Latitude and longitude coordinates are determined by figuring out where someone is on Earth in relation to the equator and the prime meridian. Because the Earth is a sphere, locations can be precisely measured in degrees, using an angle centered in the Earth's core. These coordinates are written in the formula of degrees, minutes, and seconds, with latitude measurements given before longitude measurements, like this: 38° 53′ 42.4″ N, 77° 2′ 12″ W. Fortunately for modern explorers, there are a number of electronic tools which can calculate latitude and longitude automatically, sparing people the effort of painstaking observations and calculations. Historically, the issue of finding one's correct location was extremely important, and several great scientific minds dedicated a great deal of time to this quest.
Lines of latitude run around the Earth horizontally in the form of circles that get progressively smaller as they approach the poles. A latitude coordinate is given in degrees of difference between the equator and someone's current location. There are 60 nautical miles (111.12 km) between each degree of latitude, and 90° of latitude in either direction. For example, someone standing on the equator would be at 0°, while someone at the poles would be at 90° of latitude.
By convention, people identify latitude measurements as being in the North or South, as a number like “22° of latitude” is meaningless unless one knows which hemisphere the measurement is meant to apply to. Certain lines of latitude are named, such as the Tropics of Cancer and Capricorn. Historically, people determined latitude with a tool called a sextant, which could be used to calculate the degree of difference between the observer and the equator with incredible accuracy when used well.
Longitude is the degree of difference between an observer and the prime meridian, an arbitrary line that runs more or less through England. There was some historical dispute over the placement of the line, since it was generally believed to be a great honor. Lines of longitude approach each other as they reach the poles, so the distance between degrees actually varies. Longitude measurements never exceed 180°; someone who is at 180° East is along the same line of longitude as someone who is at 180° West, and rather than saying that one was at “181° East,” one would use “179° West.”
Measurements of longitude are determined by calculating the time difference between the observer and the prime meridian, and using that different to come up with a measurement in degrees, minutes, and seconds. The time at the prime meridian is called Coordinated Universal Time (UTC), and most people have extremely accurate clocks which keep UTC and local time. Historically, the challenge of making timepieces that could accurately keep track of UTC was a serious problem for navigators.
By determining a person's latitude and longitude, it is possible to figure out where on Earth he or she is. Very precise electronic devices network with satellites to pinpoint locations within inches (or centimeters). People can also do things the old fashioned way with astronomical observations and extremely accurate clocks, but most people prefer to leave the work to electronic equipment, since a miscalculation can be embarrassing, or cause a serious problem in some cases.