Originally developed by astrologers, the astrolabe eventually adopted a second mission: helping mariners locate their position north or south of the equator. It was a huge improvement over dead reckoning. The astrolabe also relied on visual observation of the sun's zenith. But if noon was missed by even a few minutes, the resulting miscalculation might take days to discover.
The new problem was the lack of accurate clocks. Until noontime could be determined with certainty, latitude measure by astrolabe might vary by five to 10 degrees. However, despite this flaw, use of the astrolabe continued well into the 1700s.
Improvements in latitude measurement occurred incrementally. They included such devices as the cross-staff, the Davis Quadrant, and the octant--an early precursor of the device that would forever solve the problem of measuring latitude accurately.
In 1759, John Bird invented the sextant. Utilizing the newly discovered double-reflection principle of light, the sextant was the breakthrough device that solved the problem of precisely measuring latitude. At noon, the sextant is aimed at the horizon, and by using a series of mirrors, the operator manipulates the reflection of the sun until it appears to be resting upon it. A curved degree scale on the lower portion of the sextant indicates the correct latitude.
Bird's invention occurred almost simultaneously with the development of accurate marine chronometers. Once the two technologies were married, latitude could be measured with pinpoint precision.
Sextants are still in use. Both merchant ships and warships carry sextants as backups to their electronic navigational systems.
During WWII, LORAN (LOng RAnge Navigation) was developed and used extensively by both the U.S. Navy and the RAF. LORAN is a land-based system that operates by transmitting a series of electronic waves from a primary LORAN station and two "slave" transmitters.
As the waves repeatedly intersect one another, a vessel equipped with a LORAN receiver can determine its location to within roughly 500 yards. In the United States, operation of the LORAN system is under the control of the Coast Guard and provides navigational data (latitude and longitude) for the entire Great Lakes basin and U.S. coastal waters.
GPS (Global Positioning System) is the latest iteration of electronic navigational systems. Utilizing a network of 24 orbiting satellites, GPS relays navigational data with unprecedented accuracy. The current version of GPS available to the public will pinpoint their location to within about 13 meters. The GPS systems used by the U.S. military possess an even a greater decree of precision.