This new series is aimed at the same people as the Practical Astronomy Series — in general, active amateur astronomers. However, it is also appropriate to a wider audience of astronomically-informed readers. Because optical astronomy is a science that is rather at the mercy of the weather, all amateur astronomers inevitably have periods when observing is impossible. At such times they tend to read books about astronomy and related subjects. The Astronomers' Universe Series begins by assuming an appropriate level of knowledge. Basic information about the distance, the solar system, galaxies, etc. is not part of these books, which can take a basic understanding of this as their starting point.