Friedrich Bessel, born in Minden, Germany, was a self-taught astronomer. He began working as an accountant at age fifteen, but his true interests were astronomy and mathematics.
At age twenty, Bessel recalculated the orbit of Halley's comet and mailed his findings to astronomer Heinrich Olbers. Olbers got Bessel's work published, then helped Bessel obtain a post as an assistant at a private observatory. At age twenty-six, Bessel was appointed by King William III of Prussia to the directorship of the Königsberg Observatory, a position he held until his death in 1846.
Bessel worked tirelessly and had an impressive list of achievements to show for it. He catalogued over fifty thousand stars, developed a new method of mathematical analysis that could be applied to fields outside of astronomy, and created the most precise telescopes of his day
Bessel's greatest achievement was to define the parallax of a star. As the Earth orbits the sun, its position relative to any star shifts by up to 186 million miles (300 million kilometers). Thus, the apparent position of any star in the sky changes slightly throughout the year. The amount of the observed change in position is the parallax. Once the parallax is known, it is possible to calculate the distance to a star.
In 1838, Bessel found the parallax for a star called 61 Cygni, the star with the largest known range of apparent movement. He assumed that the star was relatively close to Earth, because the closer an object is, the greater its parallax would be. He calculated that the star was ten light-years away. Although we now know that this distance is actually very close for a star, it astounded astronomers in 1838. They thought the stars were much closer than that. Bessel's work turned out to be the first accurate measurement of the distance to a star, something that astronomers had been trying but failing to do for almost a century.
Discovering the concept of parallax also helped refute the notion that the universe centered around the Earth. Parallax implied that the Earth was moving, which strengthened Copernicus' theory that the earth orbited around the sun.
In 1841, Bessel noticed that the bright star Sirius wobbled in its path.The motion was unlike that due to parallax, which would be smooth. He concluded that the wobbling was caused by the gravitational tug of an invisible companion star in orbit around Sirius. Sirius and its companion are an astrometric binary system, meaning only one star in the pair is visible. Bessel's theory was shown to be correct in 1862 when Sirius' companion star was found by telescope-maker Alvan Graham Clark. Clark observed a bright, small, dense star known as a white dwarf. Since both stars were visible, the pair from then on has been considered a visual binary system.