Results of the Gaia Space Telescope were published on December 3, 2020. In the press release, the 19th century astronomer of Finnish background, Friedrich Wilhelm August Argelander (1799–1875) was also mentioned. Who was he and how is an ancient astronomer related to Gaia?
After the Finnish War (1808–1809), an astronomical observatory and an observer’s post were established at the Imperial Academy of Turku in 1817. After the sudden death of the first holder of the post, Henrik Johan Walbeck, in 1822, the Academy hired Argelander, who had recently completed his doctoral thesis supervised by Friedrich Bessel in Königsberg, Prussia (now Kaliningrad, Russia). He had Finnish roots: he was born in Memel, Prussia (now, Klaipėda, Lithuania), but his father, merchant, was born in Pernaja, Finland, and the Argillander priestly family originated from Northern Savonia.
Vigorously, Argelander undertook to arrange the commissioning of the instruments acquired for the observatory and to launch observation activities. In 1827, the Great Fire of Turku interrupted the work. Argelander’s notes on the night of September 4-5 can be found in the Observatory's diary: 'These observations were interrupted by a terrible fire that wiped out almost the entire city to ashes but left the observatory untouched thanks to God.'
Three days later, when the smoke was diminished, one can find the following words in the logbook: 'The most beautiful aurorae I have ever seen.'
Argelander was a skilful and hard-working observer. By 1831, he had observed the exact coordinates of 560 stars before the observation equipment was transferred to Helsinki, in the new observatory of the Imperial Alexander University in Finland. In 1835, Argelander published his Turku observations in the book DLX stellarum fixarum positiones mediae ineunte anno 1830 (560 star positions according to 1830), also known as Catalogus Aboensis.
Argelander observed the small changes in the positions of stars with extreme precision over five years. He combined them to observations made by other observers in previous decades, and was able to calculate the proper motions of the stars, i.e. their motion relative to each other. From these, he determined the direction of the motion of our Solar System.
This was the first time that the direction of the movement of the Solar System could be determined from the observations. Everyone can practice this in a moving boat: observe the directions of other ships and boats and the rates of change of directions. The direction of one's own movement can be determined from these observations.
In 1837, Argelander was invited to Bonn as professor of astronomy and Director of the Bonn Observatory. There he completed his second major work, a 324,198 star Catalogue, the Bonner Durchmusterung.
Gaia satellite measurements of stellar motions are basically similar to Argelander's measurements 190 years ago, though the accuracy and number of stars has increased tremendously. Argelander would hardly have imagined measurements and accuracy like Gaia in his wildest dreams, but as a pedant and a diligent observer he would certainly be pleased with what has now been achieved.
Department of Geodesy and Geodynamics