The offer paid off. The answer lay in knowing what time it is when you make your sextant measurements. For example, say your Greenwich almanac predicts that the sun is highest at noon. Your shipboard clock, synchronized to Greenwich time when you left port, says it’s 2 p.m. when your sextant measures that event. Then you must be the equivalent of two hours west of Greenwich.

In 1761 a cabinetmaker named John Harrison developed a shipboard timepiece called a chronometer, which lost or gained only about one second a day—incredibly accurate for the time. For the next two centuries, sextants and chronometers were used in combination to provide latitudes and longitudes.

In the early 20th century several radio-based navigation systems were developed and used widely during World War II. Both allied and enemy ships and airplanes used ground-based radio-navigation systems as the technology advanced.

A few ground-based radio-navigation systems are still in use today. One drawback of using radio waves generated on the ground is that you have only two choices: (1) a system that is very accurate but doesn’t cover a wide area or (2) a system that covers a wide area but is not very accurate. High-frequency radio waves (like satellite TV) can provide accurate position location but can only be picked up in a small, localized area. Lower frequency
radio waves (like FM radio) can cover a larger area, but are not a good yardstick to tell you exactly where you are.