Willem de Sitter was born on May 6th, 1872 and died on November 20th, 1934. His parents were Lamoraal and Catherine de Sitter. His family had a tradition of becoming lawyers and while he was expected to follow the family tradition he became a mathematician and astronomer. In his lifetime de Sitter received several awards including the Gold Medal of the Royal Astronomical Society. The crater de Sitter on the moon and an asteroid were named in his honor.
He based his models of the universe on Albert Einstein's general theory of relativity. De Sitter studied math at the State University of Groningen and joined the astronomical laboratory there. Under J.C. Kapeteyn's guidance he developed a liking for astronomy. He spent several years in South Africa at the Cape Observatory. Later de Sitter became a professor of astronomy at the University of Leiden. In 1919 he became director of the Leiden Observatory where he analyzed the motions of Jupiter's four Galilean satellites in attempt to determine their masses. He published several papers in London that led to Arthur Eddington's expedition to observe a solar eclipse to measure the gravitational deflection of light rays passing near the Sun.
In de Sitter's work he concluded that the universe is constantly expanding independently of matter. Later Einstein and de Sitter published a joint paper called the Einstein-de Sitter model of the universe in 1932. In their paper they present the concept of "dark matter" where light has not been emitted. De Sitter came up with an explanation to Einstein's general theory of relativity in his concept of the de Sitter space and the de Sitter universe. De Sitter's concept of expanding space became the basis of the Big Bang Theory. Sitter died of pneumonia at the age of sixty-two in Leiden.
