The speed of light in a vacuum is a constant of special relativity, electromagnetic wave theories, and astrophysical distances. However, several measurements of its speed (c) at locations on the Earth’s surface seem to vary at different times during the last century. Efforts have been made on instruments performance to achieve a unique viable value in any spacetime referential. The time-variability on c-values obtained is here addressed inside the gravity field (g) in which the measurements of c have been estimated. It appears a correlation of c and g both daily (tidal) and yearly (no-tidal) variations which suggest that the gravity acceleration control the c-variability everywhere in a spacetime referential. Implications of this model provide a sensitivity constant of c from g, and the estimates of c on planets of the solar system where g values are known. It is deduced an upper limit of gravity in black hole that can cancel the speed of light in the horizon.