If a Cepheid's period allowed the calculation of its Absolute Magnitude (M), what measurement provided the Apparent Magnitude (m)?

Answer

The telescope observation from Earth.

The apparent magnitude (m) is how bright the star looked from Earth, which was determined by observation using the telescope. This value was then compared to the calculated absolute magnitude (M) to solve for distance.

If a Cepheid's period allowed the calculation of its Absolute Magnitude (M), what measurement provided the Apparent Magnitude (m)?
measurementHubblegalaxyDistanceCepheid