# How old is Earth?

01.04.2015

Episode #2 of the course “Science questions everyone should know how to answer”

The average person could live their life over 63 million times and still not live as long as the Earth has been alive. In other words, the Earth is really old. How old? Approximately 4.54 billion years.

It’s taken scientists a very long time to come up with this number. And even after all this time, they still have a plus/minus range of about 50 million years. Meaning, the Earth is somewhere between 4.49 billion and 4.59 billion years old. So how did they finally determine this?

For a long time, scientists just grabbed rocks from around the world and calculated their age. The problem with that method is that old layers of earth are constantly slipped into the Earth’s molten core and new layers are pushed to the top. Most rocks on the Earth’s surface are only 100 million years old, a far cry from 4.54 billion.

The solution, as it turns out, could be found in space, not on Earth. When mankind first landed on the moon during the Apollo missions, we brought back samples of moon rock. The moon is not geologically active like the Earth, so it stood to reason that the rocks on the surface of the moon are the same rocks that were there from the moment it first existed. These moon rocks were 4.4 to 4.5 billion years old. Further evidence came from meteors that crashed into Earth. More than 70 meteors have been dated with the exact same range of 4.4 to 4.5 billions years old.

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