Neil F. Comins is professor of physics and astronomy at the University of Maine. His books include Discovering the Universe (eleventh edition, 2019); What If the Earth Had Two Moons? And Nine Other Thought-Provoking Speculations on the Solar System (2010); Heavenly Errors: Misconceptions About the Real Nature of the Universe (Columbia, 2003); and What If the Moon Didn’t Exist? Voyages to Earths That Might Have Been (1993).