# Friedrich Bessel

## Astronomer; Originator of Bessel Functions

Friedrich Bessel analyzed disturbances in planetary and stellar motion, which enabled him to measure the positions of numerous stars and, for the first time, the distance from Earth to a star other than the Sun. He introduced Bessel functions and showed that Earth was not a perfect sphere.

Bessel was born on July 22, 1784, in Minden, in present day Germany. He was apprenticed at age 15 to an accountant, but he dreamed of travel and studied navigation at night. From observations of Halley’s Comet recorded in 1607, he calculated the comet’s orbit and sent his notes to the astronomer Wilhelm Olbers, who published them and sent Bessel to join the Lilienthal Observatory as an assistant.

In 1808 Bessel was asked to oversee construction of a large observatory at Königsberg (now Kaliningrad); two years later he was appointed professor of astronomy at the city’s university. He directed the observatory from 1813 until his death.

From 1821 to 1833, Bessel diligently calculated the positions and observed the motions of thousands of the nearest stars, correcting errors associated with telescope imperfections and atmospheric disturbances.

In 1824 he formulated new functions, now known as Bessel functions, to assist the understanding of anomalies in planetary motion. Bessel functions represent solutions to certain differential equations.

In 1832 Bessel calculated Earth’s ellipticity, the extent to which its shape deviates from that of a perfect sphere, by measuring elements of selected meridian arcs (imaginary arcs inscribed in the sky corresponding to Earth’s longitude lines) over East Prussia.

Six years later Bessel accomplished a groundbreaking determination of the distance from Earth to the star Cygni 61. This was the first calculation of the distance from Earth to a star other than the Sun. He found the distance to be about 10.3 light years, which is within 10% of the current measurement. His method involved stellar parallax, the displacement nearby stars exhibit over time because they are viewed from different locations as Earth moves through its orbit. Bessel’s computation was the first use of parallax to make an accurate measurement of stellar distance. He published his results in 1842.

Soon thereafter, Bessel suggested that the wavelike motion of the star Sirius is due to the gravitational pull of an unseen orbiting body. He also predicted the existence of a planet beyond Uranus by observing minute irregularities in its orbit.

Bessel died on March 17, 1846, in Königsberg.

**Friedrich Bessel’s Legacy**

Bessel established the scale and framework of the universe, thus lighting the path for following generations of astronomers.

Bessel’s later predictions were confirmed soon after his death. In the summer of 1846, astronomers identified the planet Neptune as the body that disturbs the orbit of Uranus. The companion of the star Sirius, dubbed Sirius B, was detected by telescopelens maker Alvan Clark in 1862. Such twostar systems were found to be common.

Bessel’s method of calculating the distance from Earth to the nearest stars led to the best determination of the scale of the universe that had then been achieved. His star positions allowed the first accurate calculation of distances between stars and led to a means of calculating the size of stars, galaxies, and clusters of galaxies.

Bessel is credited with reforming the approach to astronomical observation and providing the key to astronomical progress with his precise methods of correcting measurement errors.

Bessel functions have applications in pure mathematics and physics. They are useful in studying the distribution and flow of heat and electricity through cylinders and in solving problems related to wave theory, elasticity, and hydrodynamics.

Friedrich Bessel – 1784-1846