Instead of telling me, “Here is the time of the equinox in Greenwich,” and allowing me to perform subtraction like an adult, Google told me it would occur at UTC−0500 in Texas-- and also at UTC−0500 in Connecticut, which I’m pretty sure is impossible.
So: what the hell is UTC?
It is, for ordinary human purposes, the modern successor to Greenwich Mean Time. GMT was based on mean solar time at Greenwich. UTC is based on atomic clocks because, inconveniently, the Earth does not rotate with perfect regularity.
And why “UTC” instead of something intelligible like “Coordinated Universal Time,” abbreviated CUT? Apparently the international timekeepers compromised between English and French abbreviations.
Fine. Whatever.
But this made me wonder why anyone needed to know the time somewhere else before cell phones or even telegraph lines.
I assumed Greenwich must have been important because it was a port. Close, but not quite. Greenwich was home to the Royal Observatory, founded in part to improve navigation. Sailors needed astronomy and accurate clocks to determine longitude at sea.
British navigators carried marine chronometers set to Greenwich time. Suppose the chronometer said it was noon in Greenwich, while the position of the sun told you it was 2 p.m. locally. That two-hour difference told you how far east or west of Greenwich you had traveled.
So Greenwich time had a very practical purpose: it helped sailors figure out where the hell they were.
By the nineteenth century, railroads and telegraphs made standardized time useful on land too. And in 1884, an international conference adopted Greenwich as the prime meridian.
Perfectly comprehensible:
Here is the time in Greenwich, England.
If you live somewhere else, add or subtract some hours.
Then modernity gave us UTC−0500.
And because that isn't complicated enough, most of the United States changes its clocks twice a year. Hawaii doesn't. Most of Arizona does not. The Navajo Nation in Arizona does. The Hopi Reservation inside the Navajo Nation doesn't.
So UTC−0500 can describe Texas in summer and Connecticut in winter, while in the southwest you can change clocks (if you're in the know) every time you cross a border. I once arrived an hour late for Torah study in New Mexico because I didn't re-set my watch on the road.
