When at sea, there are two things a captain and navigator always want to know. These are the vessel's location and how to get to the intended destination. A number of variables are always at play and the weather poses constant challenges due to changing wind and waves. At a given point on any nautical chart, a person can retrieve the coordinates, known as latitude and longitude, for the position of a vessel. Historically, a number of basic instruments were used to determine LAT/LON. Today, these are found through the help of many instruments.
Most ancient mariners found some way to find their location at sea, whether it was the Polynesians noting bird types near islands or Europeans using an astrolabe to measure altitudes of celestial bodies. Sextants, too, measure angles to celestial bodies and are a prudent tool for modern navigators to have on hand, should the electronics fail. Accuracy of the measurements of the ancient tools varied and mainly provided relative positions. Errors in math deductions or with the device itself could pose serious problems for mariners.
The GPS, or Global Positioning System, takes a lot of pressure off the celestial navigator. In an instant, anyone on earth can know his precise location, to within several feet. The accuracy of the GPS makes it an ideal tool for marine navigation provided there is a sufficient electrical supply and the sextant is kept handy. GPS also provides speed and course information. Completely reliant on satellites, the GPS provides both longitude and latitude at a glance.
Mostly usurped by GPS, the LORAN - or Long Range Navigation System - relied on low frequency radio transmitters set up in various locations. The LORAN provided coordinates that weren't as reliable as what one finds today on the GPS. A few boats may still use this type of navigational tool, but the program supporting it has been phased out in the United States, Canada and other nations. Decca and Omega were other forms of radio navigation.
Global Navigation Satellite Systems, or Sat Nav in short, has evolved considerably worldwide, and today the one in the United States has become the NAVSTAR GPS. Many cellular phones today offer some form of satellite navigation or GPS system, but boat captains tend to rely strictly on those designed for marine use.
Like a sextant, a simple protractor and a straw can give angles to celestial bodies, and though this is not an instrument a ship's captain is likely to use, it's a simple way to understand how to find latitude. The equator is at 0 degrees latitude; the north pole is at 90. If the north star (Polaris) is 50 degrees above the horizon, subtract 50 from 90 to realize your latitude is 40 degrees north. If Polaris is lying on the horizon, with no measurable angle, your vessel is located somewhere on the equator. Now you'll need to find your longitude. Actual measuring devices are encouraged.
