Home > The Transits of Venus: 1761, 1769, 2004 and 2012

The Transits of Venus: 1761, 1769, 2004 and 2012

 

My interest in Cook was initially stimulated by Peter Yeldham's TV series in which we hear about the scientific expedition to Tahiti being to observe the Transit of Venus across the Sun, but not why. Why go so far when it could be seen from sites much closer to home? What vital information could the observation produce? My reading soon revealed that the objective was no less than the measurement of the distance between the Sun and the Earth, the Astronomical Unit by which the scale of the universe could be assessed. The method depended in part on using at least two antipodean observations as a baseline approaching the diameter of our planet, for triangulation. There were more than 150 observations world wide in 1769 by several different countries, the first major international scientific effort. The pair of the Tahiti observation by British observers was at North Cape in Norway.

I tried out my planetarium program1 to see a virtual transit of Venus. The results were quite fascinating.

I can see (on a sun that nearly fills the screen) an animation of Venus crossing the orb, and obtain the predicted times of ingress and egress. In Cybersky you enter the location (either as one of the hundreds of pre-set towns, or as Latitude and Longitude) and the date. Then select the view of the Sun and zoom in until the sun nearly fills the screen. Then run an animation with a time interval of one minute (forwards or backwards), and see Venus dancing naked!

Has anyone heard of any plans to commemorate or utilise this coming Transit? The last one was in 1884, so the opportunity to do so is very infrequent. Has anyone any bright ideas on the subject? The next such event will be on 8th June 2004. My immediate thought was, wouldn't it be wonderful to have a re-enactment of 1769 with the Endeavour replica in Tahiti. But that idea received a shower of cold water when I fed the details into Cybersky: the transit will not be visible from Tahiti, although it will be partially visible from where I live near Brisbane. Here, the sun will set before the transit is complete.

There is at least one book published recently on the history of transits of Venus in preparation for the upcoming event: June 8, 2004: Venus In Transit by Eli Maor 2. I have also read The Transits of Venus: A Study of Eighteenth Century Science, by Harry Woolf 3.

Maor's work refers to Woolf, and also in particular to a predecessor, Richard Proctor, who covered much of the same ground in 1874. But Maor is more popular in style, like Dava Sobel's Longitude, much briefer - and an easier read. Technical details are explained clearly and in simple language. It is well illustrated, and has some pictures that I had not seen before. The global views of possible observation sites for the 2004 transit are easy to understand. The history and prediction of transit observations, even to the extent of putative observations from Mars, are well covered. So far so good, but now for the caveats.

On page 101, the position and importance of Banks to the expedition is totally misrepresented: "Cook's staff included Charles Green ... He in turn was assisted by Joseph Banks..." Then on page 103 we read "Banks's heavy quadrant mysteriously disappeared" - I believe the quadrant came from the Astronomer Royal.

On page 102, in giving Banks some acknowledgement, we are told "Banks Peninsula, ... New Zealand ... is named after him." True, but why just that one example? Bankstown (Sydney), the bush Banksia etc., etc., must surely rate equally.

Earth at the end  of the transit as seen from the sun
Earth at the end of the transit as seen from the sun. The hatched area will witness the complete transit. Adapted from Maor.

On page 104, Moar quotes Richard Hough's book Captain James Cook word for word, and at one point puts Hough's words into Cook's mouth! I noticed this because Moar writes "Cook reported '... conditions were not so good on the surface of Venus, sixty-seven million miles distant'. " Clearly, Cook could not have been precise about the distance of Venus, although he would have had some idea from the calculations following the 1761 transit.

Following an enquiry from the new world about William Wales who travelled with Cook on the Second Voyage and observed the 1769* transit in Canada, I noted that typical sightings in Canada on 8 June 2004 will be as follows (local times):
Toronto from 4:38 am (dawn) to 5:47 am
Quebec from 3:54 am (dawn) to 5:47 am
St Johns, Newfoundland from 4:07 am (dawn) to 7:17 am

The second times are when Venus touches the inner limb of the Sun and gives the "black drop" effect that so bothered the 1769 astronomers. All these will be incompletely observable transits because Venus and the Sun will be below the horizon part of the time, i.e. night. Since the key observations were times of ingress AND egress, a complete transit would seem to be necessary for a commemoration.

The transit will be partially observable in Tahiti on 5th June 2012, from ingress at 12:34 to sunset at 17:28 local time. Not a complete transit, so I don't think it would be any more appropriate for a re-enactment than 2004.

See also Cook's Log, page 264, vol. 7, no. 3 (1984) and page 1633, vol. 22, no. 3 (1999).

Ted Webber

With acknowledgements to Ron Ravneberg

 

Notes

  1. Cybersky shareware from http://www. cybersky.com. A more sophisticated program is The Sky from http://www.bisque.com
  2. Princeton University Press 2000, ISBN 0-691-04874-6.
  3. Princeton University Press, 1959.

Originally published in Cook's Log, page 1863, volume 24, number 3 (2001).
*Amended with the correction published in Cook's Log, page 1894, volume 24, number 4 (2001).

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