In 1804, he contacted Wilhelm Olbers (1758-1840) concerning a paper he had written on comet Halley, using data from observations made by Harriot in 1607. Olbers advised him to elaborate this work further, and it was finally published. On Olbers suggestion, Bessel accepted a post at Lilienthal Observatory, a private observatory near Bremen, owned by Johann Hieronymus Schroeter (1745-1816) in 1806. There he did observational work on comets and planets, in particular planet Saturn and its moons and the newly discovered asteroids Ceres, Juno and Vesta, studied atmospheric refraction, and started to re-investigate astrometric observations of Bradley (1692-1762).
In 1809, Bessel was appointed director of King Frederick William III of Prussia's new Königsberg Observatory, and as professor of astronomy at Albertus University in Königsberg, positions he should keep for the rest of his life. Bessel arrived in May, 1810, lectures started in summer, 1810, and the observatory was completed in 1813. In 1812, he was elected to the Berlin Academy of Sciences.
Also in 1812, Bessel married Johanna Hagen (1794-1885); they had one son, Wilhelm (1814-1840) and three daughters (Marie, 1816-1902; Elisabeth, 1820-1913; and Johanna).
The first instrumentaion of the Königsberg Observatory was purchased from the estate of amateur astronomer Friedrich von Hahn (1742-1805). In 1819, a meridian circle from Reichenbach was added, in 1829, a Heliometer built by Fraunhofer, an instrument suitable for very acurate position measurements, and in 1841, a meridian circle from Repsold.
During 36 years, Bessel was busily doing astronomical work in Königsberg, including very fundamental achievements, the most prominent being the first successful determination of a stellar parallax and distance. His students include F.W.A. Argelander, Carl August Steinheil and Heinrich Schlüter. Besides these activities, he was ordered to undertake a geodetical survey of East Prussia ("Ostpreussische Gradmessungen"); this work was performed together with J.J. Baeyer in 1831-1832, theoretically evaluated, and published in 1938. From the differences between geodetical and astronomical coordinates, Bessel derived the figure of Earth as an oblated spheroid with ellipticity 1/299.15 (Bessel Normal Ellipsoid). In 1939, his physical studies led to the introduction of a new Prussian measurement system.
Bessel also contributed significantly to mathematics and invented the so-called Bessel functions (also called cylindrical functions) in 1824, and to physics (potential theory, second pendulum).
Bessel suffered a severe loss when his son Wilhelm died in 1840 at age 27. In 1842, Bessel travelled to England and France, accompanied by his daughter Elisabeth, and visited several scientific convents and institutions. He died in Königsberg on March 17, 1846 at age 62 from a long mysterious disease which we now know was probably intestine cancer.
Friedrich Wilhelm Bessel was honored during his lifetime by academy memberships; besides Berlin, in Palermo, Petersburg and Stockholm, by memberships in the scientific societies of Edinburgh, Goettingen, Kopenhagen and London, the British Royal Astronomical and the Royal Meteorological Societies. Later, he was honored by the astronomical community by naming a moon crater after him (21.8N, 17.9E, 15.0 km diameter, in 1935). Asteroid (1552) Bessel was discovered on February 24, 1938 in Turku by Y. Vaisala; it had been provisionally designated 1938 DE1 and (from later independent findings) 1948 EH and 1951 UF.
Bessel's Contributions to Astronomy
Bessel's scientific works have been counted at 399 (Engelmann 1875-76). His contributions cover most of his contemporary astronomy; his particular issue was precision measurements.
His early works in Lilienthal include observations of comets, asteroids, planets, occultations and eclipses, as well as atmospheric effects and instrumental studies; most of them were published in Johann Bode's Berliner Astronomisches Jahrbuch.
