Friedrich Bessel measures the distance to a star (61 Cygni)

Background. Stellar parallax is the apparent shift of a nearby star against distant background stars, allowing distance determination through trigonometry. Technological progress in the second quarter of the 19th century provided sufficient accuracy for these measurements. Bessel, appointed director of Königsberg Observatory in 1810, utilized a Fraunhofer heliometer to conduct an intense observational campaign in 1837–1838, focusing on 61 Cygni, a binary star system in Cygnus.

Measurement. Bessel measured the parallax of 61 Cygni using a heliometer, publishing results in 1838. His value of 0.314 arcseconds indicated a distance of 10.3 light-years, close to the actual 11.4 light-years. This was the first reliable distance estimate for any star other than the Sun. The measurement relied on the accuracy of the Fraunhofer heliometer, one of the best instruments of the time.

Context. Bessel's work was part of a broader effort by Thomas Henderson, Friedrich Georg Wilhelm von Struve, and Bessel to measure stellar parallaxes. Henderson measured Alpha Centauri, Struve measured Vega, and Bessel measured 61 Cygni. These three results established the first reliable distance scale to stars. Bessel's announcement was a key milestone in the history of astronomy, demonstrating the feasibility of measuring stellar distances.

Instrumentation. Bessel used a Fraunhofer heliometer, a new type of instrument invented by Joseph von Fraunhofer. This device allowed for precise measurements of stellar positions. Bessel's use of the heliometer in 1837 and 1838 at Königsberg was crucial for obtaining accurate parallax values. The heliometer's precision enabled Bessel to publish findings with specific error margins, such as 369.0 ±19.1 mas for component A.