How Old Is Earth and How Did Scientists Figure It Out?

Efforts to figure out the Earth's age go back many centuries. The classical Greek philosopher Aristotle, who thought time had no beginning or end, also believed that Earth was infinitely old. Meanwhile, religious scholars in ancient India, who envisioned a universe that perpetually exploded, expanded, and collapsed only to begin anew, calculated that Earth had existed for 1.97 billion years.

During the medieval era, various Christian theologians scrutinized the Bible for clues, and came up with estimates of between 5,471 and 7,519 years, according to G. Brent Dalrymple's book "The Age of the Earth." From the 1700s and 1800s, an assortment of scientists came up with various figures based on clues ranging from Earth's rate of cooling and the accumulation of sediment, to the chemical evolution of the oceans.

Radiometric dating is a powerful scientific method used to determine the age of the Earth and various geological materials with remarkable precision. The fundamental principle behind radiometric dating lies in the decay of radioactive elements found in rocks and minerals. Certain naturally occurring radioactive elements, known as parent isotopes, undergo spontaneous decay over time into stable isotopes, referred to as daughter isotopes. By measuring the ratio of parent to daughter isotopes in a sample, scientists can calculate the time elapsed since the material's initial formation.

The breakthrough in radiometric dating came in the early 20th century when researchers, such as Bertram Boltwood, pioneered the use of this technique to estimate the age of rocks and minerals. Subsequently, radiometric dating techniques were further refined, and their application extended beyond Earth to other celestial bodies, including the moon. In fact, the age of the moon was determined by analyzing moon rocks brought back by the Apollo missions in the late 1960s and early 1970s, providing crucial evidence for the validity and reliability of radiometric ages. Today, radiometric dating remains an essential tool in unraveling the mysteries of Earth's history and the broader universe.

In the early 1950s, a California Institute of Technology geochemist named Clair C. Patterson, who had worked on the Manhattan Project to develop the atomic bomb during World War II, measured the isotopic composition of lead from the Canyon Diablo meteorite and several other pieces of space rock, which were believed to date back to the disc of material from which Earth also formed. In 1953, Patterson came up with an estimate of 4.5 billion years. Since then, research on meteorites and lunar rocks has refined that number only slightly.

Although Patterson's breakthrough made him famous in the world of science, he didn't see it as a big deal. At the time, "No one cared about it," he recalled in an oral history interview conducted shortly before his death in 1995. "Even today, people don't care how old the earth is. In fact, less today than 40 years ago, when I measured it."