Willem De Sitter was born in Sneek on May 6, 1872. He was a Dutch astronomer and mathematician. In his early years, he had done experimental work for physics at the astronomical laboratory of Jacobus C. Kapteyn. He was also involved in measuring photographic plates from the Cape Observatory in South Africa to chart the southern skies.In 1897, De Sitter began working on celestial mechanics and stellar photometry. He worked with others to measure the four Galilean satellites of Jupiter. He calculated the mass of the satellites from perturbations in its orbit. Then, he published the results in his New Mathematical Theory of Jupiter's Satellites in 1925.
In 1908, he was professor of theoretic astronomy of the University of Leiden. Then, he was appointed the chair of astronomy at the university. From 1919 up until his death, he was also the director of their Observatory.
In 1932, he and Einstein co-authored a paper where they argued the existence of dark matter, where there's a large amount of matter but they do not emit light. Afterwards, De Sitter proposed the de Sitter universe which was in contrast to the Einstein universe. Einstein had came up with a static universe from solving equations of his general relativity theory. But De Sitter came across another solution to the equation. He was able to solve it even if no matter were present. De Sitter concluded that space can't be in a stable equilibrium. So, Einstein's universe had matter but no motion while de Sitter's had motion but no matter. A few years after, two Russian mathematicians indepently came up with the idea of an expanding universe with moving matter. De Sitter's universe could now be transformed into an expanding universe. Ultimately, it combined with Einstein's universe to form the Einstein-de Sitter universe which contained normal Euclidean space and a simple curved space. It formulated the idea that the universe is expanding at a decreasing rate that gets closer and closer to zero.
In 1919, he was operated for gallstones. But the overdose of ether that was given caused his health to degenerate. He also had tuberculosis which made him spend two years in Switzerland. But, he still continued his research and carried out observational studies. In his lifetime, he received the Gold medal of the Royal Astronomical Society and the Bruce Medal of the Astronomical Society of the Pacific. From 1925 to 1928, he was the president of the International Astronomical Union. Then on November 19, 1934, he passed away from illness.
