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HM Nautical Almanac Office: Transits of Venus

1761 June 6th Transit of Venus

Background

Despite the fact that this transit took place during the latter half of the Seven Years' War (1756-1763), a worldwide conflict involving the major powers of the time and their colonies, it was observed by many astronomers from around sixty locations ranging from China to England and from South Africa to Norway as part of the world's first multinational scientific collaboration. Among the many expeditions making observations were those of Mason and Dixon at Cape Town in South Africa, Chappe at Tobolsk in Central Siberia, Pingré on Rodriguez Island, 600 kilometres east of Mauritius, Lomonosov at St. Petersburg, Russia, Winthrop at St. Johns, Newfoundland and Cassini de Thury at Vienna in Austria. Many observers failed to get to their primary observing sites due to military action. For instance, Mason and Dixon were travelling to Bencoolen in Sumatra and Le Gentil tried to reach Pondicherry in India but was forced to return to Mauritius where he chose to wait for the transit of 1769.

It was during this transit that the phenomenon known as the "black drop effect" was first described. This effect can impair observers' estimates of the internal contact times of the transit by appearing to distort the disc of Venus as it nears the Sun's limb which in turn limits the accuracy with which the solar parallax can be determined.

Visibility

Global Visibility of the 1761 Transit of Venus

Venus crossed the disc of the Sun on 1761 June 6th. The map above shows the visibility of the event. The entire transit was visible from Scandinavia, Asia, the western half of Australia and northern parts of Alaska and Canada. The Sun set while the transit was in progress in the grey area encompassing northern and western parts of North America, the Pacific Ocean, New Zealand and eastern parts of Australia. The Sun rose while the transit was in progress in the grey areas taking in Africa, Europe and northern-eastern parts of Canada. The yellow lines on the diagram show the position of the terminator, where the Sun is either rising or setting, at the key phases of the transit.

Geocentric Circumstances

Geocentric Circumstances of the 1761 Transit of Venus

The geocentric circumstances of the transit are shown in the diagram above. During the transit, the diameter of the Sun was 1890.1 arcseconds and that of Venus was 59.7 arcseconds. In other words, the diameter of Venus was 0.03 that of the Sun, making it look like a rapidly moving sunspot. The whole transit lasted just over six and a half hours.

All timings are given in Universal Time (UT).

The Transit from Cape Town

Local time in South Africa is 2 hours ahead of Universal Time.

The only successful southern hemisphere observations of the transit were made by Mason and Dixon in Cape Town. It is their view of the event that will be used as an example. The ingress phases of the transit occurred around three and half hours before the sun rose in Cape Town at 5h45m (UT). Venus reached a minimum separation from the centre of the solar disc of 562.9 arcseconds shortly before sunrise at 5h25m (UT). The latter half of the transit was visible from Cape Town in the north-eastern sky in the three hours after sunrise. Interior egress, when Venus started to cross the solar limb for the second time, occurred at 08h25m (UT), and the transit ended with exterior egress at 08h42m (UT).

The progress of the transit is summarized in the diagram below. The left-hand panel shows the movement of the Venus across the solar disc. The top of the diagram points to the zenith, the point directly overhead. The position of Venus is marked every UT hour. The right-hand panel shows the movement of the Sun in the sky.

The 1761 Transit of Venus as seen from Cape Town

Local Circumstances

Summary plots like the one shown above and animations showing the motion of Venus relative to the Sun as seen by someone observing the transit through appropriate eye protection are available for several locations worldwide. The summary gif files are ~18Kb and the animations are ~200Kb. To view the animations properly, it may be better to download the animations and view them locally.

Local Circumstances of the 1761 Transit
Location Sunrise (UT)Sunset (UT)Downloadable gifs
Cape Town, South Africa05h45m15h43mAnimationSummary
Rodriguez Island, Mauritius02h15m13h13mAnimationSummary
Vienna, Austria02h55m18h55mAnimationSummary
Tobolsk, Siberia22h26m16h25mAnimationSummary
St. Petersburg, Russia00h41m19h13mAnimationSummary
St. Johns, Newfoundland07h34m23h24mAnimationSummary

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Last modified: Tuesday, 03-May-2011 14:04:30 BST