His first major work in Königsberg was a reduction of Bradley's astrometric observations to a fixed date, 1755; this work was published in 1818 as "Fundamentae Astronomiae pro Anno MDCCLV deducta ex Observationibus viri incomparabilis James Bradley" (Foundations of Astronomy for the year 1755, deduced from the Observations of the incomparable man, James Bradley). This Latin-language work contained the reduced positions of 3,222 stars together with a complete theory of spherical astronomy and data reduction. Bessel extracted a list of 71 stars with notable proper motion.
With the new Reichenbach meridian circle, Bessel started the project to determine acurate positions for all stars to the 9th magnitude in the zone of declinations between +15deg and -15deg, in August, 1921, together with Argelander. In 1825, the range was extended to +45deg, and concluded in 1835 with a catalgue of 75,011 stars, organized in 536 zones. Later, Bessel's assistent Argelander continued this work to create the famous "Bonner Durchmusterung." Also in 1825, Bessel initiated the endeavour to create an acurate atlas, the "Akademische Sternkarten" (Academic Star Maps), carried out at various observatories and finished only in 1859. In 1833, Bessel published a catalog of 38 double stars, measured with the Fraunhofer heliometer.
Bessel was very interested in effects that can impact on acurate measurements, and studied precession, nutation, aberration and refraction. His results are summarized in the "Tabulae Regiomontanae reductionum observationum" (Königsberg Tables for reducing observations) of 1830; this work also contains the positions of Maskeleyne's 36 "fundamental stars" and Polaris from 1750 to 1850. Moreover, in 1821, he discovered the "Personal Equation," the effect of the observer's personality and circumstances on astrometrical measurements, and suspected variations of the obliquity of the ecliptic.
He also was concerned on the quality of his instruments, and effects of instrumental errors on observations, which he thought could be eliminated by expended data reduction. Engelmann counts 23 articles on his investigations of astronomical instruments for angular measurements.
Bessel's first contribution to astronomy had been on comet Halley's 1607 apparition, and he always stayed interested in comets, both by observing and by calculating their orbits; for this purpose, he improved the orbit calculation methods. Following the return of Comet Halley in 1835, which he had occasion to observe, Bessel developed the "Physical Theory of Comets," published 1836, stating that comets are mainly consisted of volatile matter. In 1839, he proposed methods to calculate meteoroid orbits from meteor observations.
His continued interest in planetary astronomy caused him to observe the orbits of the satellites of Jupiter and in particular, Saturn's moon Titan, with the Fraunhofer heliometer, resulting in acurate determinations of the masses of planets Jupiter and Saturn. In 1837, he investigated the disturbation theory of Uranus, and supported the hypothesis of a further planet. That planet was finally found in the year of Bessel's death, 1846, and named Neptune.
Bessel was the first to measure and publish a parallax, and calculate the distance to a star, double star 61 Cygni, from observations during 18 months in 1837 and 1838. Bessel's parallax value of 0.314", corresponding to a distance of 3.18 parsec or 10.4 light years, is very close to the modern value of 0.292", corresponding to 3.42 pc (11.2 ly). He had selected 61 Cygni because it had the largest known proper motion. Concerned ybout the acuracy of his parallax, Bessel re-determined the parallax of 61 Cyg in 1840, together with Heinrich Schlüter, yielding a somewhat less acurate value of 0.348", corresponding to 2.87 pc (9.4 ly). Concurrently, Thomas Henderson published a parallax for Alpha Centauri in 1839, derived from 1832-33 observations at the Cape of Good Hope, and in 1840, F.W.G. Struve of Dorpat presented his (less acurate) parallax for Vega from observations in 1835-1837.
In 1841, Bessel announced that Sirius and Procyon each had an invisible (to then) companion because of variations in their proper motion. An orbit for Sirius' companion, Sirius B, was calculated ten years later, and it was eventually found by Alvan Clark in 1862, Procyon B was not discovered before 1896, by Schaeberle with the 36-inch telescope of Lick Observatory. These two remarkable stars were later revealed to be White Dwarfs.