He determined the correct position of about fifty thousand stars: who is Friedrich Wilhelm Bessel?

He was born on July 22, 1784, in Minden, Prussia. He was the son of a civil servant with nine children, six girls, and three boys. He could only continue his secondary education, which he started in Minden, for four years. As he later said in an article about his childhood years, he was not a good student. Apart from his interest in mathematics and physics, he could not show extraordinary talents and success in any subject until the age of 15 and had difficulties in Latin lessons. However, later on, he learned this language well enough to write some of his books in Latin by working on his own.

Friedrich Wilhelm Bessel (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. Certain important mathematical functions were named Bessel functions after Bessel's death, though they had originally been discovered by Daniel Bernoulli before being generalized by Bessel.

In 1799, at the age of 15, he left school and became an apprentice in a trading house in Bremen. In a short time, he became so proficient in accounting and accounting that, according to the law, apprentices were required to work for seven years without pay, but at the end of the first year they gave Bessel a small monthly salary. Thus, gaining his financial independence, Bessel trained himself in every subject he was interested in by constantly working at night. He learned geography, Spanish, and English in order to be successful in foreign trade; He also read books on shipping in order to become a cargo officer on merchant ships. The problem of locating a ship at sea with a sextant instrument measuring the height of celestial bodies caught Bessel's interest in astronomy. Since sailing by looking only at the position of the stars without knowing the basics of astronomy could not satisfy him, he was able to study astronomy and mathematics and determine latitude and longitude in a short time.

Bessel, who began his early astronomy studies by determining the longitude of Bremen and observing a star's eclipse with the Moon, read an article on Halley's comet's transit in 1607 and considered calculating the comet's orbit based on these observations. After reading the works of Lalande and especially the German astronomer Heinrich Wilhelm Matthaeus Olbers (1758-1840), who worked on comet orbits, he presented his work, which he finished in 1804, to Olbers. Olbers was impressed by the consistency between Bessel's conclusions and Halley's calculations and suggested that Bessel further his work on this trajectory by making additional observations. The article, which was published in the journal Monatliche Correspondenz, at the end of Olbers's suggestion, was at the level of a doctoral thesis, and it had a great impact especially when it came from the pen of a self-taught, astronomy enthusiast twenty-year-old young man. This event marked a real turning point in Bessel's life and set the stage for the growth of one of the greatest astronomers of that era. Indeed, Olbers said that his greatest contribution to astronomy throughout his life was to encourage Bessel to become a professional astronomer.

In the first months of 1806, Bessel left his old job before his apprenticeship had expired and, with the support of Olbers, became an assistant at J H Schröter's (1745-1816) observatory in Lilienthal, near Bremen. During his four years at Lilienthal, he gained hands-on experience in observing comets and planets, particularly observing Saturn with its rings and moons. At the same time, he accelerated the studies of celestial mechanics and contributed to the determination of comet orbits.

When the King of Prussia Friedrich-Wilhelm III wanted a large observatory to be established in Königsberg (today Kaliningrad in the USSR), Bessel, who was appointed as the director and professor of astronomy to this observatory upon Humboldt's suggestion, took office on May 10, 1810. A committee at the University of Göttingen, chaired by Gauss, who had known Bessel in Bremen three years before that date and recognized his extraordinary talent, immediately granted the title of "doctor", which was necessary for Bessel's promotion to professor, without any formality.

Bessel had two sons and three daughters from his marriage to Johanna Hagen in 1812. He lived in Königsberg until the end of his life, led a quiet and happy life, though marred by the death of his two sons at a young age, and was in constant correspondence with Olbers and Gauss.

Bessel, whose health began to deteriorate towards 1840, made his last trip to England to attend a congress meeting. He was thrilled to meet the great British astronomers, including Herschel. When he returned to his country, with the enthusiasm of this meeting, he finished and published a series of works, regardless of his deteriorating health. After suffering for two years, he died of cancer in Königsberg on March 17, 1846, and was buried next to the observatory, which he directed for thirty-six years, starting from his founding work. He trained valuable astronomers such as Bessel Argelander, who was elected a member of the Berlin Academy of Sciences in 1812 and a member of the French Academy of Sciences in 1840.

Bessel, whom Newcomb would describe as the "founder of the applied German school of astronomy" at the beginning of the 20th century, began his astronomical observations to determine the positions of the stars with the most accurate measurements. Observation errors at that time were largely due to the inadequacy of measuring instruments, and Bessel worked meticulously to reduce the errors as much as possible by improving the instruments used.

A second and important reason for the misconceptions was that not all of the Earth's movements had yet been determined with sufficient precision. As a matter of fact, the subject of Bessel's first work, which was published in 1815 and awarded by the Berlin Academy, was to determine the values of some parameters that determine the motion of the Earth in space, according to Newton's theory.

Relying on Bessel's meticulousness of observation and accurate measurements, Olbers asked Bessel to reconsider Bradley's observations of the positions of some thirty-six bright stars between 1750 and 1762. Bessel also determined the location of Bradley's stars very accurately and precisely, correcting any measurement inaccuracies caused by the inadequacy of the observing instruments or the misleading effects of the atmosphere.

Determining the correct position of about fifty thousand stars, one of Bessel's greatest achievements was his very precise measurement of the distance of fixed stars from Earth by measuring stellar parallax angles, the measurement of which has occupied astronomers for nearly three hundred years. In the early 19th century, although Neptune and Pluto had yet to be discovered, the dimensions of the solar system were approximate, but the stars were thought to be immeasurably distant. It was thought that a star that was relatively closer to the Earth than other stars would shift relative to distant stars, and this was called the parallax angle. At the beginning of the 18th century, Bradley tried parallax measurements, but he was unable to measure the parallax of any star, despite discovering a physical phenomenon such as the declination of light during his research. Thus, after it was understood that the angles to be measured were too small, the measurement methods and which stars to choose were reconsidered. It was Herschel who tried to systematically measure parallaxes since then, but he too found many binary star systems but could not measure a single parallax.

Bessel was also a highly valued mathematician. Today, he expanded the boundaries of theoretical mathematics by defining and applying a special function known as the Bessel function, which is one of the indispensable methods of applied mathematics, physics, and engineering sciences.