Photograph of a photoheliograph, a photographic telescope, which was used to observe the 1874 Transit of Venus. Made by Dallmayer, it was part of the equipment supplied to the five British expeditions, sent around the world to view and measure this rare event. It gave astronomers an opportunity to accurately determine the size of the Solar System. This view shows the telescope in its portable observatory building and was probably taken before the expedition's departure. All the equipment and portable observatory buildings were assembled beforehand in the park at the Old Royal Observatory, Greenwich for testing. After a period of over a hundred years the next transits of Venus occur in 2004 and 2012.
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