How Old Is Earth?

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How Old Is Earth?

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Humans have wondered about the age of Earth for centuries. In the 1600s, a bishop worked backwards through generations of “begats” in the Bible to come to a very specific date for Creation: October 26, 4004 BC. At 6 PM. In the 1700s, scientists developed their own theories. Recognizing that different layers of rock represent different periods in Earth history, they calculated a much looser estimate: 1 million to 1.6 billion years old. By the early 1900s, scientists began to understand radioactivity, and found that each radioactive element has a half-life—a specific amount of time it takes to lose half its energy. They could use these half-lives to more precisely date Earth’s rock layers, pushing the age of the oldest ones to 3 billion years. With improved dating techniques, we now find rocks between 3.5 and 4 billion years old on every continent. But there are limits to this method. The surface of Earth is always eroding and renewing itself, and old Earth rocks tend to get recycled. To reach back further in time, we needed a place of the same age, but undisturbed by plate tectonics, like the moon. The oldest rocks we’ve found there date to around 4 and a half billion years. By studying these, and meteorites that landed on Earth from within our solar system, we’ve arrived at an age for Earth of 4.55 billion years.

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