Calibration of time.
We are familier with hours,minutes and seconds and even distant events such as the age of Vikings or Egyptians or Roman empire but what about on geological timescales in the millions of years,well they can be obtain in fossils by various methods.
The absolute timescale produces a calibration of rocks utilizing the RADIOACTIVE DECAY of unstable isotopes {atomic varieties}of elements . As decay proceeds sub atomic particles are emitted and these can be registered by the crackling of a Geiger counter.This provides a true clock Radioactive elements decay from an unstable state to a stable one.when half the original material has decade its [half life} has expired.Half lives varies from a few moments to millions of years,depending on the element involved.Thus radiocarbon dating works best for archaeological material because this form of familier element has a short half life.On the other hand uranium isotopes to provide very slow clocks,leisurely enough to pluck out time from the early phases of the solar system itself.All that is needed is a measure of how much the original material has decayed and by a very simple calculation based on the rate of decay the time taken to effect the change can be deduced.
For example potassium has a half life of a billion years before it changes to argon,this method is called potassium-argon dating or uranium 235 -lead for measuring in the millions of years.
Ie uranium 235 has a half life of a million years before changing to lead
. So 1, carbon dating gives a half life in 1housands of years ,measuring thousands of years.
2,Uranium lead dating measures half life of a million years measures elements millions of years old
3,Potasium has a half life of a billion years so is used to date elements in the billions of years,
A combination of those dating methods can give an accurate measure of say a fossil trapped in a rock.with an error either way of 3% or so either way.