed by the author using StarryNight Pro software.

3.             P Kenneth Seidelmann (ed.): Explanatory Supplement to the Astronomical Almanac, University Science Books, Mill Valley, California, 1992, page 700.

4.             Computed by the author using SkyMap Pro and StarryNight Pro software.

This report was originally published in the Transactions of La Société Guernesiaise (Vol. XXV, Part IV, pages 600-2).

Romanian translation of this page by Web Geek Science


This page was last updated on 14 October 2011.


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