Many great thinkers throughout history have tried to figure out Earth’s age. For example, back in 1862, Lord Kelvin calculated how long Earth might have taken to cool from its original molten state. He concluded that Earth was born 20 to 400 million years ago. Today’s scientists believe that answer is incorrect, but Kelvin’s calculations were scientific in being based on logical thinking and mathematical calculation.

Scientists tried to determine Earth’s age via our planet’s layers of rock, which must have been built over time. You’ve seen these rock layers if you’ve ever observed a cut-away section of a mountain, perhaps because a highway runs through it. But Earth’s layers of rock did not give up the secret of Earth’s age easily. Their message proved difficult to decipher. How old is Earth? In the early part of the 20th century, scientists still weren’t sure. However, from working with layer upon layer of rock laid down on Earth over long time spans, early 20th century scientists came to believe Earth not millions of years old – but billions of years old.

Modern radiometric dating methods came into prominence in the late 1940s and 1950s. These methods focus on the decay of atoms of one chemical element into another. They led to the discovery that certain very heavy elements could decay into lighter elements – such as uranium decaying into lead. This work gave rise to a process known as radiometric dating. This technique is based on a comparison between the measured amount of a naturally occurring radioactive element and its decay products, assuming a constant rate of decay – known as a half-life.

Using this technique, scientists could, for example, analyze a sample from Earth’s crust, figure out the quantities of uranium and lead, plug those values along with the half-life into a logarithmic equation, in order to compute the age of the rock. Over the decades of the 20th century, scientists documented tens of thousands of radiometric age measurements. Taken as a whole, these data indicate that the Earth’s history extends backward from the present to at least 3.8 billion years into the past.

Nowadays, scientists use radiometric dating of various sorts of rock – both earthly and extraterrestrial – to pinpoint Earth’s age. For example, scientists search for and date the oldest rocks exposed on Earth’s surface.

Also, because Earth formed as part of our sun’s family of planets – our solar system – scientists use radiometric dating to determine the ages of extraterrestrial objects, such as meteorites. These are space rocks that once orbited our sun, but later entered Earth’s atmosphere and struck our world’s surface. Likewise, scientists use radiometric dating to determine the ages of moon rocks, obtained by astronauts.

Taken together, these methods give results that suggest an age for our Earth, meteorites, the moon – and by inference our entire solar system – of 4.5 to 4.6 billion years old.