One that sits as a person who sits for a portrait or a bust; a person who babysits

(1872-1934) Dutch astronomer, mathematician, and physicist, who contributed to the birth of modern cosmology. He was influential in English-speaking countries in bringing the relevance of the general theory of relativity to the attention of astronomers.
Sitter was born in Sneek, Friesland, and studied at the University of Groningen and the Royal Observatory, Cape Town, South Africa. He was professor of theoretical astronomy at the University of Leiden from 1908 as well as director of its observatory from 1919.
In 1911 Sitter outlined how the motion of the constituent bodies of our solar system might be expected to deviate from predictions based on Newtonian dynamics if Albert Einstein’s special relativity theory were valid. After the publication of Einstein’s general theory of relativity 1915, Sitter expanded his ideas and introduced the “de Sitter universe” (as distinct from the “Einstein universe”). His model later formed an element in the theoretical basis for the steady-state hypothesis regarding the creation of the universe. He presented further models of a nonstatic universe: he described both an expanding universe and an oscillating one.