WordNet sense 1 (Dutch astronomer who calculated the size of the universe and suggested that it is expanding (1872-1934)):

Willem de Sitter

WordNet sense 2 (a person engaged to care for children when the parents are not home):

babysitter, baby-sitter

WordNet sense 3 (a person who poses for a painter or sculptor):

artist's model

WordNet sense 4 (a domestic hen ready to brood):

broody, brood hen, broody hen, setting hen