Seafarers have worked with a coordinate system for navigating the globe since the middle ages. This system uses lines of latitude and longitude. Lines of latitude are always parallel to each other and encircle the earth in concentric rings from the south to the north poles. The Equator is the 0° parallel.
The Tropic of Cancer, Tropic of Capricorn, and Arctic Circle are all parallels of latitude. Positions are either north or south of the Equator. Latitude is a function of nature and any competent sailor can estimate a rough value of their latitude. To find your position on earth, you need one more piece of information, longitude.
Meridians of longitude loop around the earth from north to south, all converging at the poles. The most well known line of longitude is the Prime Meridian at 0° running through Greenwich, England. Time zones begin at the Prime Meridian and the International Date Line is located at 180° longitude.
Directions are either east or west of the Prime Meridian. Longitude posed a real problem for mariners in the middle ages. Lines of longitude merge as they go north or south and positions could not be determined while at sea until the advent of accurate timepieces in the 1700's.
Latitude and longitude form a coordinate system. Each line of position encompasses 360°, each degree is divided into 60 minute units, and each minute is further divided into 60 seconds. A minute of latitude is approximately one nautical mile. Since longitude varies, a degree of longitude is much larger at the Equator than at the poles. Latitude is described as a number from 0° at the Equator to 90° at the north or south pole.
An example for illustration could be 45 degrees N or 30 degrees S latitude. Longitude starts from 0 at the Prime Meridian to 180 degrees east or west. Examples would include 120 degrees E or 20 degrees W longitude. Putting the two addresses together can identify any unique location on earth. The convention for describing a location is to give the value for latitude first and longitude next. Seconds units are displayed as decimal minute values with GPS receivers. An example position would look like N40 degrees 34.56 W80 degrees 33.45 on your GPS display. Here are some of the latest systems.