This longest division of geologic time doesn't have a fixed period; in astronomy, it's one billion years
The planet likely presents only one face to its star, as the moon 
does to Earth,... [DOC] [TLE] Building Planets | NASA [PAR] Apr 29, 2010 ... OSIRIS-REx will make a swing by Earth next year to gain a gravity assist that will 
... from the formation of our solar system more than 4.5 billion years ago.