Friedrich Wilhelm Bessel, a German astronomer, was born July 22, 1784 (see third image above). Bessel was the first astronomer to measure the parallax of a star. Stellar parallax is a slight displacement of a star, caused by the motion of the earth around the sun; the diagram above exaggerates the situation for clarity (second image). The key word is “slight.” Astronomers had been trying to measure parallax for centuries, but they simply couldn’t detect any change in position. In 1838, Bessel was successful. He focused on a small star in the constellation Cygnus, named 61 Cygni (first image), and over the course of six months he detected a tiny angular shift, which he measured to be 0.3 seconds. That is 83 millionths of a degree–no wonder so many people had tried and failed to detect parallax! Once he had the parallax, Bessel could calculate the star’s distance, which turned out to be 66 trillion miles. The light-year as a unit hadn’t been invented yet, but it would be shortly, making the star 11 light-years distant. Within the year, now that Bessel had shown the way, other parallaxes were measured, including that of alpha Centauri, and it was thus determined that our closest stellar neighbor is 4.3 light-years away.
Bessel was commemorated on a German postage stamp in 1984, but it was for his invention of Bessel functions, which is far less interesting than measuring the distance to the stars (fourth image).
Dr. William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to firstname.lastname@example.org.