An anonymous commenter asks from where the school teacher got his evidence for a slowing speed of light. We can only guess! Perhaps he keeps up to date with news on Louise Riofrio's
blog. Alternatively, he may have looked at wikipedia and observed that the 1926 experiment
measured a value of 299796 km/sec, compared to the modern value of
299792.458 km/sec. This modern value is now the standard for $c$, which since 1983 has been used to define the metre.
Anyway, the lattes all worked well today and I'm enjoying a clear spring evening on this holiday weekend.