Van Maanen claimed that his astrometric measurements of spiral nebulae revealed detectable internal motions, supporting the idea that these nebulae were local, stellar and gaseous systems that existed in our galaxy.

The competing theories were the subject of the Great Debate between Harlow Shapley and Heber Doust Curtis in April 1920.

Curtis agreed that if van Maanen's results were correct, the spiral nebulae could not be distant galaxies.

Later astronomers re-examined van Maanen's measurements, and concluded that he had made a serious error.

As it was widely believed that the "spiral nebulae" were relatively nearby and therefore ought to have a detectable rotation, a result which appeared to support that would not have been subject to as much scrutiny as one which contradicted it.