How many geologic eons are there?
Geologic time scale
The primary defined divisions of time are eons, in sequence the Hadean, the Archean, the Proterozoic and the Phanerozoic. The first three of these can be referred to collectively as the Precambrian supereon. Eons are divided into eras, which are in turn divided into periods, epochs and ages.
Eos family
The Eos family asteroids have semi-major axes between 2.99 and 3.03 AU, eccentricities between 0.01 and 0.13, and inclinations between 8° and 12°. Currently there are about 4,400 members known. The inner orbit of the family is bracketed by the 7/3 mean-motion resonance with Jupiter at 2.96 AU. The orbital range also includes the 9/4 mean-motion resonance with Jupiter at 3.03 AU. Most of the family members lie within the latter orbital distance. The distribution of asteroid sizes suggests that the family is about 1–2 billion years old.