His work spanned astronomy’s transition from a focus on observation and location measurement to astrophysics.

His career was a fruitful combination of research and teaching at major observatories, in his own classrooms and through his textbook.

After receiving his Master of Arts in 1906, he began his doctoral studies at the University of California under the director of the Lick Observatory William Wallace Campbell,[1] and in 1909 defended his dissertation on the Cepheids Y Sagittarii and RT Aurigae.

[3] Much of his later work was closely linked to the Mount Wilson Observatory, which he visited for the first time in 1920–21, and where he spent the summers from 1922 to 1949 as a volunteer.

[4] For his 1920 photograph of the Eagle Nebula, he provided the first published description of the structures now known as the Pillars of Creation.