Are you asking when the speed of light was first estimated, or are you asking when the speed of light was first actually measured?
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I don't believe there was any serious attempt to measure the speed of light before Galileo. That was a couple of millennia after the golden age of the Greek philosopher 'scientists', who, I believe, assumed the transfer of light to be instantaneous, and the perception of distant events to be simultaneous with the event.
Ole Romer
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Galileo attempted to measure the speed of light using lanterns positioned at known distances and observing the time it took for light to travel between them. He would uncover the lanterns simultaneously and use a telescope to try and detect any delay in the light reaching his eyes. However, his methods were not sensitive enough to accurately measure the speed of light.
speed of light is constant velocity and does not accelerate so there is no g force