Longitude Map of Earth showing vertical lines of longitude Longitude, sometimes denoted λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. Longitude is given as an angular measurement ranging from 0° at the Prime Meridian to plus or minus 180°. Unlike latitude, which has the equator as a natural starting position, there is no natural starting position for longitude. Therefore, a reference meridian had to be chosen. While British cartographers had long used the Greenwich meridian in London, England, other references were used elsewhere, including: Ferro, Rome, Copenhagen, Jerusalem, Saint Petersburg, Pisa, Paris, and Philadelphia. In 1884, the International Meridian Conference adopted the Greenwich meridian as the universal prime meridian. Each degree of longitude is further sub-divided into 60 'minutes'. In navigation a minute may be sub-divided into tenths. Thus a fully qualified longitude may be expressed thus; 23° 27.5' E A specific longitude may then be combined with a specific latitude to give a precise position on the Earth's surface. As opposed to a degree of latitude, which always corresponds to ca. 111 km, a degree of longitude corresponds to a distance from 0 to 111 km: it is 111 km times the cosine of the latitude. Longitude may be determined by calculating the time difference between the location a person is in and Coordinated Universal Time (UTC). Since there are 24 hours in a day and 360 degrees in a circle, the sun moves 15 degrees per hour (360°/24 hours = 15° per hour). So if the time zone a person is in is three hours ahead of UTC then that person is at 45° longitude (3 hours × 15° per hour = 45°). In order to perform this calculation, however, a person needs to have a chronometer (watch) set to UTC and needs to determine local time by solar observation or astronomical observation. The details are more complex than described here: see the article on Universal Time for more details. A line of constant longitude is a meridian, and half of a great circle. This measurement is important to both cartography and navigation; the discovery of how to measure it accurately was among the important discoveries of the 1600s and 1700s. The first effective solution for mapmaking was achieved by Giovanni Domenico Cassini starting in 1681, using Galileo's method based on the satellites of Jupiter. For application without a professional astronomer at hand, and in particular measurement at sea, the problem was more difficult; see Dava Sobel's book: Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time for a good historical overview. This genius was John Harrison who eventually received the Longitude Prize. See also celestial navigation dead reckoning latitude latitude and longitude navigation sextant time zone great circle distance explains how to find that quantity if one knows the two latitudes and longitudes. External links Look-up Latitude and Longitude Resources for determining your latitude and longitude Most Popular Articles: Music    Musical Instruments    Guitar    Classical Music    History    Music Definition    Country Music    Rock Music    Pop Music    Hip Hop    Rap    DJ    Video Games    Casino Games    Poker    Blackjack    Casinos    Real Estate    Loan    Auction    eBay    Marketing    Search Engine Optimization    File Sharing    MP3    Music Industry    Kazaa    Internet    Supermodel    Fashion    Fashion Designers    Erotica    Modeling    Clothing    Boots    Shoes    Diamond Engagement Rings    Diamond    Rugs    Gospel Music    Affiliate    E-Commerce    Radio This article is licensed under the GNU Free Documentation License at http://www.gnu.org/copyleft/fdl.html You may copy and modify it as long as the entire work (including additions) remains under this license. You must provide a link to http://www.gnu.org/copyleft/fdl.html for use of this content. To view or edit this article at Wikipedia go to http://www.wikipedia.org/wiki/Longitude