How old is the Earth?

In 1862 the physicist Lord Kelvin worked out how long the Earth would take to cool from molten rock to its present state, and got somewhere between 20 and 400 million years. He was a careful scientist and his sums were right for what he knew. What he did not know was that rocks contain a hidden clock, and a source of heat. Both are radioactivity, discovered at the end of the century.

Atoms that keep time

Most atoms last forever. A few kinds, unstable isotopes, break down into other atoms. You cannot tell when any one atom will go, but in a large crowd of them a fixed fraction goes in each stretch of time. The time it takes for half to go is the half-life. After one half-life, half is left. After two, a quarter. After three, an eighth.

Heat, pressure and chemistry make no difference to these rates that matters for dating rocks. So if you know how much of the original element a crystal started with, and how much it has now, you know how long it has been.

Different clocks suit different jobs. Carbon-14 has a half-life of about 5,700 years, so it is good for wood, bone and charcoal from the last few tens of thousands of years, and useless for rocks. Uranium-238 has a half-life of 4.468 billion years, about the age of the Earth itself. That makes it ideal for the oldest rocks there are. It is also why about half of the uranium-238 the Earth formed with is still here, while uranium-235, with a half-life of 704 million years, is down to about 1% (our own sums).

The perfect crystal

The best clock of all is a mineral called zircon. When a zircon crystal grows, it takes in uranium but almost no lead. So any lead inside it now came from uranium decaying, and it started its count at zero. Zircons are also very tough, and survive being worn down, buried and heated. The oldest piece of the Earth ever found is a zircon from Jack Hills in Western Australia, 4,404 million years old. The oldest rock, the Acasta Gneiss in Canada, is about 4,030 million years old.

Why the answer comes from space

Those dates give a minimum, not the age itself. Plate tectonics, weathering and melting have recycled the Earth's first crust, so no rock from the very beginning is left. Meteorites have not been through any of that. They are left-over pieces of the same cloud the Earth formed from, and dozens of them have been dated to between 4.53 and 4.58 billion years.

In 1956 the American geochemist Clair Patterson measured lead in meteorites and showed that Earth's lead fitted the same pattern. That put the Earth and the meteorites on one timeline, and gave an age of 4.55 billion years. Refined since, the accepted figure is 4.54 billion years, give or take less than 1%. The oldest solid grains in the Solar System, found inside meteorites, are slightly older still: 4,567 million years.

1. 1862Lord Kelvin20 to 400 million years, from how fast the Earth should cool
2. 1913Arthur HolmesAt least 1.6 billion years, from uranium in rocks
3. 1956Clair Patterson4.55 billion years, from lead in meteorites
4. TodayThe accepted age4.54 billion years, give or take less than 1%

Now walk through all 4.54 billion years, one metre at a time.