1884-1946

Dutch-American astronomer known for his careful measurement of distant astronomical objects. Beginning in 1916 he claimed to have detected motion in certain spiral nebulae. If true, these spiral nebulae were thought to be within our own Milky Way and not galaxies distinct from it. But in 1932 these results were shown to be in error, and in 1935 he admitted his mistake. It was then shown that those nebulae were in fact independent galaxies.