The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The equation was written in 1961 by Frank Drake not for purposes of quantifying the number of civilizations, but intended as a way to stimulate scientific dialogue at the world's first SETI meeting, in Green Bank, West Virginia. The equation summarizes the main factors which scientists must contemplate when considering the question of other radio-communicative life. The Drake equation has proved controversial since several of its factors are currently unknown, and estimates of their values span a very wide range. This has led critics to label the equation a guesstimate, or even meaningless.
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in the journal Nature with the provocative title "Searching for Interstellar Communications." Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 centimeters (1,420.4 megahertz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the radio spectrum.
Seven months later, radio astronomer Frank Drake became the first person to start a systematic search for intelligent signals from the cosmos. Using the 25 meter dish of the National Radio Astronomy Observatory in Green Bank, West Virginia. Drake listened in on two nearby Sun-like stars: Epsilon Eridani and Tau Ceti. In this project, that he called Project Ozma, he slowly scanned frequencies close to the 21 cm wavelength for six hours per day from April to July 1960. The project was well designed, cheap, simple by today's standards, and unsuccessful.
Via Nicko Gibson