Oxford heliometer, 1840-48.
© Science Museum / Science & Society Picture Library
Completed in 1848, this heliometer was made by A & G Repsold of Hamburg, Germany and installed at the Radcliffe Observatory in Oxford. It was the first and last large example of its kind to be used in England. The heliometer is a split-lens micrometer, originally developed for measuring the angular diameter of the Sun. This instrument was later adapted to make precise measurements of star positions, and was used to detect tiny shifts in nearby stars caused by parallax as the Earth orbited the Sun. In 1838, the German astronomer Friedrich Besel (1784-1846) was able to make the first accurate parallax measurements and hence calculate the distance to the nearby star of 61 Cygni. The heliometer is shown on display at the Science Museum in London.