Before the 17th century scientists believed that there was no such thing as the "speed of light". They thought that light could travel any distance in no time at all. Later, several attempts were made to measure that speed:
Speed Of Light
Relative Speed Of LightAdvanced
Suppose that you have a clock and a ruler (which is not rotating with respect to stars) and that you are not accelerating (inertial). Locally (where you are) you will always measure the speed of light at 299792.458 km/sec. However in the presence of gravity if I am at a different location than yours then I could measure the speed of light at your location to be any value smaller than or greater than 299792.458 km/sec. It depends on where I am and where you are (it depends on locations). So in the presence of gravity the speed of light becomes relative (variable depending on the reference frame of the observer). This does not mean that photons accelerate or decelerate; this is just gravity causing clocks to run slower and rulers to shrink.
We discovered that when the geocentric frame is inertial 12000 Lunar Orbits/Earth Day becomes equivalent to the speed of light.