Charles Greeley Abbot (May 31, 1872 - December 17, 1973) was an American astrophysicist, astronomer and Secretary of the Smithsonian Institution. He was born in Wilton, New Hampshire.

Abbot graduated from MIT in 1894, and being skilled at laboratory work, he came to the attention of Samuel Pierpont Langley, who was looking for an assistant at the Smithsonian's Astrophysical Observatory. Under Langley, Abbot flourished as a creative designer and builder of delicate devices for measuring solar radiation. As Langley focused more and more on his aeronautical experiments, Abbot became responsible for maintaining the observatory's solar program, including an expedition to observe the 1900 solar eclipse in Wadesboro, North Carolina where Abbot applied a vastly improved bolometer to take readings of the Sun's inner corona. He was also a leading member of the American eclipse expedition to Sumatra in 1901. Abbot proved to be a reliable observer and impressed many astronomers who encountered him in various places. A music lover, Abbot sang and also played the cello.

He became director of the Smithsonian's Astrophysical Observatory in 1907. Abbot knew that the Smithsonian value for the solar constant was too high, but he carefully avoided the issue until he was in charge. Then, he quickly announced results from observations at Mount Wilson, California, that reduced the solar constant first to 2.1 and then to 1.93, largely through the introduction of improved, standardized methods and better thermal isolation for his pyrheliometers and bolometers and overall paid close attention to detail.

When Abbot retired as APO director and as Smithsonian Secretary in 1944, setting a precedent as the first Smithsonian Secretary not to die in office, most but not all of the great cataloguing projects were gone and the discipline was undergoing profound change. Problem-oriented research, informed by modern physical theory, dominated the discipline. Yet the Smithsonian's Astrophysical Observatory pursued its single mission all along, elaborating on its purpose not by a broadening of its astronomical base but by refining its instrumentation and technique, searching for evidence that Earth's meteorology and biology were intimately connected to variations in the Sun's output of energy. Although he eschewed physical theory, Abbot was thoroughly modern in his problem-oriented approach to research. Thus, his failure to broaden the astrophysical scope of the APO during his long tenure has to be appreciated as due to a complex set of factors centered on his singular sense of mission, which transcended disciplinary lines between astronomy, geophysics, meteorology, and biology.

Even before he assumed the directorship of the APO, Abbot was among the astronomical elite. In a 1903 census by the AAAS he was listed among the top thirty astronomers by his peers. Langley was among the first rank, and both scored even higher among physicists who were polled. Abbot won the prestigious Draper Gold Medal of the National Academy of Sciences in 1910 and the Rumford Medal of the American Academy of Arts and Sciences in 1916.

With his solar-constant critics vanquished, Abbot focussed more on Langley's ultimate goal: to search for evidence of variations in the solar constant and to show that these cycles influenced cycles in weather and climate. He believed that such evidence was already at hand from the findings of H. H. Clayton, the chief forecaster of Argentina and a colleague of A. Lawrence Rotch of the Blue Hill Meteorological Observatory outside Boston, Massachusetts. Clayton had excitedly written Abbot in 1912 with what he believed was proof that changes in the world's weather correlated with changes in the solar constant that he had gleaned from published Smithsonian data. Clayton soon became one of Abbot's closest allies, and over the next three decades, confirming these clues would define Abbot's mission.

In 1918 Abbot was designated Assistant Secretary of the Smithsonian under Walcott with responsibilities for the Smithsonian library and the venerable International Exchange Service. The latter was a world-wide clearinghouse for the diffusion of scientific literature, which had been set in motion by Joseph Henry and fostered by Congress to keep open lines of communication between governments. Abbot succeeded Walcott in 1928 and guided the Smithsonian through the Great Depression and World War II. Despite Abbot's extreme focus on the APO, by the late 1930s the Smithsonian had weathered the Depression intact but not undamaged. Walcott's campaign for a $10 million dollar endowment was too short-lived to be effective and after his death was not supported by the Smithsonian Regents.

During World War II, Abbot directed Smithsonian resources to the war effort, forming the Smithsonian War Committee to disseminate the Smithsonian's technical knowledge and expertise in fields such as aviation, entomology, geography, desert and Arctic conditions, and anthropology. The Smithsonian created a series of twenty-one pamphlets describing the lands where the war was being fought. Called "War Background Studies," they were published in the hundreds of thousands. The Smithsonian also joined the National Research Council, the State Department, and other governmental and private organizations to form the Ethnographic Board and the Institute for Social Anthropology, both housed at the Smithsonian, to use the social sciences for national goals. He also left the workings of the National Museum largely to his Assistant Secretary, Alexander Wetmore, who succeeded him as Secretary in 1944.

Dr. Abbot provided most of the research funds for the work of Robert Goddard, who was the developer of the liquid-fuel rocket and is considered the father of the space age. The USSR, having mapped the back side of the moon for the first time, wanted to name a major crater the "Charles Greeley Abbot Crater" and found at a meeting of the International Astronomical Union that this was not yet possible since he was still alive. At the next meeting an exception was made and the crater named for him. Dr. Abbot obtained his last patent at the age of 101, a sign of enduring competence and optimism. He was also the oldest inventor to ever receive a patent.