Which geometric method was necessary for the initial calibration step to determine the true baseline luminosities of nearby Cepheid variables?
Answer
Parallax
The entire sequence of distance calculations depended on accurately calibrating the intrinsic luminosities of Cepheid variable stars. This calibration required measuring the true distances to relatively nearby Cepheids using geometric techniques. Specifically, parallax measurement—observing the slight apparent shift of a star against a distant background as the Earth orbits the Sun—was the method used to establish this initial baseline luminosity standard.
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