What was known about the Speed of Light by the 1800’s?
Ole Roemer (1644-1710), a Danish astronomer, found in 1675 that the observed orbital period of Jupiter’s moons varied with time.
Newton’s theory predicts the orbital period to be a constant.
Should we conclude Newton’s theory is wrong?
No. Instead the evidence for Newton theory is so strong that Roemer’s observation was used to say something about light!