Argelander, Friedrich Wilhelm August

(b. Memel, Prussia, 22 March 1799; d. Bonn, Germany, 17 February 1875)

astronomy.

Argelander’s father, Johann Gottlieb Argelander, was a wealthy merchant from Finland; his mother, Dorothea Wilhelmine Grünlingen, was German. The boy grew up in the easternmost corner of Prussia, and the Napoleonic Wars brought him into close contact with the major political events of his day. After the defeat of Prussia in the campaign of 1806. Queen Louise and her sons fled from Berlin to Memel. The crown prince, who later became Friedrich Wilhelm IV, and Prince Friedrich lived in the Argelander home, and young Argelander formed a lasting friendship with both of them. After attending the secondary school in Elbing and the Collegium Fridericianum in Königsberg, Argelander entered the University of Königsberg in 1817 in order to study political economy and political science. At this time he attended the lectures of the astronomer Friedrich Wilhelm Bessel and took an active part in the calculating work of the observatory at Königsberg. When the results of this work were published, Bessel referred to Argelander as one of his “most outstanding students.”

When he was twenty-one, Argelander was completely won over to astronomy by Bessel and was appointed an assistant in the observatory. He took part in a newly initiated project, designed to determine the exact positions of all the brighter stars of the northern sky between 15° southern declination and 45° northern declination; this was the first part of Bessel’s projected survey of the entire northern sky. In 1822 Argelander earned his doctorate with a study of the older Greenwich observations (which were made with sextant and quadrant), De observationibus astronomicis a Flamsteedio institutis, and in the same year he earned his lectureship with a study entitled Untersuchung über die Bahn des grossen Kometen von 1811 (“Investigation of the Orbit of the Great Comet of 1811”). In both these works he demonstrated his great proficiency in the critical evaluation of astronomical observations.

Bessel recommended him for the post of observer at the small, just rebuilt observatory in Åbo (now Turku), Finland. Thus Argelander, who had just turned twenty-four, went into the Russian civil service. Equipped with excellent instruments from the best German factories, so that the Åbo observatory could compete with Königsberg and Dorpat, Argelander made it famous as the northernmost station of the “three-point constellation” of east European observatories. Here he demonstrated his ability, later tested again and again, to achieve optimum results in spite of limited resources and an unfavorable climate. While Bessel continued to contribute to the knowledge of the positions of stars through observations of zones, Argelander dedicated his observatory to the study of the alteration of the positions in time—the “proper motions” of the fixed stars.

In 1718 Halley, by the comparison of older and more recent positions, had discovered that some brighter stars, such as Sirius, Arcturus, and Aldebaran, showed deviations that could be explained only through proper spatial motions and not through errors in observation. In 1760 Johann Tobias Mayer found that of eighty stars he investigated, fifteen or twenty showed proper motions of several arc-seconds in 100 years. Argelander devoted all his energies to this problem. In a few years he collected over 10,000 observations with the transit circle of several hundred stars that were suspected of proper motion. The catalog of 560 stars, which was based on these observations (1835), is incontestably the most exact of the contemporary catalogs.

Argelander was forced to abandon these observations in 1827 because of a fire that destroyed most of Åbo. Even if the observatory itself had been saved, a transfer of the entire university to Helsinki would still have been unavoidable. Argelander, who in 1828 was named full professor at the new university, was assigned the task of planning and building a new observatory. In 1832 he was finally able to move to Helsinki. Here his earlier work proved fruitful. The ingenious investigation Über die eigene Bewegung des Sonnensystems, hergeleitet aus den eigenen Bewegungen der Sterne (“Concerning the Peculiar Movement of the Solar System as Deduced From the Proper Motions of the Stars,” 1837) is one of the few theoretical works in which Argelander found the basis of his observations conclusive enough to make certain deductions.

Through all his other works Argelander has become renowned as a master of practical investigations of fundamental importance. It would, however, be wrong to disregard his theoretical ability. His foresight and critical strength protected him from the danger of drawing premature and insufficiently proved conclusions from imperfect material. Lalande had already concluded from theoretical considerations that the sun, like the other fixed stars, has a progressive motion in space in addition to its rotation. All other bodies of the solar system, the planets and comets, participate in this movement; therefore the peculiar motion of the sun is only to be recognized as relative to the fixed stars outside the solar system. The motions of the fixed stars consist of the apparent perspectivic changes in position, which are caused by the motion of the sun, and of the real, progressive motions of the fixed stars through space. Only the most exact observations of a greater number of stars can allow the separation of these two aspects.

