Before venturing into outer space, let us look at our own Earth and see if we can determine some of its basic features, such as its size, age, and mass. The question of the Earth’s size,—specifically, its circumference—was a question posed by the Greek mathematician Eratosthenes of Cyrene over 2200 years ago when knowledge of the world was extremely limited. The main mathematical tool he used was some basic geometry espoused by Euclid some years earlier.