Interesting statement in the article: "Though modern science finds minute variations in the speed of light in a vacuum or empty space..." I thought that this was a largely discredited hypothesis - is there more recent research in this area?
The vacuum isn't empty. Virtual particles are constantly popping in and annihalating nanoseconds later with their antipairs. They affect light.
Great article about the discovery of light spectra lines and their impact on astronomy. But the article should have also mentioned they lead to quantum physics.