Answer:
People with a casual interest in astronomy.
Step-by-step explanation:
The book "Great Astronomers" by Robert Stewall Ball was written to provide basic information on some important astronomers for the evolution of what we know about astonomy. The author uses very simple and easy-to-understand language in these books and talks about the lives of astronomers we have heard of such as Nicholas Copernicus and some contemporary astronomers of this century.
Since the language is simple, without the use of technical terms and basic information, we can say that the target audience for this book is people with a casual interest in astronomy.