results of van Maanen were correct, Shapley was right. But Curtis rejected the results of van Maanen due to unrealistic accuracy claimed by him. Later astronomers have re-examined the measurements of Van Maanen, and concluded he had made a serious error. The origin of van Maanen's measurement errors emerged from his stereo blink comparator, which van Maanen used to compare new plates with plates some 10–20 years old. By blinking between those two plates he could detect small discrepancies on the positions of the objects. His reference objects were the field stars fairly near the edge of the plates. However he didn't consider that due to optical effects these stars had been smeared out a little towards the edges, slightly differently between the two plates. This caused systematic errors resulting in imaginary movements. Another possible explanation is that van Maanen simply saw what he had been trained to see for years: that the "spiral nebulae" were relatively nearby and therefore ought to have a quite detectable rotation. This belief was quite widespread in the early 20th-century and therefore very hard to ignore. This does not, however, explain supposedly corroborating findings from Mount Wilson, Lowell Observatory, Russia, and the Netherlands. References Notes Further reading External links Angular Rotations of Spiral Nebulae, Hubble, E., ApJ, 81, 334 (1935) – NADS Internal Motions in Spiral Nebulae, Van Maanen, A., ApJ, 81, 336 (1935) – NADS Adriaan van Maanen and internal motions in spiral nebulae: A historical review, Quarterly Journal of the Royal Astronomical Society, Vol. 13, 1972 Adriaan van Maanen on the significance of Internal Motions in Spiral Nebulae, Hetherington, N.S., J Hist Astron, 5. 52 (1974) – NADS On Spiral Nebulae, van Maanen et al. Chronological bibliography of articles on astrometrical measurements of spiral nebulae. 1884 births 1946 deaths 20th-century American astronomers American people of Frisian descent 20th-century Dutch astronomers People from Sneek Dutch emigrants to the United States Members of the Royal Netherlands Academy of Arts and Sciences Utrecht University alumni Academic staff of the University of Groningen