To measure the passage of long periods of time, scientists take advantage of a regularity in certain unstable atoms.In radioactive atoms the nucleus will spontaneously change into another type of nucleus.
When plants absorb carbon-dioxide in the photosynthesis process, some of the carbon dioxide has the carbon-14 atom in the molecule.
For example, looking at a series of layers in the side of a cliff, the younger layers will be on top of the older layers.
Or you can tell that certain parts of the Moon's surface are older than other parts by counting the number of craters per unit area.
Here are the steps: a result of radioactive decay (call that isotope ``B'' for below).
Isotopes of a given element have the same chemical properties, so a radioactive rock will incorporate the NONradioactively derived proportions of the two isotopes in the Multiply the amount of the non-daughter isotope (isotope B) in the radioactive rock by the ratio of the previous step: (isotope B) × R = initial amount of daughter isotope A that was not the result of decay.'' or ``what if the rock had some daughter material at the very beginning?