This longest division of geologic time doesn't have a fixed period; in astronomy, it's one billion years
[DOC] [TLE] Years and years - Crossword Clue Answers - Crossword Solver [PAR] how long something has existed; "it was replaced because of its age"; a time of 
life (usually ... "Young men senesce"; a prolonged period of time; "we've known 
each other for ages"; ... long period of time; "oh, that happened eons ago"; the 
longest division of geological time ... Astronomical periods ... One billion years, in 
geo [DOC] [TLE] Roche limit - Chapter 12 [PAR] Less than one-third the mass of Jupiter, Saturn is still an enormous planet, at 
least ... At some times during Saturn's orbital period of 29.5 Earth years, the rings 
.... planet has a large reservoir of heat left over from its formation--doesn't work for 
Saturn.