Willem de Sitter was a Dutch astronomer and physicist born on March 6, 1872. He is best known for his work in cosmology and his contributions to the understanding of the universe's structure. De Sitter formulated the de Sitter universe, a solution to Einstein's field equations in general relativity, which describes a universe dominated by a cosmological constant. In addition to his theoretical work, de Sitter made significant contributions to observational astronomy. He served as the director of the Leiden Observatory and was involved in various international scientific organizations. His research laid the groundwork for future studies in cosmology and the expansion of the universe.