As a method of dating, archeologists have long found counting tree rings especially useful because it is so precise.Since there is a ring for each year, counting can be accurate to a year or two for an event thousands of years old.

In theory, that's simple enough. Bu t in practice it's a complicated process of studying the ring patterns of a single tree, comparing them to others of the same species, then fitting the results into a chronology of trees that grew before and after.

Now, in perhaps the most ambitious dendrochronological (tree-dating) undertaking ever, a Cornell professor named Peter Kuniholm is attempting to piece together a master chronology going back to 7000 B.C. Supported by the National Geographic Society, the National Science Foundation and others, he is working with such materials as a slie off a 6th century B.C. bed leg, Byzantine church beams, chunks of charred logs and hundreds of other wood fragments.

If he succeeds, archeologists will have an independent "calendar" that could help pinpoint ancient events whose dates are now known only vaguely.

"Right now, the classical archeologist in many cases is forced to round off a date to the nearest millennium or qualify his findings with plus-or-minus 500 years," says Kuniholm. "It is not uncommon to have at least a hundred years' wobble either way -- or simply guess."