In the nineteenth century, the emergence of transcontinental railroads further complicated matters.

One "day" was simply the amount of time between two consecutive "noons." Most cities on the planet set their clocks to that cycle, and all was good — at least within any specific city.

The problem was, each city experienced noon at their own (apparent) pm.

Somewhere around 150 BCE, Hipparchus of Nicea, a Greek mathematician and astronomer, proposed a global grid of longitude and latitude lines to measure position.

It was a coordinate system for locating points on the surface of a sphere.

And of course, to use either system effectively, it's helpful to know the clock times at both the sender's and receiver's locations.

Latitude and longitude Before we explain how time zones solved these clock problems, let's do a quick review of latitude and longitude.

But beyond that, our teachers didn't say much more.

That's probably why I keep getting questions like these on my blog: Frequent international travelers are comfortable with the IDL. They're used to the idea of changing the day and date, yet even they would struggle with most of those questions. The lyrics from that classic song come to mind: "Does anybody really know what time it is? " As you will soon discover, the answer to both questions is "yes." There are no equations, but you might encounter a few new terms.

The vertical axis measured "latitude," and the horizontal axis "longitude." Though prescient, his idea languished for over a millennium.

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