Since 1950, radiometric methods have been developed to a very sophisticated level in several countries, including Canada.
It has been demonstrated that when rocks which have led an undisturbed history are analysed, all methods reveal the same age.
The vast majority of atoms (each composed of a nucleus surrounded by electrons) are stable. Because of this particle emission, the original radioactive parent atom changes its identity, becoming a different, stable daughter atom.
The Earth certainly must be older than the oldest terrestrial rocks found.
Samuel Bowring, now of the Massachussetts Institute of Technology, and his coworkers Ian Williams and William Compston of the Australian National University at Canberra have shown that a small area of metamorphic rock in northern Canada, known as the Acasta gneiss, is the oldest known intact solid piece of the Earth's crust.
Using the uranium-lead technique they dated zircon crystals from the gneiss (located southeast of Great Bear Lake in the NWT) and showed that it was formed almost 4 billion years ago.
Therefore it is clear that the Earth is over 4 billion years old.
It has even been possible to work out a time scale of the reversals of the Earth's magnetic field.
This "radiometric" approach has superseded all other techniques for determining absolute ages. Their nuclei tend to emit particles spontaneously - ie, they are radioactive.
These discussions were rendered obsolete by the discovery of radioactivity in 1896 by the French physicist Henri Becquerel.
The existence of radioactivities of various kinds in rocks has enabled earth scientists to determine the age of the Earth, the moon, meteorites, mountain chains and ocean basins, and to draw up a reasonably accurate time scale of evolution.
This uniformity demonstrates that the principle is reliable.