Bessel, Friedrich Wilhelm (1784-1846), a German astronomer, was the first to measure stellar parallax, an apparent change in a star's position as a result of the earth orbiting the sun. Bessel functions, the mathematical functions that he derived, are still used extensively in physics.
While an apprentice at a mercantile firm, Bessel taught himself navigation and astronomy. In 1804, he calculated the orbit of Halley's Comet based on observations made in 1607. He sent his work to astronomer Wilhelm Olbers, who encouraged him to make further observations and improve his calculations. Olbers then saw that Bessel's work was published and recommended him as assistant at the observatory in Lilienthal. Germany. After four years there, Bessel became director of the observatory at Königsberg, where he remained the rest of his life. In 1810, he also became professor of astronomy at the University of Königsberg.
Around 1817, Bessel introduced a set of mathematical functions, now called Bessel functions, to help understand the movements of three bodies under mutual gravitation. Bessel's contributions to geodesy, the mathematics related to the size, shape, and gravitational field of the earth, included his 1841 calculation of the amount by which the earth's shape deviates from a sphere.
Bessel laid the foundations for establishing the scale of the universe. He accurately measured the positions of about 50,000 stars and observed very small motions of one star relative to another. He showed that 61 Cygni, the star with the greatest proper motion known to him, apparently moved in an ellipse yearly. Bessel attributed this motion, called parallax, to the motion of the earth around the sun. By calculating the distance from the earth to 61 Cygni, Bessel became the first to accurately measure the distance of a star other than the sun. He also predicted the existence of another planet beyond Uraunus—Neptune—which was discovered after his death.