The book alternates between a typically absurd Discworld story and serious scientific exposition. The purpose of the book is both to entertain and educate. In having fictional sections in which observers from a very different world with a very different set of rules look with confused eyes upon the Earth ("Roundworld"), the authors are able to expand upon things we take for granted, such as planets being round and stars being far away, in a manner which is free of a line of thinking which states "But that's obvious".