Four centuries ago, James Ussher calculated our planet’s birthdate. Ussher was an archbishop and a professor at Trinity College in Dublin. Using clues from the Bible, he decided that Earth was born on today’s date in 4004 BC. That would make Earth 6,026 years old. Other scholars of the day, including Isaac Newton, came up with similar dates. Ussher was using the best evidence available at the time. Since then, though, scientists have come up with new evidence. Studies over the past century have put Earth’s age at 4.54 billion years, with a margin of error of just one percent. To find that age, scientists have used both Earth rocks and space rocks — meteorites that have fallen to Earth. Radioactive elements in the rocks decay at a predictable rate, changing to other elements. Comparing the ratios of those elements reveals a rock’s age. The oldest Earth rock found so far clocks in at 4.4 billion years. That means our planet can’t be younger than that. But most of Earth’s original rocks a
