The main purpose of this book is to serve as a university textbook for a first course in astronomy. However, we believe that the audience will also include many serious ama teurs, who often find the popular texts too trivial. The lack of a good handbook for am ateurs has become a problem lately, as more and more people are buying personal com puters and need exact, but comprehensible, mathematical formalism for their programs. The reader of this book is assumed to have only a standard high-school knowledge of mathematics and physics (as they are taught in Finland); everything more advanced is...