The expansion of the universe gives an age for the universe as a whole: 13.7 billion years old.
The half-life is the time that it takes for half the radioactive sample to change from one element into the other.
Some isotopes have short half-lives of minutes or years, but Potassium-40 has a half-life of 1.3 billion years.
Like the tree rings, this method can be verified by comparison to historical records for weather, as well as to records of volcanic eruptions around the globe that left thin dust layers on the glaciers.
Scientists have drilled ice cores deep into glaciers and found ice that is 123,000 years old in Greenland In your high school science classroom, you may have seen a large poster of the periodic table hanging on the wall.
Using the known rate of change in radio-active elements (radiometric dating), some Earth rocks have been shown to be billions of years old, while the oldest solar system rocks are dated at 4.6 billion years.
Astronomers use the distance to galaxies and the speed of light to calculate that the light has been traveling for billions of years.
The periodic table shows the types of atoms that make up the world around us.
An element in the periodic table can come in different flavors called isotopes.
Many independent measurements have established that the Earth and the universe are billions of years old.