For millennia, man navigated across the seas and oceans using nothing but the stars and the sun, or by staying within sight of land at all times. With technologies such as the compass, radio, and, ultimately, global positioning systems (GPS), maritime navigation has over the years become ever more precise and reliable, with increasing levels of redundancy in case of equipment failures. Few would disagree that the ubiquity, reliability, and simplicity of GPS makes it the single best way to navigate in open waters.
The ancient Polynesians managed to travel thousands of miles from their home islands in the South Pacific to Australia, by relying on the position of the stars alone. Navigating by the stars, known as dead reckoning, obviously remains a viable option for sailors who know how to do it. Of course, with a clear night sky being a necessity for this technique, it can be a dangerous and unpredictable option. Increasingly, it is a last ditch resort for all but the most committed of traditional, purist sailors.
Around the time of the Renaissance, the invention of the compass and the sextant made traveling beyond sight of land infinitely more predictable. By measuring a ship's position relative to that of the sun, location and course could be plotted on a map overlaid with latitude and longitude lines. This was perhaps the greatest leap forward in maritime navigation until the computer age, and made possible the period known as the Age of Sail, for the great sailing ships that traversed the world on military, exploratory, and commercial missions. Like dead reckoning, manual plotting remains a viable, but complex, tool for the properly trained.
Maritime navigation remained greatly unchanged throughout the 19th century, though refinements in existing techniques, maps, and charts, and progress in steam propulsion led to improvements in the reliability and speed of oceanic travel and navigation. Since the first GPS satellites were put into orbit in the second half of the 20th century, however, satellite navigation has become the standard for maritime navigation. Small GPS receivers are cheap and reliable, and can locate one's position to within a matter of feet or meters. GPS navigation is mandated on most commercial vessels, such as cruise ships, but are equally essential pieces of equipment for hobbyist or amateur sailors.
There is another kind of modern navigation technique that does not require satellites, known as lines of position (LOP) navigation. This is, in essence, a highly evolved amalgamation of classic techniques, and uses landmarks, known compass bearings, and other reference points to determine location and course. Submarines often use sonar readings in this way to navigate while underwater. It is a good fallback when GPS is for some reason unavailable, but suffers many of the same drawbacks as dead reckoning — namely a complete reliance on ideal conditions and skill of the navigator.