One of the leading astronomers of our time has here explained in non-technical language what his science is about. His method has been to discuss what each of the great astronomers has contributed to our knowledge of the universe, from the earliest Greek star-gazers down to Eddington, Jeans, and Einstein. He thus gives both a history of astronomy and an account of its purposes and its ways of working. The basis of the book is the series of Lowell Institute Lectures which Professor De Sitter delivered before popular audiences last winter; his development of those Lectures has been calculated to make his material even more informative to the general reader.