One customer was Joseph Ellicott,

One customer was Joseph Ellicott, a Quaker surveyor, who needed an extremely accurate timepiece to make correct calculations of the locations of stars. Ellicott was impressed with Benjamin's work and lent him books on mathematics and astronomy. Banneker began his study of astronomy at age 58. He was able to make the calculations to predict solar and lunar eclipses and to compile an ephemeris for the Benjamin Banneker's Almanac, which an anti-slavery society published from 1792 through 1797. He became known as the Sable Astronomer. Banneker and Ellicott worked closely with Pierre L'Enfant, the architect in charge. However L'Enfant could not control his temper and was fired. He left, taking all the plans with him. But Banneker saved the day by recreating the plans from memory. In early 1791, Joseph Ellicott's Quaker brother, Andrew Ellicott, hired Banneker to assist in a survey of the boundaries of the future 100 square-mile District of Columbia, which was to contain the federal capital city (the city of Washington) in the portion of the District that was northeast of the Potomac River. Because of illness and the difficulties in helping to survey at the age of 59 an extensive area that was largely wilderness, Banneker left the boundary survey in April, 1791, and returned to his home at Ellicott Mills to work on his ephemeris.