Background
Van Maanen was born on March 31, 1884, in Sneek, Netherlands, the son of Johan Willem Gerbrand van Maanen and Catharina Adriana Visser.
Heidelberglaan 8, 3584 CS Utrecht, Netherlands
Van Maanen received his Bachelor of Arts degree in 1906, Master of Arts in 1909, and Doctor of Science in 1911 from the University of Utrecht.
Van Maanen was born on March 31, 1884, in Sneek, Netherlands, the son of Johan Willem Gerbrand van Maanen and Catharina Adriana Visser.
Van Maanen received his Bachelor of Arts degree in 1906, Master of Arts in 1909, and Doctor of Science in 1911 from the University of Utrecht.
From 1908 to 1911 van Maanen worked at the University of Groningen, where he met J. C. Kapteyn. In 1911 he 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 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.
Nothing is known of van Maanen's family.