Sagan also insisted those who do not believe that two old English guys made all the circles are not skeptical enough, despite the fact that the phenomenon dates back two centuries and that the modern interest begins with a documented circle in Australia in 1966. The speaker was Dr. Carl Sagan, a 28 year old assistant professor of astronomy at Harvard University. His audience consisted of several hundred members of the American Rocket Society, gathered for his luncheon address. The equation was his way of expressing the mathematical probability that intelligent beings from outer space have visited earth. Sagan soberly explained that in his equation N stands for the number of advanced civilizations in the universe possessing the capability of interstellar communication.
R is the mean rate of star formation averaged over the lifetime of the galaxy.
FP is the fraction of stars with planetary systems. NE is the mean number of planets in each system with environments favorable for the origin of life. FL is the fraction of such inhabited planets on which intelligent life with manipulative abilities rises during the lifetime of the local sun. FC is the fraction of planets populated by intelligent beings on which an advanced technical civilization rises. And L is the lifetime of this technical civilization.