John Harrison had invented a clock that could accurately tell time at sea. This made it possible to finally determine longitude at sea. But it would still take many years before the clocks could be produced in enough numbers to make them common possessions among seamen. Even by the year 1815 there were only about 5,000 sea clocks in existence. How many clocks do you think there are now?

Take a moment and just think about how often we use clocks and watches every day of our lives just to keep track of time! Alarm clocks wake a lot of people up each morning. School starts and stops each day according to the clock, and so do many people's workdays. But, now you know that a clock can tell you much more than just the time. It can help you find your location.

With the availability of sea clocks in the 1800's, a sailor needed only two instruments to determine his location: a sea clock and a sextant (as well as the tables of different "heavenly" events, such as sunrises, sunsets, and the highest point of the sun for the different latitudes). This method of determining location carried the world into the twentieth century. But technology has provided us even easier, more convenient ways of finding our location. That's what we'll look at in the next section.