Which astronomer discovered the crucial relationship that provided a 'cosmic yardstick' for measuring distances?

Answer

Henrietta Swan Leavitt

Henrietta Swan Leavitt discovered the period-luminosity relation among Cepheid variables. This relationship linked the time it took for a Cepheid to pulsate (its period) to its true intrinsic brightness (absolute luminosity).

Which astronomer discovered the crucial relationship that provided a 'cosmic yardstick' for measuring distances?
measurementHubblegalaxyDistanceCepheid