1200th Ordinary General Meeting

"Transit of Venus 2012 - what we and others saw"

Dr. Andrew Jacob, Assistant Curator, Sydney Observatory

Wednesday 6 June 2012 at 6.30 pm

Sydney Observatory, Observatory Hill, Sydney

Meeting report by Donald Hector

Early in the 17th century, Johannes Kepler predicted that every 120 years or so Venus would pass between the Earth and the Sun and on each occasion, there would actually be two transits about seven years apart. This was a particularly important prediction: Kepler's third Law had provided the means to accurately calculate of the relative distances of each of the planets from the sun but there was no way to determine the absolute distance between them. By observing a transit of Venus from different points on the Earth, observing the times at which the transits start and finish and the exact location of the observation it is relatively simple to calculate the absolute distance between the Earth and Sun (referred to as one Astronomical Unit). From Kepler's third Law it is then possible to calculate the distance of the other planets from the sun but, more importantly, it then enables the calculation of distant astronomical bodies using the Astronomical Unit as a baseline.

The Society was fortunate to be able to mark the transit of Venus at the Sydney Observatory with a talk given by Dr Andrew Jacob, the Observatory's assistant curator. Not only is the transit of Venus an important and rare astronomical event, astronomy and, in particular, the transit of Venus, play and important part in both the history of Australia and history of the Society.

The first predicted transit was in 1631 but there is no record of any successful observation is being made. The next, in 1639, was observed by Jeremiah Horrocks who was able to calculate the Astronomical Unit to an accuracy of about 50%. The next pair of transits were in 1761 and 1769. Lieutenant James Cook was ordered to sail to Tahiti to observe the 1769 transit which he did successfully and the rest, as they say, is history.

The next pair of transits in the 19th century (1874 and 1882) were observed using much more sophisticated instruments, including photography and these observations enabled very precise estimates of the Astronomical Unit.

The first of the 21st-century transits in 2004 was noted as an interesting phenomenon but not of any particular scientific importance. However, in the few years between 2004 and the 2012 transit, the discovery of hundreds of "exo-planets" (planets orbiting far-away stars) led astronomers to realise that precise observations of the phenomena caused by the transit of Venus could allow a much more precise and detailed characterisation of exo-planets.

It is notable that the Royal Society of NSW traces its origins to 1821 when the Philosophical Society of Australasia invited the Governor, Sir Thomas Brisbane, to become its first president. Brisbane was a keen astronomer and made important contributions to the science both in Australia and when he returned to Scotland.

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