I just finished reading Marcia Bartusiak's new book, The Day We Found the Universe. It is a wonderful look at the people and discoveries that led to our understanding of our size and place in the universe.
The book tells the story of the hunt to unravel the mystery of spiral nebulae. Was the Milky Way the entire universe and were the spiral nebulae a just a small part of it? Or were the spiral nebulae individual island universes (galaxies) rivaling our own? The truth came out from the hard work of astronomers at the Lick, Lowell and Mt. Wilson observatories. Marcia Bartusiak follows the efforts (not all of them successful) of astronomers such as James Keeler, Heber Curtis, Harlow Shapley, Vesto Slipher, Adriaan van Maanen, Edwin Hubble and more. The progress of astronomers solving the mystery of spiral nebulae was paralleled by the work of Albert Einstein and led directly to the discovery of the expanding universe by Georges Lemaître.
Bartusiak does an excellent job of weaving together the story of what was a very exciting period in astronomy. For those who want explore the history of this period further, the book also contains an excellent set of notes and bibliography which has already pointed me to some sources that I was unaware of.
The Day We Found the Universe is a must read for anyone interested in the history of astronomy or anyone who wants to know in how we came to understand what galaxies are and our sense of place in the universe.