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: http://star.arm.ac.uk/venustransit/exhibit/section-III-IV.html
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The distance of the Sun from Earth is the basic unit of all astronomical distances. In astronomical parlance it is known as The Astronomical Unit in recognition of the fact that all other distances in the Universe depend upon it. The 18th century transits of Venus allowed a proper geometrical determination of this unit to be made for the first time.
However, in spite of the great care astronomers took to observe these events, certain fundamental problems, which were not well understood at the time, caused their results to be less accurate than expected. One of the most famous of these problems is the black drop effect. This effect, which was noticed by all observers, resulted from the tendency for the black spot (Venus) to appear to draw a black thread from the surface of the Sun as Venus reached the edge of the Sun's disk. This illusion, which affected different people in different ways, made it impossible to time the instant of contact accurately.
In the 19th and early 20th centuries another method gained favour, that of direct trigonometrical observation of the Minor Planets, and in particular of the asteroid Eros. This technique was not affected by the black drop effect. Thus, though the transits of Venus of the 18th century ultimately provided values for the solar distance which were reasonably close to the modern value, they have been superseded by more modern and accurate techniques such as Radar observations of Venus.
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Section IV - The 19th century transits of Venus
Transits of Venus occurred in 1874 and 1882, and on these occasions, though they were no longer considered so important for the determination of the size of the solar system, they were observed by a number of astronomers in Ireland. Astronomers at Dunsink and Armagh Observatories, University College Cork, Markree Observatory, County Sligo and Daramona Observatory, County Westmeath observed the transit on 6th December 1882.
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Last Revised: 2009 November 18th |