How old is the Earth? In this lecture I review the ideas of cyclic and linear time, and how this determines whether or not the question of the age of the Earth is meaningful. I then review various ways people have tried to estimate the age of the Earth, starting with historical ages that equate human history with the physical history of Earth. We then look at physical estimates of the Earth's age that do not make an appeal to human history, but instead seek physical processes that play out over time to make the estimates. This brings us to a discussion of radiometric age dating techniques that use the radioactive decay of isotopes trapped in minerals to identify the oldest Earth rocks and meteorites, and hence establish a radiometric date for the formation of the Earth some 4.55+/-0.05 Billion Years ago. Recorded 2007 Oct 29 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.