Whereas, in order to the finding out of the longitude of places for perfecting navigation and astronomy, we have resolved to build a small observatory within Our Park at Greenwich.
So decreed King Charles II on June 22, 1675, so that the British Navy could calculate longitude at sea. Longitude is a lot trickier than latitude, but if sailors had good star and moon charts they could calculate how far east or west of Greenwich they were, given an accurate clock set to Greenwich time.
There were a few glitches, though, including what waves at sea tend to do to a pendulum clock, and increasing air and light pollution as the centuries went by – not to mention that if you stand at the Prime Meridian of the world, as Greenwich was declared in 1884, your GPS will show you’re several hundred feet off zero degrees longitude.
Not to worry though. Now the prime meridian is determined statistically.
At least we still have Greenwich Mean Time – well, actually, it’s only called that. First of all, the observatory moved twice, first to Sussex and then to Cambridge; and then, to add insult to injury, the world now uses atomic clocks.