How can they tell the earth is 4.5 billion years old?

Question: How can they tell the earth is 4.5 billion years old?
- Keywords:
  billion,
  earth,
  planet,
  time
Asked by courteney to Ed, Katie, Sam, Steve, Vera on 17 Jun 2011.
- Ed Morrison answered on 16 Jun 2011:
  
  I believe scientists can work this out by using a technique called radiometric dating. Basically they find rocks with certain elements in them which break down at a certain rate. By looking at the amount of element in the rock, they can work back to tell when the rock was first formed. The oldest rocks on earth are about 4.5 billion years old using these techniques.
  
  It’s hard to even think about that much time isn’t it?
- Sam Tazzyman answered on 16 Jun 2011:
  
  Scientists can use the same techniques to look at meteorites to see how old the Solar System is, because the Earth can’t be older than the Solar System. Using this method they also get an age of about 4.5 billion years.
- Vera Weisbecker answered on 16 Jun 2011:
  
  I agree with Ed and Sam!
- Katie Marriott answered on 16 Jun 2011:
  
  Yup Sam and Ed have the same thoughts as me!
  
  A atom is split into different parts called eletrons, protons and neutrons. As the atoms break down new atoms the number of neutrons they contain decreases and so you get new isotopes. Scientists look at the number of atoms of each isotope and then can work out how old the Earth is.
- Steven Daly answered on 17 Jun 2011:
  
  Just think, 4.5 billion years is only a third of the time the Universe has been around. It is a truly unimaginable amount of time.
  
  I forget exactly now; perhaps someone else will know, but there was a David Attenborough series where he looked at all life on earth through the ages. On that it said that if you imagine that the length of time the Earth had been around was a year, animals only evolved in december, and humans at around 23:59 on 31st December.

Question: How can they tell the earth is 4.5 billion years old?

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