US astrophysicist who studied the rings of Saturn and the abundance and structure of nebulae. He demonstrated 1888 that nebulae resembled stars in their pattern of movement.

Keeler was born in La Salle, Illinois, and studied at Johns Hopkins University and in Germany at Heidelberg and Berlin. He was appointed astronomer at the Lick Observatory on its completion 1888; became professor and director of the Allegheny Observatory 1891; and returned to the Lick as director 1898.
In 1895, Keeler made a spectroscopic study of Saturn and its rings, in order to examine the planet's period of rotation. He found that the rings did not rotate at a uniform rate, thus proving for the first time that they could not be solid and confirming Scottish scientist James Clerk Maxwell's theory that the rings consist of meteoritic particles.
After 1898, Keeler devoted himself to a study of all the nebulae that William Herschel had catalogued a hundred years earlier. He succeeded in photographing half of them, and in the course of his work he discovered many thousands of new nebulae.