What are secular equations? The term “secular” comes from the latin “saecularis” which is related to “saeculum”, which means “century”. So secular refers to something that is done or happens every century. It is also used to refer to something that is several centuries old. It appeared in mathematics to denote equations related to the motion of planets and celestial mechanics. For instance, it appears in the title of a 1829 paper of A. L. Cauchy (1789–1857)*“Sur l’ équation à l’ aide de laquelle on détermine les inégalités séculaires des mouvements des planètes”*(Oeuvres Complètes (IIème Série), v 9