The wave nature of light is not part of students' common experiences, so often physics teachers and textbooks will add a historical anecdote about how scientists, too, were tricked by light. A common one is how, in the early 19th century, Poisson declared that since Fresnel's ideas on the wave nature of light implied that the shadow cast by a disk would contain a bright spot at its center, Fresnel's ideas were obviously flawed. The spot was later detected, proving Fresnel right! But recent studies of Galileo's work have brought to light a story about diffraction that may displace Poisson's spot as the favored historical anecdote, for it seems that diffraction tricked Galileo, too. Diffraction of light caused Galileo to mismeasure the distances to the stars.