Section III - The Astronomical Unit

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ultimately provided values for the solar distance which were reasonably close to the modern value, they have been superseded by more modern and accurate techniques such as Radar observations of Venus.

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27. Nevil Maskelyne (1732-1811) the Astronomer Royal for England for most of the second half of the 18th century. Maskelyne who had observed the 1761 Transit of Venus from St Helena, was the principal source of motivation for the British campaign in 1769. He was instrumental in setting up the new Observatory at Armagh. (Courtesy of the Royal Society)

28. An Essay on the Present State of Astronomical Certainty .. etc by James Archibald Hamilton (1748-1815), first Director of Armagh Observatory. In this account, J.A. Hamilton and his assistant Robert Hogg give their deductions on the radius of the Earth (based on trigonometric surveys) and the distance to the Sun (determined from observations of the Transits of Venus in 1761 and 1769). They calculated the mean distance to the Sun to be 95 million miles whereas the currently accepted value is close to 93 million miles. Hamilton showed further that, if this distance scale is extrapolated beyond the solar system, the nearest stars must be further away than 39 million million miles as none at that time had been found to have an annual parallax in excess of 1 second of arc. (Transactions of the Royal Irish Academy}

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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29. The Transit of Venus in 1874 photographed by Charles Burton, later of Dunsink Observatory, Dublin from Rodriguez in the Indian Ocean where he accompanied an expedition organised by the Royal Greenwich Observatory.

Last Revised: 2009 November 18th