61 Cygni

Königsberg, 1838: 183 comparisons

At Königsberg Observatory in 1838, Friedrich Wilhelm Bessel used a Fraunhofer heliometer to measure 61 Cygni against two comparison stars, completing the decisive observations by October. The instrument was the last heliometer Joseph von Fraunhofer made.

He had picked an inconspicuous binary rather than a brilliant star, because its unusually large proper motion suggested it might be nearby. By October he had 183 relative-position comparisons behind him, across nearly 100 nights.

0.3136 arcseconds, plus or minus

Bessel reported an annual parallax of 0.3136 arcseconds with a stated mean error of ±0.0202. That put 61 Cygni about 10.4 light-years away.

He described the result to John Herschel in a letter dated 23 October 1838, and the Royal Astronomical Society published an English translation on 9 November.

The regular observations, the explicit uncertainty and the close fit to the expected annual pattern are what made this the first stellar distance astronomers broadly accepted as reliable.

First accepted, not first attempted

Friedrich Georg Wilhelm von Struve had announced a tentative parallax for Vega in 1837, a year earlier. Thomas Henderson had observed Alpha Centauri in 1832–33 but published only after Bessel's announcement.

The figure was close rather than exact: Bessel's inferred distance runs roughly 9–10 percent below the modern one, against a modern parallax of 0.2859 ± 0.0001 arcseconds.

There was also no single moment at which the measurement was "made" — it came from observations spread across the seasons, the October letter, the November translation and a fuller German paper afterward.