In 1783 Sir William Herschel had deduced, on the basis of the proper motions of only seven bright stars, that the apex of the movement of the sun was somewhat north of the star λ Herculis. A repetition of his calculations in 1805/1806 by the aid of a somewhat larger number of stars yielded data that did not essentially deviate. While Herschel himself had made no observations of his own for this purpose and had frequently encountered doubt and contradiction, Argelander saw in his observations an opportunity to derive anew the motion of the sun. He felt this would be worthwhile because of the progress both in the construction of instruments and in critical calculation methods that, as far as possible, freed observations from all accidental and systematic errors. His result, based on no fewer than 390 proper motions, verified the accuracy of Herschel’s pioneering work. The spatial movement of the sun has since then been proved beyond doubt.

It would have been obvious to a confirmed theorist that a further question should be asked: What is the law that governs the motion of the sun and the other stars through space? Are all bodies, Argelander asked, subject only to their mutual attractions, or do all of them obey the attractive force of a large central body? In other words, is the solar system only the smaller model of a larger, similarly constructed stellar system? Argelander did not address himself to this question; he did not indulge in idle speculation, as did other astronomers who claimed that now Sirius, or again the Pleiades, or even a central sun as yet unknown was the center of the realm of the stars. His merit lies much more in the recognition of the significance of the dynamics of the stellar system and in his provisions that enabled later astronomers to solve the problem. Everything he did in the next four decades served to increase the knowledge of the positions and proper motions of the stars, in the hope that someday a second Kepler might be successful in revealing the more complex laws of the orbits of the fixed stars. In November 1836 Argelander was appointed to the professorship of astronomy at the Prussian University of Bonn. He was also promised a new, richly fitted observatory, and his old friend the crown prince, who ascended the throne several years later, supported the project.

During his first years at Bonn, Argelander worked in an old bastion situated directly on the Rhine, where a provisional observatory was erected. Under these limitations Argelander again showed himself to be the ingenious improviser who, even with modest resources, could achieve lasting results. Although without measuring instruments, he created in these years one of his finest works, the Uranometria noi’va (1843). The main feature of this work was not the determination of exact positions, but the recording of all stars visible to the naked eye and a settlement of the nomenclature that had been used arbitrarily up to that time, as well as a demarcation of the constellations of the stars. At the same time, this atlas and the accompanying catalog fulfilled the task of a reliable representation of the magnitudes of the stars.

The exact observation of stellar magnitudes was not yet possible in Argelander’s time because there was no suitable photometer. By means of the method of “estimation by steps,” developed by Argelander, it was nevertheless possible, even without instruments, to obtain reliable magnitude data with an exactness of about one-third of a magnitude class. For this purpose the magnitudes were, by means of mutual comparison with neighboring stars of slightly different magnitudes, arranged in an arbitrary scale which could be gauged with the aid of stars of known magnitudes. This simple method, which is based on the ability of man’s eyesight to perceive very slight differences in brightness, proved itself especially useful in the investigation of the changing brightness of variable stars, of which only eighteen were known at that time. By qualitatively determining the changing brightness of these stars, Argelander opened a completely new field of research which soon earned an important place in the working program of many astronomers; and, at his suggestion, it became of interest to many amateurs as well. It has to a considerable degree enriched our knowledge of the physical nature of the fixed stars. Argelander devoted himself to the continuous observation of these stars and inspired a similar zeal in his students. At the time of his death, the number of variable stars with known period of changing light had increased to almost 200.

When a five-foot transit instrument, the first larger instrument ordered for the new observatory, arrived from Ertel, it was immediately set up at the provisional observatory. With it Argelander extended the work begun by Bessel at Königsberg (which reached only to 45° up to 80° northern declination. In more than 200 nights he measured the exact positions of 22,000 stars. The work was later continued on the same instrument at the new observatory to the south from 15° southern declination up to 31° southern declination with 17,000 additional stars. With this work, in conjunction with the Besselian zones, a first, although not wholly complete, inventory of the entire sky visible in the northern latitudes was made. This inventory included many faint stars which were invisible to the naked eye, but it was not completed to a limited magnitude, which is necessary to permit the investigation of certain questions of stellar statistics. Argelander stands out above all other astronomers of his century not only because of his perception of this important problem but also because he followed with all his energy the only practicable way to its solution. In order not to stretch this undertaking to an impossible extreme, he set the limit of completeness at stars of the ninth magnitude. His goal was a uniform registration of all stars up to this magnitude and the cataloging of their positions and magnitudes with an exactness sufficient for further identification.

