The speed of light was measured before 1801 . It was still considered as a ballistic phenomenon. So the movement of the light source could be added to the speed of light in the Galilean sense.
However young and Fresnel and Arago's established the undulatory explanation of light propagation. Ths was finally established by Arago's colleagues when they measured the speed of light as slower in a denser medium in exactly the amount required to refract light, and account for diffraction.
Newton had proposed attractive forces causing the light to speed up on entry into a denser medium and slow down in a les dense one, but quite how this refracted light I have yet to discover. On the face of it this should result in light refracting in the opposite way to that observed.
The aether was the assumed medium of undulation, but of course it was a potentially special aether, the luminiferous aether. This was an attempt to distance the scientific or physical notion of aether from the widely held religious or occult versions by the same name..
Arago's demonstrated that undulations were propagated more slowly in dense media, and it was assumed that the aether penetrated less easily into denser media. At the same time the aether was being constructed as a kind of fluid, and the theoretical formulations could not decide whether the aether was absolute and stationary, or relative and in motion.
The aether concept was in fact in disarray. Everyone believed in some form of aether but no one actually knew what it was.
In this context the Michelson Morley experiment has little to do with defining the aether. It's design was to see if light travelled at different speeds in if fervent directions, one with an additional velocity component and one without.
The results were said to be null. The experimental design has been ingloriously reviewed and re evaluated, and the results are still said to be null.
At the time this presented a problem for the specific aether hypothesis that was being tested. It later was generalised as a problem for all aether concepts.
Physics was in disarray. The successes of the time were in jeopardy of being untenable. So when Einstein proposed several unusual explanations of experimental results , which turned out to be measurable and verifiable in 1905, it was with alacrity that his other insights were lapped up and presented as a solution!
Lorentz, a crazy experimentalist who always redacted his equations to fit the data,exploring a different elliptical geometry was able to make things agree with observations if he adopted a few simple geometrical rules. Light was to be constant in sledding, and then like Doppler suggested for sound wavelengths could shorten or elongate.
Einstein derived a similar principle, but instead of making it physical he made it a fundamental principle. The difference is, fundmental principles are the foundation of experimental design, and not questioned!
Einsteins reasoning is faulty if taken as derived from Galileos mathematical rules. However, he did not take the mathematics he took the principle: physics should be the same in any frame of reference if they are in uniform relative motion.
The consequence of this is that certain distinct formulas are forced to fit in observably different situations. Then to make it all seemingly work, the Doppler shift is plied. In that scenario lengths alter if measured in different frames. This means time alters and these two fundamental dimensions applied to momentum and mass lead to wired contractions in physical measurements.
This seemed to work, and during the war years the scientists were more concerned about blowing the enemies brains out than any deep analysis of why.
In the meantime Einstein decided to tackle gravity relativistic ally, and got in a muddle. He could not resolve it and so he buried it under arcane mathematics.
This was good and bad. The aether was replaced by a complex mathematical model of its behaviour, and so he preserved the Aether in mathematical form during turbulent times. Later he would make an impassioned plea for the return of the aether into physics. The bad was that his muddle made it ifficulties to resolve the new quantum physics with his new theiry. And quantum physicists, seeing the freedom of mathematics presented their theories in arcane mathematics too.
Feynman freely admitted that he did not know what he was calculating but the calculations were proven right by experiment!.
The Doppler effect is a real physical phenomenon. Wave lengths and frequency do alter, lthough which is which is debate able. This means that clocks will run at different Peres because the the extra velocity component does physically slow things down, but to extrapolate that to living longer and going less rapidly is a stretch too far. If a chemical reaction acts at a slower speed then a system is more likely to fail than o slow down to accommodate!
The polarisation of light is another important non isolated factor.
If a bullet is accelerated in a train it will have greater or less kinetic energy depending on which frame it is measured in. When it hits its target it depends in which frame the target is positioned. If the train is moving and the target is not thn the target will reflect this in how deep the bullet penetrates. This will reflect whether the train is moving toward or away from the target..
Now it is usually assumed that the light is reflected from a mirror . This means that if light is moving toward a mirror with n additional velocity, if the mirror reflects it it also has this additional velocity on return. However if the light is absorbed nd re radiated it loses this afditional velocity and returns at a slower speed. . Similarly if light is radiating with a reduced velocity due to the source moving. On re radition it returns with a greater peed!
However, that is the ballistic explanation. In the medium of propagation light will indeed be constant , but it will have a different frequency depending on the frame in which It is measured. On retaliation the light will have a given frequency which again depends on the frame in hich it is measured.
Combining these 2 effects the interpretation of the null result has never been thus analysed.
For example, a wave with an additional velocity component cannot be generated in a closed system. This is clear because the medium in which the light travels is also moving at the speed of the source and the reflectin and polarising mirror. If the incident light is absorbed and retaliated. Any additional velocity is lost, and again the wave is travelling in a medium that is in the same reference frame as the source nd receiver.
When Fred Hoyle measured light from galaxies and saw the red shift and bloe shift he immediately used the oppler nalysis. . So we know that light in a vacuum oes have a fixed speed. But when we bounce a laser off the moon we have an opportunity to measure 2 different frames of reference where the intervening medium is not moving in the same reference frames as the earth and moon.
The light that comes back does show variation and changes as described. However this is usually averaged out and massaged to remove" experimental error!
http://www.fesg.bv.tum.de/91872-bD1lbg-~fesg~forschung~llr.htmlhttp://www.mcise.uri.edu/sadd/mce565/Ch12.pdf