The Science of the Perfect Second - a meta-article about us measuring time with insane accuracy - it made me think of a bit from the book Eifelheim (which plays with first contact with aliens during medievel germany times) - but in the present era a scientist notices old measurements of the speed of light were faster...
"Think it through, Jackson. Light speed is frequency times wavelength. So if c is dropping and wavelength is constant, frequencies must be increasing."and I wonder if there could be anything like that
"So," she said, her excitement building, "atomic frequencies govern the rate at which atomic clocks tick. Of course, the speed of light has been constant since they began using atomic clocks to measure it. The instrument is calibrated to the thing it's measuring!