From these considerations there arose the Bonner Durchmusterung, which has since provided the working basis for every observatory. It consists of a three-volume catalog of stars and a forty-plate atlas in which are recorded the positions (exactness ±0.1’) and magnitudes (±3.0^m) of 324,198 stars between the northern celestial pole and 2° southern declination. The observations were made with a very small instrument, a so-called comet seeker with only a 7.8-centimeter aperture and a 63-centimeter focal distance with a ninefold magnification. The observations, which lasted over a period of 625 nights, from 1852 until 1859, extended for terms of five, six, seven, eight, and occasionally twelve hours, were mostly made by his assistants Eduard Schönfeld and Adalbert Krüger. The plan of observation, thought out to the smallest detail, comprised not only the tremendous number of individual observations, which exceeded a million, but also the extensive revisions of doubtful cases, which were carried out by Argelander himself on the meridian circle. In addition, it included the detailed calculations for the final catalog and for the plates of the atlas. Thus the results, despite the small circle of collaborators, were published after a few years and became the foundation for all future astronomical work.

The next step was the improvement of the preliminary determination of the positions of stars, utilizing the high development of the art of measurement, but it could no longer be the task of a single observatory.

Therefore, in 1867 Argelander proposed to the Astronomische Gesellschaft that several observatories acting as a team undertake according to a uniform plan the observation of the most exact positions possible on meridian circles of all stars in the Bonner Durchmusterung up to the ninth magnitude. Argelander lived to see the project, in which seventeen observatories took part, set in motion in the 1870’s.

Some idea of Argelander’s renown as an astronomer may be gained from the fact that he was a member of the academies of St. Petersburg, London, Berlin, Stockholm, Paris, Vienna, Boston, and Brussels; of the Societas Fennica in Helsinki; of the Royal Astronomical Society of London; of the National Academy of Sciences of the United States; and a charter member of the Astronomische Gesellschaft, of which he was a member of the governing body from 1863 to 1871 and chairman from 1864 to 1867.

Argelander’s achievement does not lie in substantial discoveries, but in the single-minded and systematic planning of his lifework and his unique resoluteness and skill in its execution. His main endeavor consisted in offering extensive and complete numerical data for the study of the construction of the heavens and the motions of the fixed stars in the stellar system. Without his work, which was continued at the Bonn Observatory, by Schönfeld and Küstner, the knowledge attained in our century about the structure of the universe would not have been imaginable.

BIBLIOGRAPHY

1. Original Works. Argelander’s writings include De observationibus astronomicis a Flamsteedio institutis, his dissertation (Königsberg, 1822); Untersuchung über die Bahn des grossen Kometen von 1811 (Königsberg, 1822); DLX stellarum fixarum positiones mediae ineunte anno 1830 (Helsinki, 1835); Über die eigene Bewegung des sterne systems, hergeleitet aus den eigenen Bewegungen der Sterne (St. Petersburg, 1837); Uranometria nova (Berlin, 1843), with seventeen charts and a catalog of stars; Bonner Sternverzeichnis, 3 vols. (Bonn, 1859–1862), Vols. III-V in the series Astronmische Beobachtungen auf der Kördlichen gestirten Sternwarte zu Bonn; and Atlas des nördlichen gestirnten Himmels für den Anfang des Jahres 1855 (Bonn, 1863), with forty charts.

II. Secondary Literature. Works providing information about Argelander include B. Sticker, Fr. W. Argelander und die Astronomie vor hundert Jahren (Bonn, 1944); E. Schönfeld, obituary, in Viertelijahrsschrift der Astronomische Gesellschaft, 10 (1875), 150–178; articles on him in Allgemeine deutsche Biographie, Vol. XLVI; Neue deutsche Biographie, Vol. I; and Westermanns Monatshefte, 51 (1906), which concerns Argelander’s family.

Bernhard Sticker