We show that the speed of a longitudinal-extended, elastic (variable length), and massive particle, emitted by a source during an emission time T, at speed u (escape speed from all the masses in space), is invariant for every real measurement, (intending a measurement requiring an interaction light-matter), in spite of any reciprocal motion source-Observer. Thus we may argue that the light has to be composed of such particles (photons) moving at speed c = u. Compliance of these photons with Newtonian mechanics is shown for many effects, (like the Doppler effect, redshift, time dilation, etc.), highlighting the differences versus the Relativity. In the 2^{nd} part, on the assumption that the electron charge can be considered as a point-particle fixed to the electron surface, always facing the atom nucleus during the electron revolution, we revised the light-matter interaction, showing that it only depends on the particular impacts between these photons and the circling electrons: for instance, on H atom, we found 137 circular orbits only, the last one being the ionization orbit, where the electron orbital speed becomes v_{i}= c/137^{2}. [Classical mechanics implies that orbiting electrons produce an electro-magnetic radiation causing their fall into the nucleus: on Section 3.5, the reason why the electron circular orbits are stable].

Nasa Extragalactic Database: (i.e. Galaxy M86 Has z ≅ -0.001 with s ≅ 16 Mpc; M99 Has z ≅ + 0.008 with s ≅ 15 Mpc; NGC0063 Has z ≅ +0.004 with s ≅ 20 Mpc; VCC0815 Has z ≅ -0.0025 with s ≅ 20 Mpc).