Final answer:
The author of 'Great Astronomers' targeted an audience with an interest in the historical aspects of astronomy and seminal figures in the field, suitable for students and general readers with some foundational knowledge.
Step-by-step explanation:
Based on the content and style of the excerpt from Great Astronomers by Robert Stewall Ball, the author had in mind an audience that is primarily interested in the historical development of astronomy and the contributions made by notable figures in the field. This audience would likely include students specializing in the study of astronomy, as well as individuals with a more general interest in the history of science. The detailed development of the subject matter, with reference to key figures like Ptolemy, Copernicus, Galileo, and Newton, as well as the inclusion of their philosophical and historical impact, suggests an educational yet accessible tone suitable for an audience with at least a foundational understanding of astronomy or history of science.