Introduction

Can you imagine what life would be like if we couldn’t tell time? How would we mark the days or years? How would we know when to be at work or school? How would we even know how old we were?

Time exists whether we notice or not, but the measuring of time is a human invention. Chronology—the measuring of time—allows us to record the past and plan the future.

Measuring time is not just a matter of marking off particular years, days, hours, minutes or seconds. Chronologies are equally important. A chronology is a listing of events. Each event is listed in the order in which it happened. Each is given a date. That date can be exact. Or, it can just be a range of years.

History is a chronological story of past events. For that reason, historians depend on accurate chronologies. Of course, history is far more than a simple chronology. Historians need to explain the meaning of past events. They need to look for big patterns. Still, they cannot do these things without a reliable chronology.

The First Chronometric Revolution—Writing

The earliest humans had no system of writing. They used storytelling to pass down the history of their people. Their stories included details about when things happened. Unfortunately, oral histories cannot be used to create a good chronology. They simply are not reliable. They change a bit every time they are told.

The invention of writing was a big step forward. It was the first chronometric, or time-measuring, revolution. For the first time, information became fixed, once it was written down. It no longer constantly changed over time. It then became possible to create a useful chronology of past history.

Written records give us a picture of the past. However, that picture is far from complete. The many thousands of years before writing appeared are left out. So are communities that did not develop writing.

Written records have another big drawback. They are written by certain kinds of people. In ancient times, very few people could read and write. They were the richest and most powerful members of society. The written record is their story. The opinions and experiences of most people are left out.

The written record also overlooks the natural world. It ignores plants and animals. We cannot truly understand the past with such limited information.

Modern historians have looked beyond the written word. Science has helped them greatly.

The Second Chronometric Revolution—Radioactivity

In the early 1900s, British scientist Ernest Rutherford discovered something important about atoms. He proved that they lose energy over time. They do this by giving off radiation. This is known as radioactive decay.

Later, American scientist Willard Libby built on Rutherford’s discovery. Libby figured out carbon-14’s rate of decay. Carbon-14 is a radioactive isotope.

Isotopes are slightly different forms of an element. Regular carbon is carbon-12. It has 6 protons and 6 neutrons in its nucleus. It is stable and does not decay over time. Carbon-14 has 6 protons and 8 neutrons in its nucleus. Those extra 2 neutrons are why it is radioactive. It is unstable and decays.

All living things contain carbon. When a plant or animal dies, its carbon-14 begins to decay or break apart. The rate of this decay can be measured. In turn, this measurement shows the age of the plant or animal. The greater the decay, the older the plant or animal.

Libby’s discovery was published in 1946. It marked the second chronometric revolution. Suddenly, it was possible to date ancient remains.

Carbon-14 dating has limits, though. It cannot be used for anything older than 60,000 years. After 60,000 years, so much carbon 14 is gone that there is not enough left to measure. Fortunately, several other isotopes can be traced much further back.

Scientists have also found other ways to measure time. One is by tree-ring dating. The rings on a tree tell a story about every season in which the tree has lived. An old tree can provide a lot of information. For example, it can show us a great deal about climate changes.

Then there is genetic dating. This method is based on the study of genes. It allows scientists to trace the chronology of animal and human evolution.

Historians can now date events that happened long before writing developed. Not too long ago this was impossible. We now know more about human history than we ever did before. We also know much more about the history of the Earth itself. Much of this is because of new ways of measuring time.