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HM Nautical Almanac Office: Transits of Venus

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1882 December 6^th Transit of Venus

Background

The second transit of the 19^th century again stimulated considerable efforts by astronomers from countries worldwide. However, some astronomers did not take part in the international collaboration, most notably Russia and Austria, because they were not convinced that the transit was the best way to determine the solar parallax. Astronomers from the United States went on eight expeditions including Asaph Hall to San Antonio and Simon Newcomb to Wellington in South Africa. British observers went to Southern Africa, Australia, Canada and the Caribbean. French observers mounted ten expeditions ranging from Fort Marion in Florida to Cape Horn and from Puebla in Mexico to Fort Tartenson in Martinique. In 1890, Newcomb determined a value for the solar parallax of 8.79 arcseconds from observations made in 1874 and 1882 which compares very favourably to current best estimates.

Visibility

Venus crossed the disc of the Sun on 1882 December 6^th. The map above shows the visibility of the event. The entire transit was visible from South and Central America as well as the eastern half of North America. The Sun set during the transit in the grey area encompassing Africa and Europe including the British Isles. The Sun rose while the transit was in progress in the grey area taking in northern and western parts of North America, the Pacific Ocean and eastern Australia. 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

The geocentric circumstances of the transit are shown in the diagram above. During the transit, the diameter of the Sun was 1946.6 arcseconds and that of Venus was 63.1 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 lasts just over six and a quarter hours.

All timings are given in Universal time (UT).

The Transit from Wellington

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

In Wellington, the transit began with exterior ingress at 13^h52^m (UT). Venus took 20 minutes to cross the limb of the Sun with interior ingress at 14^h12^m (UT). Venus reached its minimum separation of 630.0 arcseconds from the centre of the solar disc at 17^h02^m (UT). Interior egress occurred at 19^h54^m (UT). Venus took 20 minutes to cross the limb of the Sun for the second time with exterior egress occurring at 20^h14^m (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. At the beginning of the transit, the Sun was in the west at an altitude of 66°. By the time of minimum separation, the Sun was in the south west at an altitude of 7°. Sunset occurred at 17^h44^m (UT).

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.

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Last modified: Tuesday, 03 May 2011 at 14:07:27 BST