For hundreds of years, transits of Venus have been important for scientific research. From the seventeenth century onward, Venus transits provided observers with data that eventually led to a very close estimate of the astronomical unit-the distance between Earth and the Sun.

Transits of Venus occur in pairs that are eight years apart, then don't happen again for more than a century. The last two Venus transits were in 1874 and 1882, so no one alive today has seen one. After transits in 2004 and 2012, there won't be another until 2117.

Watch a Quicktime animation of the 1882 Venus transit made from glass plate negatives:

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Courtesy Lick Observatory