Sommerfeld Fine Structure: a0(not "CONSTANT") AND Not "Rationalized"... with 4p.
   1. Historically, a0  = "ratio of the velocity[dL/t] first Bohr-Orbit" ...
"to the speed[L/t] of light,c, in Einstein's Energy[LLm/tt] = mcc."
   1a. Thus: velocity[dL/t]/speed[L/t] = [d]direction.<==Surprise to
TextBooks (Figure-A, below).
20th-Century Physics TextBook (SadStuff):
<==20th-Century TextBooks Figure-A
    1) Sommerfeld Fine Structure (not "constant"), a0= d0 /137.039 = e2/hce0  = ce2m0/h.
permittivitye0  & permeabiitym0... <==(VacuumOnEarthSurface) Snell (Unit)[d]Direction ...
   The Above "Table D"... SHOWS that 1960s-TextBooks WERE "Fuzzy/FAULTY(!!)"...