1769 TRANSIT of VENUS

Background

This transit was observed by Captain James Cook at Point Venus, near Papeete on Tahiti in French Polynesia and at two other sites on neighbouring small islands during his first great voyage of discovery. Other notable expeditions were those of Pingré to Santo Domingo, Chappe to Cabo San Lucas in Baja California and Le Gentil to Pondicherry in India. Expeditions in the 18^th-century could be both frustrating and hazardous - Chappe died during a yellow fever epidemic at his observing site and the hapless Le Gentil failed to see the transit due to cloud. If he had gone to his first choice site of Manila, he would have seen the transit!

Visibility

Venus crossed the disc of the Sun on 1769 June 3^rd-4^th. The map above shows the visibility of the event. The entire transit could have been seen from the northern and western parts of North America, north-west Mexico, the Pacific Ocean and northern and eastern Asia. The Sun set while the transit was in progress in the grey area encompassing the eastern parts of North America, Central and South America. The Sun rose while the transit was in progress in the grey area taking in Australasia, southern Asia, and parts of Scandinavia. 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 given in the diagram at the right. During the transit, the diameter of the Sun was 1890.7 arcseconds and that of Venus was 57.8 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 quarter hours.

All timings are given in Universal Time (UT).

The Transit as seen from Tahiti

Local time in Tahiti is 10 hours behind Universal Time.

In Tahiti, the transit began well after sunrise. Taking Papeete as an example, sunrise occurred at 16^h22^m (UT). The transit began with exterior ingress at 19^h21^m (UT) when the disc of Venus began to cross the limb of the Sun. Interior ingress occurred at 19^h40^m (UT), when the whole of the disc of Venus had crossed over the limb of the Sun. Venus then moved across the solar disc, reaching a minimum separation from the centre of the Sun of 623.5 arcseconds at 22^h25^m (UT). Interior egress occurred when Venus started to cross the solar limb for the second time at 01^h10^m (UT), and the transit ends with exterior egress at 01^h29^m (UT). Sunset occurred two hours later at 03^h30^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 north east at an altitude of 35°. By the end of the transit, the Sun was in the north west at an altitude of 24°.

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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HM Nautical Almanac Office UK Hydrographic Office E-mail: hmnao@nao.rl.ac.uk Last revision was made on 2007 November 5