Willem de Sitter. Portrait of Willem de Sitter (1872-1934), Dutch astronomer & mathematician. De Sitter studied the motion of stars and the moons of Jupiter. In 1916 Einstein published his theory of relativity and De Sitter's interest in it led to the theory being publicised in English- speaking countries. Einstein had found a solution to the relativity equations that yielded a static universe. De Sitter demonstrated that the equations were also solved by an expanding universe that contained no matter. De Sitter's solution was modified to produce a description of an expanding universe with matter; this model is known as the Einstein-de Sitter universe