When Aristotle (384-322 BC) observed that fossil seashells from rocks were similar to those he found on the beach, he concluded that those fossils were once living animals. He further deduced that the positions of land and sea had changed over time and thought these changes occurred over very long periods.
Of course, Aristotle was in no position to accurately determine the length of those long time periods. Nowadays however, thanks to radiometric dating (the measurement of decay in radioactive isotopes), we know that the Earth is approximately 4.54 billion years old.
“The geologist takes up the history of the earth at the point where the archaeologist leaves it, and carries it further back into remote antiquity.” – Bal Gangadhar Tilak, The Arctic Home in the Vedas
In order to accurately document the enormous history of the Earth, a system known as the Geologic Time Scale has been developed to divide the history of our planet into units. The Geologic Time Scale is made up of the following units of time:
- Supereon (approximately 4 billion years)
- Eon (half a billion years or more)
- Era (several hundred million years)
- Period (between tens and one hundred million years)
- Epoch (tens of millions of years)
- Age (millions of years)
Because each unit of time summarizes the major events and characteristics of a certain geological time span, there are no fixed time frames for the different units of time.
“Geology gave us the immensity of time and taught us how little of it our own species has occupied.” – Stephen Jay Gould, Ever Since Darwin: Reflections on Natural History
See other: History of the Earth
Knowledge Guild 
[Aristotle} “deduced that the positions of land and sea had changed over time” – How much more perceptive was he than those who concluded, not that mountains had risen from beneath the sea, but that seashells found on mountaintops were left there by a global flood?
