The writer John McPhee once came up with a striking way to think about the age of the Earth. He said to imagine that the length of your arm represents the planet's history. Your shoulder is the farthest back in time - 4.567 billion years ago, scientists say - your elbow would be a little more than 2 billion years ago, and so on. And here's his point: If you gently brushed the tip of your fingernail with a nail file, you would erase all of human history.
Filing your nails should never be the same again.
But what's with that 4.567 billion year birthday? The age of rocks can be measured with great precision by studying the results of radioactive decay, but the oldest rocks ever found are dated only a little over 4 billion years. During its infancy, the Earth was constantly being bombarded with debris as the solar system pulled itself together, and no rock could form while that blasting kept the planet in a molten state.
So how do we get precisely 4.567 billion years? The answer is both simple and elegant: by dating rocks that are on the Earth, but are not of the Earth.
As the planets were being formed, countless other bits and pieces of matter never got a chance to join up, Those small, lonely asteroids just kept zooming around, pulled slightly this way and that whenever they neared a planet, but generally were doomed to isolation - until they found themselves aimed squarely at a crash landing on a planet such as the Earth. Over the eons, a great many of them have done just that.
Studies of the ages of these rocks show ... 4.567 billion years.
If you held such a rock in your hand, your fingers would be curling around not only an object exactly as old as the Earth, but a third as old as the universe itself.