DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Illustration showing the Koenigsberg heliometer, an intrument designed by Friedrich Bessel, director of the Koenigsberg observatory, and built for him by Joseph von Fraunhofer. A heliometer is a refracting telescope in which the objective lens is divided into two parts, each half producing a separate image. The two halves of the objective can be moved to bring the two images together. The amount of movement needed is used to determine the size of an object or the separation of two different objects. Bessel used this instrument to measure the first stellar parallax, that of the faint star 61 Cygni. From this he could calculate its distance; about six light years from Earth.
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