Caption: Friedrich Bessel (1784--1846).

And no, it's NOT Ludwig van Beethoven (1770--1827): all early 19th century guys looked like this.

In 1838, Bessel measured the first stellar parallax (see No-419--420; Wikipedia: Stellar parallax: 19th and 20th centuries). But there are some complications in claiming first. Thomas Henderson (1798--1844) obtained data in 1832--1833 for Alpha Centauri (Α Centauri) that allowed for a stellar parallax determination, but he did NOT actually make that determination until 1839. Friedrich Georg Wilhelm von Struve (1793--1864) may have made a determination of the stellar parallax for Vega in 1837, but the references give contradictory statements.

As to Bessel's stellar parallax measurement, he obtained 0.314 arcseconds ('') stellar parallax for the nearby binary star system 61 Cygni (No-419).

The modern value for 61 Cygni stellar parallax is 0.28588(54) arcseconds ('').

Thus,

  d(61 Cygni) = (1 AU)/(0.28588 arcseconds)
              = 3.4978 pc  .  

How did Bessel know that 61 Cygni was close by, and thus was good target for a stellar parallax measurement? 61 Cygni has a high proper motion (i.e., high angular velocity on the sky relative to the background fixed stars). If local stars all have of order the same velocities in space relative to the Sun, then the closest ones should have high proper motion---and 61 Cygni fit the bill.

Stellar parallax had been sought since classical antiquity since it was understood that stellar parallax would be a disproof of Aristotelian cosmology and would strong evidence for a moving Earth.

By the by, Bessel is also famous for his systematization of what we now call Bessel functions---they are the darndest things.

Credit/Permission: Perhaps derived from a portrait by Johann Eduard Wolff (1786--1868) (see Portrait of Friedrich Bessel (1784--1846), 1834). The two portraits have different faces, but nearly the same cravat. / Public domain.
Download site: St. Andrews Mathematician Biographies: Friedrich Wilhelm Bessel.
Image link: Itself.
Local file: local link: friedrich_bessel.html.
File: Astronomer file: friedrich_bessel.html.