How old is the Earth? This lecture reviews the idea of cyclic and linear time, since how you view time determines whether the question of the age of the Earth is even meaningful. We then review various ways people have estimated the age of the Earth, starting with historical ages that equate human history with the history of the Earth proper, and then see how various physical estimates, which do not make an appeal to human history, were made. This brings us to the technique of radioactive age dating of the oldest rocks, leading to our current best estimate of 4.5+/-0.1 Billion years for the age of our planet. Recorded 2006 Oct 30 in 100 Stillman Hall on the Columbus campus of The Ohio State University.