Cartoon schematic of a scattered‐light echo. Echoes lie on the locus of points equidistant in light travel from the source and observer—i.e., an ellipsoid—and appear as arcs or rings on the plane of the sky, with a one‐to‐one mapping between their two‐dimensional image positions (right) and their three‐dimensional locations in space (left). Note that the distances are not to scale: D is much larger than z, such that the segments from Earth to the scattering dust are nearly parallel to the line of sight.