How are the ages of the Earth and universe calculated?
Many independent measurements have established that the Earth and the universe are billions of years old. Geologists have found annual layers in glaciers that can be counted back 740,000 years. 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 expansion of the universe gives an age for the universe as a whole: 13.7 billion years old.
(Updated April 16, 2012)