Friedrich Bessel makes the first accurate measurement…
1838 CE
Starting in 1819, Bessel has determined the position of over fifty thousand stars using a meridian circle from Reichenbach, assisted by some of his qualified students.
The most prominent of them is Friedrich Wilhelm Argelander.
With this work done, Bessel iss able to achieve the feat for which he is best remembered today: he is credited with being the first to use parallax in calculating the distance to a star.
Astronomers have believed for some time that parallax would provide the first accurate measurement of interstellar distances—in fact, in the 1830s there has been a fierce competition between astronomers to be the first to measure a stellar parallax accurately.
In 1838 Bessel wins the race, announcing that 61 Cygni has a parallax of 0.314 arcseconds; which, given the diameter of the Earth's orbit, indicates that the star is 10.3 light years away.
Given the current measurement of 11.4 light years, Bessel's figure has an error of 9.6%.
Nearly at the same time Friedrich Georg Wilhelm Struve and Thomas Henderson measure the parallaxes of Vega and Alpha Centauri.
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