A little off topic. I watched “Telescope” on the Discovery Channel and they gave a really bad explanation of using Cephid Variables to gauge distance.
You have a much better explanation in your book, but I’m still wondering — how did they know (and especially how did they know back in 1912) that Cephid variables have a consistent Period-Luminosity relationship, or was this simply a conjecture? (And also, how did they measure brightness accurately back in 1912? My guess is by stacking neutral-density filters until they could no longer see the star, but even that is subjective, and dependent on how used to darkness your eye is, where on the retina the light is falling, etc.)