FAS Astronomers Blog, Volume 31, Number 11. The Earth’s history is described by geologic time, which is broken into eons, eras, periods, and epochs. The first three eons (Hadean, Archean, and Phanerozoic) are often combined into the Precambrian supereon. Today, we find ourselves in the Cenozoic era of the fourth (Phanerozoic) eon. However, as we…