The free, online, open-source, introductory textbook Astronomy, published by the nonprofit OpenStax project at Rice University, has recently reached the milestone of having been read by over half a million students since it was published in 2017. The book was written by astronomers Andrew Fraknoi, David Morrison, and Sidney Wolff, with the help of about seventy other astronomers and educators. The authors are at Foothill College, NASA Ames Research Center, and the National Optical Astronomy Observatory, respectively. Fraknoi and Morrison are both fellows of CSI.

The book is part of an initiative, funded by the Gates, Hewlett, and other major foundations, to provide a high-quality, free textbook for every introductory course taken by students in the first two years of college, thus reducing the often onerous costs of a college education. The publisher estimates that the Astronomy text alone has already saved students in North America approximately $40 million.

As you might expect from authors associated with CSI, a key focus of the book is to teach students skeptical thinking and to ask “How do we know?” about our current picture of the universe. The book includes:

• a section in which students are encouraged to think how we know Earth is round and not flat (and then shown various experiments that establish this);
• a clear discussion of astrology, how it differs from astronomy, and what tests show that astrology is not a working hypothesis;
• a link (in the chapter on the Moon) to a skeptical article that explains the evidence that NASA really did go to the Moon and the Moon landings were not faked;
• a chapter devoted to the scientific search for life elsewhere in the universe and the experiments that are under way to find it.

The book is available, completely free to anyone, at http://openstax.org/details/astronomy.