Allan Sandage and the Distance Scale
  Allan Sandage returned to the distance scale and the calibration of the
Hubble constant again and again during his active life, experimenting with
different distance indicators. In 1952 his proof of the high luminosity of
Cepheids confirmed Baade's revision of the distance scale (H0 ~ 250 km/s/Mpc).
During the next 25 years, he lowered the value to 75 and 55. Upon the arrival
of the Hubble Space Telescope, he observed Cepheids to calibrate the mean
luminosity of nearby Type Ia supernovae (SNe Ia) which, used as standard
candles, led to the cosmic value of H0 = 62.3 +/- 1.3 +/- 5.0. Eventually he
turned to the tip of the red-giant branch (TRGB) as a very powerful distance
indicator. A compilation of 176 TRGB distances yielded a mean, very local value
of H0 = 62.9 +/- 1.6 and shed light on the streaming velocities in the Local
Supercluster. Moreover, TRGB distances are now available for six SNe Ia; if
their mean luminosity is applied to distant SNe Ia, one obtains H0 = 64.6 +/-
1.6 +/- 2.0. The weighted mean of the two independent large-scale calibrations
yields H0 = 64.1 km/s/Mpc within 3.6%.
