Bessel Measures the First Stellar Distance
A tiny wobble in the position of a faint star finally puts a number on the distance to the stars
Quick facts
- Star measured
- 61 Cygni
- Announced
- 1838
- Bessel's parallax value
- 0.314 arcseconds (modern value: 0.292 arcseconds)
- Derived distance
- c. 10 light years
What happened
Friedrich Bessel used the astronomical technique of parallax, measuring the tiny apparent shift in a star's position against more distant background stars as Earth orbits the Sun, to determine the distance to the star 61 Cygni, announcing his result in 1838. Bessel selected 61 Cygni deliberately: it had the greatest proper motion of all the stars he had studied, and he correctly deduced that this large apparent motion across the sky meant the star was relatively nearby. Using a Fraunhofer heliometer to make the measurements, Bessel announced a parallax value of 0.314 arcseconds, which, combined with the known diameter of Earth's orbit, gave a distance of about 10 light years; the modern accepted parallax value for 61 Cygni is 0.292 arcseconds. The astronomer John Herschel called Bessel's achievement the greatest and most glorious triumph which practical astronomy has ever witnessed, and the Royal Astronomical Society awarded Bessel its gold medal for the result.
Why it matters
Before Bessel, astronomers had no direct measurement of how far away even the nearest stars actually were, only the assumption that they had to be extremely distant given the absence of any detectable parallax with earlier instruments. Bessel's 1838 measurement gave astronomy its first real ruler for the universe beyond the solar system, opening the door to measuring the true scale of the galaxy and, eventually, of the universe as a whole.
How we know
Bessel published his parallax measurement and methodology in a paper communicated to the Royal Astronomical Society, and the observational technique and result were quickly checked and confirmed by other astronomers of the period using independent instruments.
Sources
- MacTutor History of Mathematics, University of St Andrews. Friedrich Wilhelm Bessel · Reputable sourcemathshistory.st-andrews.ac.uk · The domain "mathshistory.st-andrews.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- MacTutor History of Mathematics, University of St Andrews. Friedrich Wilhelm Bessel - RAS obituary · Reputable sourcemathshistory.st-andrews.ac.uk · The domain "mathshistory.st-andrews.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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