Explainer: Understanding geologic time

Here’s how scientists have organized time throughout Earth’s 4.6-billion-year history

Imagine the nearly unimaginable: 4.6 billion years. That’s how old the Earth is — a mind-boggling length of time. And to measure it, scientists use special terms, most of which focus on the planet’s changing geology. That’s why, in fact, it’s known as geologic time.

To grasp just how old Earth is, imagine fitting its entire history into one calendar year. If Earth formed on January 1, the earliest primitive life (think algae) wouldn’t appear until March. Fish first swam onto the scene in late November. Dinosaurs stomped around from December 16 until December 26. The first modern humans — Homo sapiens — were real late-comers. They didn’t show up until just 12 minutes before midnight on New Year’s Eve. Read more.