Bancroft Walker Sitterly, 81, professor emeritus of physics of American University and former chairman of the department of physics there, died Tuesday at his home in Washington after a long illness.

Born in Madison, N.J., he graduated from Princeton University. After serving as a civilian at the Aberdeen Proving Grounds in Maryland in World War I, he returned to Princeton to study astronomy, earning master's and doctorate degrees.

Mr. Sitterly, whose primary interest was the study of binary stars, was an instructor for a year at the University of Missouri and then taught at Wesleyan University in Connecticut until 1945.

During World War II, he also was a staff member at the radiation laboratory at the Massachusetts Institue of Technology.

After he was, Mr. Sitterly spent a year as an associate astronomer at the Naval Observatory here before joining the faculty of American University, where he served in the department of mathemcatics.

From 1955 to 1961, he was chairman of the department of physics, then continued an professor of physics until he retired in 1965.

Mr. Sitterly was a fellow of the Royal Astronomical Society in London and of the American Association for the Advancement of Science.

He belonged to the American Astronomical Society, the American Association of Physics Teachers, the Philosophical Society of Washington, the Washington Academy of Sciences and Phi Beta Kappa.

He is survived by his wife, Charlotte Moore Sitterly, of the home, and four sisters, Mrs. Albert C. Rogel, of Massapequa, N.Y., Mrs. Edwin G. Heitbrink, of Chatham, N.J., Mrs. Gobin J. Stair, of Kingston, Mass., and Mrs. Geranld T. Lorentz, of McConnellsburg, Pa.