An edge detector is provided for determining the axial position of the leading or trailing edge of a rotating blade within rotating machinery. The edge detector includes a set of axially spaced tip detection probes locatable within a casing of the machinery. Each probe is arranged to detect whether or not the blade tip passes over that probe. The edge detector determines the axial position of the edge of the blade as the position of the boundary between those tip detection probes which detect passage of the blade tip thereover and those tip detection probes which detect no passage of the blade tip thereover.