The Galilee boat has been dated by a method called radiocarbon dating. This method could be used because the boat was made of wood, a carbon-containing material.
To understand radiocarbon dating, it’s important to understand some basic facts about the world of nature. Carbon in the atmosphere exists in three forms—called isotopes—that differ in the weight of their atoms but not in their chemical behavior, so organisms use them as if they were exactly the same. The most abundant form is carbon 12, but for every 1012 atoms of carbon 12 there is one atom of the heaviest form, radioactive carbon 14. Carbon 14 is constantly being produced in the atmosphere.
To say that carbon 14 is radioactive means that it decays to the stable, non-radioactive nitrogen at a constant rate. This decay accounts for the fact that the number of carbon-14 atoms in the atmosphere and in living organisms, which continuously exchange carbon with the atmosphere as part of the biological processes of life, does not increase without limit but remains approximately constant.
As long as an organism is alive, the carbon within it is composed of the same proportion of carbon 12 and carbon 14 as the carbon in the atmosphere. But when an organism dies, such as a tree cut for use of its wood, the exchange process stops and decay of carbon 14 proceeds without any replenishment of the supply from the atmosphere.
For any particular radioactive isotope, it is possible to measure its half-life, or the time it would take for one-half of the original radioactive atoms in a sample to decay to a stable form. For carbon 14 the half-life is 5,568 years. This half-life makes it useful for archaeology because the changes are large enough for meaningful measurement in the time periods archaeologists care about.
In the case of the Galilee boat, we assume that the wood used to make it was cut within a short time of the boat’s construction. Therefore, the radiocarbon age of the wood represents the true age of the boat.
Samples of the Galilee boat’s wood, each weighing several grams, were removed and sent to a laboratory. Using gas proportional counters, which count the radioactive decay events that occur within the carbon 14, the amount of radioactive carbon 14 in the sample relative to the amount of stable carbon 12 was measured. Knowing how long it takes for half the atoms of carbon 14 to decay—namely 5,568 years—it was possible to calculate, based on the current proportion of isotopes present, how old the boat was when it was made (that is, when the living trees used for it were cut). Ten samples from different parts of the boat were counted. The result was that the boat began its life as a fishing vessel on the Sea of Galilee in 40 B.C., plus or minus 80 years, or between 120 B.C. and 40 A.D.