A long time ago when adventurers set out in ships to explore new lands, they often lost their way at sea. Sometimes they would crash their ships on rocks because they didn't know where they were. Things were so bad that in 1714 the English Parliment announced a prize for the first person to devise a method to accurately measure longitude or east-west location.

The ancients had figured out how to locate their ship's north-south position or latitude by measuring the altitude of the Sun or stars. But you need both latitude and longitude to know where you are. It was John Harrison who solved the problem and won the prize to accurately determine longitude. He used time to do it.

The Earth rotates 15 degrees from west to east every hour. You need a clock that accurately keeps time on your voyage and is set to the local time at a special place (Greenwich, England). Measure the local noon time, when the Sun is directly overhead, and compare it to your clock. The difference in hours tells you how many longitude degrees you are from Greenwich, England (15 degrees for every hour).

The problem long ago was that the clocks weren't accurate enough, but John Harrison solved that. He made very accurate clocks using springs, gears, precision workmanship and clever ideas.

What if you want to adventure out into space? The distances are greater and you need more accurate clocks. Scientists have determined how to use atoms to make very accurate clocks. The cesium atom emits light waves that oscillate back and forth once every one nine billionths of a second. That's 0.00000000011 seconds! These clocks have been our time standard for several years. Because of these accurate clocks, satellites in space can tell us where we are on Earth to within a few meters.

Global Positioning System (GPS) satellites determine their distance from you by measuring the time for their radio signal to get to your receiver. Distance is velocity times time. The velocity of the radio waves is the well-measured velocity of light. Given an accurate time measurement, you get an accurate distance measurement. But how does the distance from one satellite tell you your location?

It doesn't. You need the distance from at least three satellites. The GPS uses 24 satellites spread evenly around the Earth. With three distances from three different satellites, you can determine your location by trilateration. See the activity for more information.

Can we do better? Yes, scientists have improved the atomic clock using laser cooling. Hotter atoms move faster. Atoms can be slowed down by hitting them with laser beams, just like you could slow down a cocker spaniel by throwing pillows at it. The time for a single oscillation of the light waves emitted by atoms can be measured more accurately when the atom is cooled. So we have better clocks.

It turns out that slow cold atoms are the best of all. NASA plans to put laser-cooled atomic clocks on the International Space Station. These clocks with slower cooled atoms will be the best yet and will provide the accuracy to help spacecraft navigate even greater distances out into space. The age of exploration has come a long way from ships that sailed the seas and accurate time measurement is the key.