Towards a unified definition of solar limb during central eclipses and   daily transits
  The diameter of the Sun has been measured using Baily's beads during central eclipses, observed with portable telescopes. A blend of tiny emission lines produced in the first several hundred kilometers above the photosphere gives a light signal which prolonges the light curves of the beads. The simple criterion of light OFF/ON adopted in the previous approaches to define the timing of photosphere's disappearance/reappearance is modified. The technique of the limb darkening function reconstruction from the Baily's beads light curves is introduced here. 
Costantino Sigismondi, Andrea Raponi, Cyril Bazin and Richard Nugent
astro-ph.SR astro-ph.IM