Some 250 years ago, a British astronomer published the first of a series of guidebooks that would ultimately establish Greenwich, England, as the world's zero-degree longitude location, the "prime meridian."

Nevil Maskelyne's 1763 The British Mariner's Guide—and his follow-up The Nautical Almanac and Astronomical Ephemeris, published annually beginning in 1766—would set the standard for nautical navigation for decades to come.

And though the world has largely forgotten the Guide and Almanac, modern technology has not. Any device that uses maps and geolocation—from smartphones to global positioning system (GPS) devices to GPS-enabled trucks, airplanes, and ships—still hails the prime meridian flag that Maskelyne planted at his observatory, the Royal Observatory in Greenwich, two-and-a-half centuries ago.

That's because defining zero degrees longitude—which effectively divides the world into East and West—is the first step toward finding every other point of longitude on the map, and finding your way in general. And that was once a very big deal.

A Longitude Problem

Indeed, before the Guide and Almanac, the world had a deadly longitude problem. Navigation at sea was extremely difficult, resulting in countless disasters and disappearances over the centuries because ships' captains simply could not figure out where they were.