And Now You Don’t

“IF PAINTERS WERE OBLIGED TO WHITEWASH entire scenes every time slight flaws in a finger or an eye were pointed out to them, the whole story would be very slow to emerge.” So wrote Galileo Galilei in 1624, conceding in the interest of preserving the grand structure of the heliocentric world system a small error on the part of Nicolas Copernicus. The comparison, typical in its artsy, aggressive tenor, involves what Galileo depicted as a trivial aspect of the orbit of Venus. This letter, however, was largely concerned with another issue, parallax, the variation in the apparent position of an object viewed from different perspective points. Now you see it, and now you don’t: the positional shift that is parallax only appears under certain conditions, and its value as a quantitative index of the size of the cosmos or of the distance of its components was a fluctuating one. Galileo sought to establish and to use this measurement over the course of his career; that story, more than any other account of his research agendas, is filled with gaps and obvious erasures. Rather than the specter of the bare, ruined wall to which