The Smithsonian Astrophysical Observatory (SAO) is a research bureau of the Smithsonian Institution and part of the Harvard-Smithsonian Center for Astrophysics based in Cambridge, Massachusetts. SAO observing facilities are located around the world. The SAO was founded in 1890 by Samuel Pierpont Langley, the Institution's third Secretary, primarily to study the Sun and its influence on the Earth. Langley invented the bolometer and surveyed the infrared spectrum of the Sun. His successor, Charles Greeley Abbot, maintained Langley's mission to monitor solar radiation for some 50 years. In 1955, SAO moved from Washington, D.C., to Cambridge, Massachusetts, to affiliate with the Harvard College Observatory and to expand its staff, facilities, and, most important, its scientific scope. Fred Whipple, the first director of SAO in this new era, accepted a national challenge to create a worldwide satellite-tracking network, a decision that would establish SAO as a pioneer -- and leader -- in space science research. In 1973, the ties between Smithsonian and Harvard were strengthened and formalized by the creation of the joint Harvard-Smithsonian Center for Astrophysics (CfA).