Astronomer and mathematician, born near Baltimore, Maryland, USA, the grandson of an Englishwoman and a freed black slave, but son of a slave father and freed black mother. He was allowed to attend a local elementary school, where he showed a talent for mathematics and science. Although his main occupation was as a farmer, he devoted his spare time to applied sciences, publishing an almanac (1792--1802) that used his astronomical and tide calculations and his weather predictions, along with proverbs, poems, and essays contributed by himself and others; this almanac was often cited by opponents of slavery as evidence of African-Americans' abilities. Thomas Jefferson had him hired in 1791 to assist the surveyors laying out the new capital and the District of Columbia. He in turn did not shrink from urging Jefferson to abolish slavery and to adopt more progressive policies for black Americans, of whom he was probably the best known in his day and for some decades after.