"I am sure you could get closer today with modern instruments."
Oh, no doubt, but it's still flawed in several ways. There are numerous influences on the speed of light in real space. Many of those cannot be measured from Earth, but are easy enough to understand.
Given those influences, which include gas clouds, gravitational influences, and even Earth's atmosphere, the measurement can only be an approximation.
Nope. Actually "c" (the speed of light) is a defined constant. Everything else adjusts to that constant.
One thing a scientist understands is that all measurements can only be approximations. The goal is to reduce the errors in the measurements.
| Date | Author | Method | Result (km/s) | Error |
|---|---|---|---|---|
| 1676 | Olaus Roemer | Jupiter's satellites | 214,000 | |
| 1726 | James Bradley | Stellar Aberration | 301,000 | |
| 1849 | Armand Fizeau | Toothed Wheel | 315,000 | |
| 1862 | Leon Foucault | Rotating Mirror | 298,000 | +-500 |
| 1879 | Albert Michelson | Rotating Mirror | 299,910 | +-50 |
| 1907 | Rosa, Dorsay | Electromagnetic constants | 299,788 | +-30 |
| 1926 | Albert Michelson | Rotating Mirror | 299,796 | +-4 |
| 1947 | Essen, Gorden-Smith | Cavity Resonator | 299,792 | +-3 |
| 1958 | K. D. Froome | Radio Interferometer | 299,792.5 | +-0.1 |
| 1973 | Evanson et al | Lasers | 299,792.4574 | +-0.001 |
| 1983 | Adopted Value | 299,792.458 |