How Old Is The Earth?

The Earth is clearly old, but exactly how old was difficult to work out. People long believed the Earth had only existed for several thousand years. Then, in the 1800's, a study of geological features (including fjords) led scientists to the conclusion that the planet must be much older - at least millions of years old. Later, the discovery of radioactivity provided a mechanism by which the Earth's core could be continually heated. This meant it was much older than previous estimates of 20-40 million years based on the cooling rate of the Earth. Today through many different methods we have established that the Earth is roughly 4.54 billion years old!