The Transits of Venus of 1761 and 1769

Introduction

During his 13-month sojourn on the island of St. Helena in the southern Atlantic Ocean, the English astronomer Edmond Halley (1656-1742) succeeded in observing a complete transit of the planet Mercury over the Sun on 7 November 1677 [28 October O.S.]. 

In a letter written some weeks later, probably addressed to his patron Sir Jonas Moore, Halley speculated that his measurements when compared to those made in Europe would provide a better estimate for the solar parallax, the measure for the mean distance of the Earth to the Sun. In fact, the Scottish mathematician James Gregory (1638-1675) had already pointed out in 1663 that observations of a transit of an inner planet could be used for determining the solar parallax.

In a paper read before the Royal Society of London on 23 September 1691, but not published in the Society’s Philosophical Transactions until in 1716, Halley described a practical way for determining the solar parallax from observation of the times of ingress and egress of a transit of an inner planet across the solar disk.

Visibility Regions for the Transits of Venus of 1761 and 1769

The following diagrams depict the regions of visibility for the transits of Venus on 6 June 1761 and 3/4 June 1769.

Circumstances of the transit of Venus of 1761
 
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map legend   map legend  
 
Begin of the transit (6 June, 02:02 UT) End of the transit (6 June; 08:37 UT)
Sub-Venus point: λ = +149º 18'; φ = +22º 35' Sub-Venus point: λ = +50º 06'; φ = +22º 30'
Apparent diameter of the Sun = 31.50' Apparent diameter of the Sun = 31.50'
Apparent diameter of Venus = 0.96' Apparent diameter of Venus = 0.96'

This transit took place just after Venus passed its descending node (5 June; 13:32 UT) and occurred on the southern half of the solar disk.

Circumstances of the transit of Venus of 1769
 
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map legend   map legend  
 
Begin of the transit (3 June, 19:15 UT) End of the transit (4 June; 01:35 UT)
Sub-Venus point: λ = –109º 08'; φ = +22º 39' Sub-Venus point: λ = +155º 26'; φ = +22º 34'
Apparent diameter of the Sun = 31.51' Apparent diameter of the Sun = 31.51'
Apparent diameter of Venus = 0.96' Apparent diameter of Venus = 0.96'

This transit took place just before Venus passed its descending node (4 June; 15:23 UT) and occurred on the northern half of the solar disk.

References


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