Trees undergo spurts in growth in the spring and summer months while becoming somewhat dormant in the fall and winter months. When a tree is cut down, these periods are exhibited in a cross section of the trunk in the form of rings. Simply counting the number of rings will give one a fairly good idea of the age of the tree. Periods of heavy rain and lots of sunshine will make larger gaps of growth in the rings, while periods of drought might make it difficult to count individual rings. When determining the ages of very old objects, the only suitable clocks we have found involve the measurement of decay products of radioactive isotopes.
Isotopes are atoms of the same element with different amounts of neutrons.
Some isotopes are stable, whereas others are radioactive and decay into other components called daughter isotopes. Radioactive isotopes decay according to a power law, and the typical unit given for this is called the half-life of the isotope. For all practical purposes, the original isotope is considered extinct after 6 half-life intervals. Note that there are vast ranges of time exhibited in the decay rates, allowing a suitable measure if one knows or guesses the approximate age. However, there will also be some unknown amount of 87 Sr that was in the meteorite when it formed.