American astronomer who made the first observations of the flash spectrum of the Sun, during the solar eclipses of 1869 and 1870.

He studied the Sun extensively, particularly with the spectroscope, and wrote several important books on astronomy, of which the best known was General Astronomy (1888). In 1879 he made accurate measurements of the diameter of Mars. He was professor of astronomy at Princeton University from 1877 to 1905.