he taught for another 14 years at the University of Arizona. The first edition of his widely-used textbook appeared in 1916; the fifth edition appeared in 1955. His Lick Ph.D. thesis on the orbits of two cepheid variables was the first step toward the abandonment of the widely-held binary star theory of cepheid variation. During a 28-year association with the Mount Wilson Observatory he measured the expansion of the Crab Nebula (1921), discovered three variable stars in external galaxies (1922) and took a superb series of pictorial photographs of galaxies, nebulae and Milky Way fields. (M 12457) Name proposed by F. K. Edmondson.

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