Intro | German astronomer and mathematician |

A.K.A. | Friedrich Wilhelm Bessel |

Countries | Prussia |

Occupations | Astronomer Mathematician University teacher Geodesist Physicist |

Gender | male |

Birth | 22 July 1784 (Minden) |

Death | 17 March 1846 (Königsberg) |

Star sign | Cancer |

Education | University of Göttingen |

Notable Works | Fundamenta Astronomiae |

**Friedrich Wilhelm Bessel** (German: [ˈbɛsəl]; 22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist and geodesist. He was the first astronomer who determined reliable values for the distance from the sun to another star by the method of parallax. A special type of mathematical functions were named Bessel functions after Bessel's death, though they had originally been discovered by Daniel Bernoulli and then generalised by Bessel.

## Life and family

Bessel was born in Minden, Westphalia, administrative center of Minden-Ravensberg, as second son of a civil servant. He was born into a large family in Germany. At the age of 14 Bessel was apprenticed to the import-export concern Kulenkamp at Bremen. The business's reliance on cargo ships led him to turn his mathematical skills to problems in navigation. This in turn led to an interest in astronomy as a way of determining longitude.

Bessel came to the attention of a major figure of German astronomy at the time, Heinrich Wilhelm Olbers, by producing a refinement on the orbital calculations for Halley's Comet in 1804, using old observation data taken from Thomas Harriot and Nathaniel Torporley in 1607.

Two years later Bessel left Kulenkamp and became Johann Hieronymus Schröter's assistant at Lilienthal Observatory near Bremen. There he worked on James Bradley's stellar observations to produce precise positions for some 3,222 stars.

In January 1810, at the age of 25, Bessel was appointed director of the newly founded Königsberg Observatory by King Frederick William III of Prussia. On the recommendation of fellow mathematician and physicist Carl Friedrich Gauss (with whom he regularly corresponded) he was awarded an honorary doctor degree from the University of Göttingen in March 1811.

Around that time, the two men engaged in an epistolary correspondence. However, when they met in person in 1825, they quarrelled; the details are not known.

In 1842 Bessel took part in the annual meeting of the British Association for the Advancement of Science in Manchester, accompanied by the geophysicist Georg Adolf Erman and the mathematician Carl Gustav Jacob Jacobi.

Bessel married Johanna, the daughter of the chemist and pharmacist Karl Gottfried Hagen who was the uncle of the physician and biologist Hermann August Hagen and the hydraulic engineer Gotthilf Hagen, the latter also Bessel's student and assistant from 1816 to 1818. The physicist Franz Ernst Neumann, Bessel's close companion and colleague, was married to Johanna Hagen's sister Florentine. Neumann introduced Bessel's exacting methods of measurement and data reduction into his mathematico-physical seminar, which he co-directed with Carl Gustav Jacob Jacobi at Königsberg. These exacting methods had a lasting impact upon the work of Neumann's students and upon the Prussian conception of precision in measurement.

Bessel had two sons and three daughters. His eldest daughter, Marie, married Georg Adolf Erman, member of the scholar family Erman. One of their sons was the renowned Egyptologist Adolf Erman.

After several months of illness Bessel died in March 1846 at his observatory from retroperitoneal fibrosis.

## Work

While the observatory was still in construction Bessel elaborated the *Fundamenta Astronomiae* based on Bradley's observations. As a preliminary result he produced tables of atmospheric refraction that won him the Lalande Prize from the French Academy of Sciences in 1811. The Königsberg Observatory began operation in 1813.

Starting in 1819, Bessel determined the position of over 50,000 stars using a meridian circle from Reichenbach, assisted by some of his qualified students. The most prominent of them was Friedrich Wilhelm Argelander.

With this work done, Bessel was 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 had believed for some time that parallax would provide the first accurate measurement of interstellar distances—in fact, in the 1830s there was a fierce competition between astronomers to be the first to measure a stellar parallax accurately. In 1838 Bessel won the race, announcing that 61 Cygni had a parallax of 0.314 arcseconds; which, given the diameter of the Earth's orbit, indicated that the star is 10.3 ly away. Given the current measurement of 11.4 ly, Bessel's figure had an error of 9.6%. Nearly at the same time Friedrich Georg Wilhelm Struve and Thomas Henderson measured the parallaxes of Vega and Alpha Centauri.

As well as helping determine the parallax of 61 Cygni, Bessel's precise measurements using a new meridian circle from *Adolf Repsold* allowed him to notice deviations in the motions of Sirius and Procyon, which he deduced must be caused by the gravitational attraction of unseen companions.
His announcement of Sirius's "dark companion" in 1844 was the first correct claim of a previously unobserved companion by positional measurement, and eventually led to the discovery of Sirius B.

Bessel was the first scientist who realized the effect later called *personal equation*, that several simultaneously observing persons determine slightly different values, especially recording the transition time of stars.

In 1824, Bessel developed a new method for calculation the circumstances of eclipses using the so-called Besselian elements. His method simplified the calculation to such an extent, without sacrificing accuracy, that it is still in use today.

Bessel's work in 1840 contributed to the discovery of Neptune in 1846 at Berlin Observatory, several months after Bessel's death. On Bessel's proposal (1825) the Prussian Academy of Sciences started the edition of the *Berliner Akademische Sternkarten* (*Berlin Academic Star Charts*) as an international project. One unpublished new chart enabled Johann Gottfried Galle to find Neptune near the position calculated by LeVerrier in 1846.

In the second decade of the 19th century while studying the dynamics of 'many-body' gravitational systems, Bessel developed what are now known as Bessel functions. Critical for the solution of certain differential equations, these functions are used throughout both classical and quantum physics.

Bessel is responsible for the correction to the formula for the sample variance estimator named in his honour. This is the use of the factor *n* − 1 in the denominator of the formula, rather than just *n*. This occurs when the *sample mean* rather than the *population mean* is used to centre the data and since the sample mean is a linear combination of the data the residual to the sample mean overcounts the number of degrees of freedom by the number of constraint equations — in this case one. *(Also see Bessel's correction)*.

An additional field of work was geodesy. Bessel published a method for solving the main geodesic problem. He was responsible for the survey of East Prussia which joined the Prussian and Russian triangulation networks and he obtained an estimate of increased accuracy for the figure of the Earth, nowadays referred to as the Bessel ellipsoid.

Despite lacking a university education, Bessel was a major figure in astronomy during his lifetime. He was elected as member of the Prussian Academy of Sciences in 1812, the French Academy of Sciences in 1816, foreign member of the Royal Swedish Academy of Sciences in 1823, and fellow of the Royal Society in 1825. In 1832, he was elected a Foreign Honorary Member of the American Academy of Arts and Sciences. In 1827 Bessel became member of the Royal Institute of the Netherlands, predecessor of the Royal Netherlands Academy of Arts and Sciences.

Bessel won the Gold Medal of the Royal Astronomical Society twice in 1829 and 1841.

The largest crater in the Moon's Mare Serenitatis and the main-belt asteroid 1552 Bessel were named in his honour.

## Publications

- Latin

- German