BornHelsinki, (Finland), 29 May 1841
Hugo Gyldén, director of the Stockholm Observatory, was a leading theorist of celestial mechanics and planetary perturbations. He was born into the family of professor Nils Abraham Gyldén and baroness Beata Sofia Wrede. Gyldén was admitted to the University of Helsinki and earned his doctoral degree in 1861.
Gyldén’s academic teacher, Lorenz Leonard Lindelöf, guided the young scholar into celestial mechanics. Gyldén then went to Gotha in Germany (1861–1862) as a postdoctoral student of Peter Hansen , one of the leading researchers in celestial mechanics. There, Gyldén drafted a dissertation on the orbit of the planet Neptune, which had been discovered 15 years earlier.
To continue his studies, Gyldén relocated to the Pulkovo Observatory in Russia on a grant from the University of Helsinki. There, he determined the declinations of fundamental stars with the vertical circle. In this work, Gyldén had to take into account refraction caused by the Earth’s atmosphere. In turn, he developed a new model of refraction, and with it drafted improved refraction tables that were widely used afterward. In 1863, Gyldén was appointed a “permanent astronomer” at the Pulkovo Observatory. He married Therese Amalie Henriette von Knebel in 1865; the couple had four children.
At the Pulkovo Observatory, Gyldén did not neglect celestial mechanics. He began to develop the theory of perturbations. In actual practice, the necessary calculations became insurmountably lengthy. Gyldén tried to shorten the calculations by the use of elliptic functions. With the help of these and suitable differential equations, he was able to make the series converge faster than before, so that there were not so many terms to be calculated. Gyldén implemented these methods in the 1870s and applied them to the orbits of periodic comets.
In 1871, the Royal Swedish Academy of Science offered Gyldén the directorship of the Stockholm Observatory. There, he actively developed the observatory and its instruments while continuing his research on celestial mechanics. Gyldén’s aim was to find mathematical forms describing the orbits of the planets, and with the help of these forms to account for their motions during arbitrarily long periods of time. In this way, it would be possible to answer the question of whether the Solar System has a permanent structure.
At first, Gyldén replaced the elliptical orbits of the planets with curves of higher order. In these intermediate orbits, as he called them, disturbances caused by other planets were taken into account. Soon, however, Gyldén noticed that an intermediate orbit was not accurate enough. He then tried to find as general a form as possible for the orbits of the planets, which he called absolute orbits. While an ordinary elliptical orbit was determined by six constants, the “orbit constants” of an absolute orbit must be expressed by time-dependent periodical functions. Gyldén hoped to show that no deviation of a planet’s orbit, beyond a certain small value, could ever occur.
Gyldén intended to publish his research on orbital theory as a three-volume work; the first volume was printed in 1893. But he fell ill and died before the second volume could be completed; it was published posthumously in 1908. Afterward, it was demonstrated that Gyldén’s notions concerning the existence of absolute orbits are not binding. Nonetheless, his accomplishments in the field of celestial mechanics are undeniable, and influenced other investigators, such as Marie Andoyer .
Gyldén was a delegate to the Astrographic Congress in Paris (1887), at which the Carte du Ciel project was launched. Many scientific societies and academies appointed him an honorary or corresponding member. Gyldén was also a member of the board, and finally the chairman, of the international organization of astronomers, the German Astronomische Gesellschaft.