Dendrochronology (or "How old is that tree?")

Thank goodness for gloomy days that let you stay inside and think.
There is an old oak tree that stands at the back of our property. A huge, broad, still fairly healthy, old tree. When we first moved here and scouted out all the "cool" features, we found it and stood as a family, hand in hand, barely reaching around the trunk. The girls were much smaller then, so that's not an accurate measure of its size, but quite a specimen it is!
This year, my goal is to determine its age. I've taken the critical measurement--circumference at 4 1/2 feet above ground. But, that's only the beginning.
We plan to clear a path to it this year. It will be our thinking tree--a place to go and sit beneath its massive arms. But, for now, I sit and think and read about big trees.
http://www.americanforests.org/resources/bigtrees/