Concluding remarks on the Cepheid Distance Scale
The Cepheid calibration story has been a long and complex one. Even in the post-Hipparcos era, the endless cycle of revision and counter-revision shows no sign of drawing to a close. There has nevertheless been a gradual convergence towards a zero point, Webb (1999 p179) wrote:
It seems unlikely that we will see another change in the period-luminosity relation of the size introduced by Baade. The debate is at the level of 0.15 mag not 1.5 mag.
However, whilst the zero point has converged, scientific opinion has been slower to do so. Like the ‘Great Debate’, the controversy surrounding the true nature of colour terms is likely to be resolved only through improved observational techniques. Multiwavelength and IR technologies show great promise in this respect, and might finally decouple the combined effects of extinction, metallicity and temperature. Nevertheless, an outstanding feature of the PL relation throughout its history has been its essential robustness. Despite numerous attempts to complicate it in the past, and increasingly sophisticated scrutiny in the modern era, there does appear to be a relatively straightforward underlying relation between period and luminosity for these stars.
We will leave the final cautionary note to Fernie (1969). He recalls Hubble’s remarks on the Cepheid scale at a lecture in 1935 before Baade’s great correction:
"Further revision is expected to be of minor importance."