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Twice in a lifetime Venus - Page 2

© Rodolfo Astrada

Sir Edmond Halley of cometary fame was mapping the southern skies in St. Helena when he happened to observe a much more frequent Mercury transit. It dawned on him this kind of events could be put to good use to determine a long sought value, the Earth - Sun distance. Venus should be more useful than Mercury being farther out from the Sun, thus affording a more favorable geometry for the sake of accuracy. Tragically, he was well aware the upcoming 1761 and 1769 events were beyond his life span, yet laid the foundations for the experiment further perfected by Joseph-Nicolas Delisle.
The 1761 event was attacked with a full fledged campaign involving expeditions to Siberia, India, Madagascar, South Africa and St. Helena. As politics should have it, the Seven Years' War raged then, precluding for example nobleman Guillaume Le Gentil to land in his intended observation post of Pondicherry, Mauritius under British siege. Le Gentil waited out overseas for the next opportunity in 1769 at last in again French controlled Pondicherry but June 3 was ... clouded. Destiny had it when Le Gentil returned to France after a voluntary 10 years scientific exile, to have been long assumed dead and his possessions in process of division. Eventually he was able to marry and rebuild his life.

1769 found Great Britain transiting the climbing side of the road to the Empire where the Sun never sets, so it dispatched expeditions to Hudson Bay in Canada and the better known Endeavour captained by James Cook to Tahiti, while king George III arranged for an observatory to be built in his gardens for that purpose. Fair weather rewarded observers with excellent data except for unexpected disturbances as we'll see.

Transits gone, German astronomer Johann Encke determined in 1824 after careful analysis of 1761 and 1769 timing data, the Earth - Sun distance to be 153,34 million km (95.28 million miles), 2.5% higher than the correct 149,59 million km.

A movie shot before the invention of cinema

The 17th Century observations unveiled problems that marred the otherwise excellent precision the method could provide for the Astronomical Unit determination. For one, precision rested on the accuracy of timing contacts between the Sun's limb and Venus limb - a fact that was found to be subject to individual response times (remember, it was then clockwork era) - and then there was atmospheric disturbances, Venus' atmospheric bright halo, and the so called "black drop" effect masking the contact point at the most critical instant.

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