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Adriaan van Maanen (March 31, 1884, Sneek – January 26, 1946, Pasadena) was a Dutch-American astronomer.

Van Maanen, born into a well-to-do family in Friesland, studied astronomy at the University of Utrecht (earning his Ph.D. in 1911) and worked briefly at the University of Groningen. In 1911, he came to the United States to work as a volunteer in an unpaid capacity at Yerkes Observatory. Within a year he got a position at the Mount Wilson Observatory, where he remained active until his death in 1946.

He discovered Van Maanen's star.

He is well known for his astrometric measurements of internal motions in spiral nebulae. Being of the belief that nebulae were local, stellar and gaseous systems that existed in our galaxy, his measurements came to be at odds with Edwin Hubble's discovery that the Andromeda Nebula and other spiral nebulae were extragalactic objects. The speed at which he calculated the nebulae to rotate would have had the Cepheid stars, that Hubble had used to calculate the distance to the spirals, moving at speeds faster than that of light. In 1935, it was decided that since Hubble's calculations of extragalactic Cepheid distances were correct van Maanen's astrometric measurements had to be incorrect.

What was the origin of Van Maanen's measurement errors? Van Maanen used a stereo blink comparator comparing new plates with plates some 10–20 years old, and 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 different for the two plates. This caused systematic errors resulting in apparent movements which weren't real.

Another possible explanation is that Van Maanen simply saw what he had been trained to see for years. The belief that the "spiral nebulae" were relatively nearby and therefore ought to have a quite detectable rotation was quite widespread in the early 1900s and therefore very hard to ignore. Van Maanen then simply found what he was looking for.

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