Maanen, Adriaan van

(b. Sneek, Netherlands, 31 March J884; d. Pasadena. California, 26 January 1946)

astronomy.

Descended from a long line of aristocrats, van Maanen was the son of Johan Willem Gerbrand and Catharina Adriana Visser van Maanen, He received his B.A. (1906), M.A. (1909), and Sc.D. (1911) from the University of Utrecht. From 1908 to 1911 he worked at the University of Groningen, where he met J. C. Kapteyn. In 1911 van Maanen joined the Yerkes Observatory as a volunteer assistant; and in 1912, on Kapteyn’s recommendation, he was appointed to the staff of the Mt. Wilson Observatory. His job there —to measure the proper motions and parallaxes of stars—employed the skills he had acquired while working on his thesis, “The Proper Motions of the 1418 Stars in and Near the Clusters h and x Persei.”

From its completion in 1914, van Maanen used the sixty-inch telescope at the eighty-foot Cassegrain focus for parallax determinations, the first such use of a reflector for such delicate measurements. In the 1920’s he also began using the 100-inch reflector at the forty-two-foot Newtonian focus. To measure either parallax or proper motion from a photograph, van Maanen employed a stereocomparator. After superimposing sets of comparison stars, he measured the distance separating the two images in question with a movable micrometer thread.

Van Maanen’s study of the parallaxes of more than 500 stars can be divided into two parts. He used one group, comprising stars of apparent magnitude +5 to +7 with moderate proper motions, as a distance standard for measuring spectroscopic parallaxes of more distant objects. Another group, made up of faint stars with great proper motions, led to better under standing of the luminosity function. During these observations he discovered the second known white dwarf (now named “van Maancn’s star,”), which yielded new information about such objects. A skillful determiner of parallaxes, his average probable error in those measurements was given as ±0.˝006.

Van Maanen studied the proper motions of planetary nebulae, globular and open clusters, faint stars in or near the Orion nebula, near bright stars with large proper motions, faint stars in forty-two of Kapteyn’s Selected Areas, and spiral nebulae. These studies yielded important fundamental information about several then puzzling phenomena, among them measures of the distances and absolute magnitudes of planetary nebulae, identification of stars as members of the Orion system and of the Pleiades and h Persei clusters, and rudimentary distances to 125 Cepheid variables.

In 1916 van Maanen published the results that he derived from the displacements of eighty-seven nebular points on two pairs of plates of M10I. He detected a rotation rate of 0.˝02 per year at a distance of 5’ from the nucleus, a finding he checked by having Seth Nicholson, a meticulous observer, measure half the points. All nine of the spirals that van Maanen eventually investigated seemed to show motions outward along their arms. When Knut Landmark in 1927 measured van Maanen’s plates of M33, he found a rotational component only one-tenth as large as van Maanen’s; moreover, van Maanen himself later obtained rotations only half as large as his earlier ones, But although other information strongly indicated that spirals are remote (and hence could not be rotating as fast as he had found), he continued to trust his calculations. In the famous Shapley-Curtis debate in 1920, Shapley, van Maanen’s lifelong friend, cited van Maanen’s findings as proof that spirals are relatively nearby. Fifteen years later, after plates of several spirals had been taken at longer intervals, Edwin Hubble and van Maanen published papers in Astrophyskai Journal stating that van Maanen’s results on rotations had been incorrect, apparently because of systematic errors.

Van Maanen also attempted to measure the general solar magnetic field, an undertaking begun in 1908 by G. E. Hale. The technique involved measuring the weakly polarized components of Zeeman-split lines. Several staff members at Mt. Wilson worked on the project, but the reduction of the observations was specifically van Maanen’s responsibility. Initially he found an overall field strength of roughly fifty gauss, which was later revised to about twenty. Recently J. O, Stenflo analyzed van Maanen’s plates by computer and found that van Maanen’s visual measurements apparently had again involved systematic errors. It should be noted that the field strengths being measured are so slight (about 1 gauss) that no one was able to determine them reliably until new techniques were introduced in about 1952.

Van Maanen’s entire career dealt with visually measuring almost imperceptible changes on photographic plates. In parallax observations he ranked high, but his results on the proper motions of spirals and the magnetic field of the sun were largely incorrect. Numerous explanations have been offered as to why he made such errors. Whatever the reason, the fundamental fact remains that the changes he was attempting to measure were at the very limits of precision of his equipment and techniques. Ironically, even though some of his findings caused confusion for over a decade, they also provided a healthy catalyst for research and discussion and demonstrated the ever-present dangers of systematic error, for even a careful observer.

BIBLIOGRAPHY

I. Original Works. Van Maanen published more than 150 papers, about fifty of them in the Contributions from the Mount Wilson Solar Observatory. Aside from research papers he also wrote popular pieces and biographical sketches and collaborated in the publication of Hemel en Dampkring. Many of his most important papers appeared in Astrophysical Journal, among them “List of Stars With Proper Motion Exceeding 0.˝50 Annually,” 41 (1915), 187–205; “Preliminary Evidence of Internal Motion in the Spiral Nebula Messier 101,” 44 (1916), 210–228; and “The General Magnetic Field of the Sun. Apparent Variations of Field-Strength With Level in the Solar Atmosphere,” 47 (1918), 206–254, written with G. E. Hale, F, H, Scares, and F. Ellerman.

II. Secondary Literature. Obituaries of van Maanen include Dorrit Hoffleit, in Sky and Telescope5 (1946), 10; Alfred H. Joy, in Popular Astronomy,54 (1946), 107–110; and Frederick H. Scares, in Publications of the Astronomical Society of the Pacific,58 (1946), 88–103, with portrait.

Van Maanen’s errors in the determination of the rotations of spirals have been discussed by numerous investigators, including Knut Landmark, in Uppsala Astron. Obs. Mcdd., no. 30 (1927); Edwin Hubble, “Angular Rotations of Spiral Nebulae,” in Astrophysical Journal,81 (1935), 334–335; Walter Baade, in C. Payne-Gaposchkin, ed., Evolution of Stars and Galaxies (Cambridge, Mass., 1963), 28–29; Harlow Shapley, Through Rugged Ways to the Stars (New York, 1969), 50, 55-57, 80; and J. D. Fernie, “The Historical Quest for the Nature of the Spiral Nebulae,” in Publications of the Astronomical Society of the Pacific,82 (1970), 1212–1213, 1218–1219.

Commentaries on the difficulties of measuring the general solar magnetic field and van Maanen’s attempts to do so arc given by K. O. Kiepenheuer, in G. P. Kuiper, ed., The Sim (Chicago, 1953), 361 ff.; C. de Jager, in Handbuch der Physik, II (Berlin, 1959), 340 IT.; Einar Tandbcrg-Hanssen, Solar Activity (Waltham, Mass., 1967), 75 ft,; and Jan Olof Stenflo, “Hale’s Attempts to Determine the Sun’s General Magnetic Field,” in Solar Physics,14 (1970), 263–273.

Correspondence with and about van Maanen is contained in “The George Ellery Hale Papers, 1882–1937” (Pasadena, Calif., 1968), microfilm ed., California Institute of Technology.

Richard Berendzen

Carol Shamieh