Units of Longitude and Latitude
The demarcation of the longitude coordinate is done with lines going up and down are called the meridians. A figure to the right shows a few meridians. Longitude ranges from 0° to 180° East and 0° to 180° West. The longitude angle is measured from the center of the earth as shown in the earth graphic to the right.
The zero point of longitude is defined as a point in Greenwich, England called the Prime Meridian. (Why Greenwich of all places?) 180° away from the Prime Meridian is the line called the International Date Line. Unlike the Prime Meridian, the International Date line isn't straight for political/social reasons.
The demarcation of the latitude coordinate is done with circles on the globe parallel to the equator. These parallel circles, fittingly enough, are called parallels of latitude. The figure to the right shows several parallels of latitude. Latitude goes for 0° at the equator to +90° N at the North Pole or -90° S at the South Pole where the angle is also measured from the center of the earth as shown in the earth graphic to the right.
There are a few named parallels of latitude. The reason for their definition is is explored in the Seasons and Ecliptic Simulator.
| Arctic Circle | 66°33’ N |
| Tropic of Cancer | 23°27’ N |
| Equator | 0° |
| Tropic of Capricorn | 23°27’ S |
| Antarctic Circle | 66°33’ S |
The primary unit in which longitude and latitude are given is degrees (°). There are 360° of longitude (180° E ↔ 180° W) and 180° of latitude (90° N ↔ 90° S). Each degree can be broken into 60 minutes (’). Each minute can be divided into 60 seconds (”). For finer accuracy, fractions of seconds given by a decimal point are used. A base-sixty notation is called a sexagesimal notation.
For example, a spot of ground in upstate New York can be designated by 43°2’27” N, 77°14’30.60” W. Sometimes instead of using minutes and seconds to measure the fraction of a degree, a decimal value is used. With such a convention the coordinates above are 43.040833° N, 77.241833° W. The first number was converted by taking the minutes divided by 60 and the seconds divided by 3600 and adding them together. That is: 43.040833° = 43° + 2’ × (1°/60’) + 27” × (1°/3600”). Try converting some values in the calculator below: