Transits of Venus ( Major Trends In The History of Astronomy)
The desire to detect both solar and stellar parallaxes provided great incentive for observational astronomers during the eighteenth century. Lunar distances also were important because observations of the Moon were felt to be a viable approach to the problem of determining longitude at sea.
One of the most elaborate scientific expeditions of the eighteenth century was concerned with the accurate measure of solar parallax through observations of the transits of Venus in 1761 and 17(50. Attention had been drawn to these important events by Edmond Halley in 1716 in an attempt to unite and stimulate young astronomers by the accuracy the method offered. Halley’s idea (mentioned earlier by Kepler and others) was based on his observation of the transit of Mercury in 1677. Essentially it uses the circumstance that the apparent path of Venus across the Sun is not the same for observations conducted at different points on Earth’s surface. Hence, two different observers at different latitudes will see two different paths the difference depending on the size of the Earth, position of the observers and the scale of the Solar System. Since the first two of these factor* were known, the third, the distance to the Sun. could be deduced from the differences between the observed paths
Governments, scientific societies and individuals went to incredible trouble and expense to prepare for these transits since their occurrence, due to the inclined paths of Venus and the earth, is rare. Expeditions were dispatched throughout the world to ensure the best possible results . Unfortunately a combination of effects optical, instrumental and atmospheric- led to results that did not live up to expectations.
For the first transit (1761), values tor the solar parallax ranged from 7.5 to 10.5 seconds of arc: for the second, values were much better, ranging mostly from 8.5 to 8.8 seconds of arc. Some astronomers deduced values close to the presently accepted one but the data were uncertain, so that such results held no more weight than others. The whole set of observations was reanalysed by Encke in 1835, who deduced a value of 8.571 seconds of arc.