# 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: