The "Transit of Venus," among the rarest of astronomy's predictable events, occurs twice, eight years apart, with more than a century passing in between pairs. Thus, for the vast majority of earthlings, this was the final one in their lifetime. The last one occurred in June 2004 and the first of the next pair will not happen until December 2117.
The planet, appearing as a small black dot, began its traverse across the solar disc shortly after 3 p.m. local time, and completed it just before 10 p.m., and so was visible with the help of protective solar lenses until near sunset at 8:24 p.m.
The first known scientific observation of a transit by Venus dates from the 17th century, when, as a result, the most accurate estimation to date of the distance between Earth and the sun - known as the astronomical unit (AU) - was able to be made.
In 1769, during Venus' second transit, observers in different geographical locations, acting on the prediction of astronomer Edmond Halley (who had passed away in 1742), pooled their measurements of the time for Venus to cross the sun and, through triangulation and parallax, produced an even more accurate calculation of the AU.
The value was further refined during Venus' transits in the 19th century.
To this day, the AU forms the basis for the calculation of distances throughout space.
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