This book would be considered to be an expert source due to the credibility of the author and publisher. The author of this book is John Robert Christianson is a research professor at Luther college in Iowa where has taught world history and has sat as the chairman of the history department for 15 years. He has helped write, edit and translate nine different books and over one hundred articles on Scandinavian and Scandinavian- American topics. He has also helped to translate over 20 publications about Tycho Brahe that were published in scientific journals all over the world. The publisher of this book is the Cambridge University Press, which is a well known publish company of many educational journals and books. Cambridge has been around since
