Transit of Venus instrument, 1876.
© Science Museum / Science & Society Picture Library
Photograph of a portable transit instrument that was used to observe the 1874 Transit of Venus. The instrument was used regulate accurate timepieces, called regulator clocks, by measuring when stars cross the north-south meridian. Made by Troughton & Simms, 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 After a period of over a hundred years the next transits of Venus occur in 2004 and 2012.