Latitude and longitude form a geographic coordinate system. Latitude specifies the north-south position of a point on Earth, while longitude defines its east-west position. Lines of latitude are parallel to each other and are horizontal. Lines of longitude are vertical. These lines are not parallel, so they are furthest apart at the equator and closest near the poles. Lines of latitude and longitude form a model called a graticule.
Latitude
Latitude (ϕ or phi) specifies the north-south position of a point on the Earth’s surface. Lines of latitude encircle the Earth, running parallel to the equator, so they are also called parallels. Lines of latitude run from 0° to 90° (North or South).
Lines of Latitude
Technically, there are many lines of latitude or parallels that divide the Earth. There is a total of 180° of latitude, but not all of the lines neatly fall onto degrees. There are 7 main lines of latitude:
- Equator (0°)
- Tropic of Cancer (23.5° north)
- Tropic of Capricorn (23.5° south)
- Arctic circle (66.5° north)
- Antarctic circle (66.5° south)
- North Pole (90° north)
- South Pole (90° south)
There are 5 main lines of latitude if you omit the North Pole and South Pole, which are really points rather than lines.
The lines of latitude line up with the ecliptic (the plane containing the Sun and Earth) at the vernal and autumnal equinox. But, the Earth is tilted on its axis. The lines are tilted 23.5° with respect to the ecliptic on the summer and winter solstice.
Longitude
Longitude (λ or lambda) specifies the east-west position of a point on the Earth’s surface. Each line of longitude runs from pole to pole and is called a meridian. The Prime Meridian passes near the Royal Observatory in Greenwich, England and has a defined value of 0°. Longitude runs 180° east and 180° west. In addition to location, longitude also defines Greenwich Mean Time.
How Big Are Latitude and Longitude Degrees?
Because lines of latitude are parallel to each other, each degree represents the same distance. Each degree of latitude is about 111 kilometers (69 miles or 364,000 feet). A second of latitude is about 30.7 meters (101 feet).
Degrees of longitude vary. They are largest at the equator and smallest near the poles. The degrees of longitude are 111 kilometers (60 miles) apart at the equator. At 38 degrees of latitude (North or South), one degree of longitude is about 88 kilometers (54.6 miles or 288,200 feet), one minute is 1463 meters (just under 1 mile or 4800 feet), and one second is 24 meters (80 feet). Right at the poles, the degrees are 0 meters or feet.
How to Read Latitude and Longitude (GPS Coordinates)
Map coordinates given in latitude and longitude use a geographic coordinate system or GPS. Be aware there are actually multiple methods that use latitude and longitude and give slightly different positions. The most common methods are the Degrees, Minutes, Seconds (DMS) and Decimal Degree (DD) systems.
GPS coordinates give latitude first, followed by longitude. A comma or space separates the two values. For example, coordinates of 10°N, 70°W specify a location that is 10° north of the equator and 70° west of Greenwich, England. These are the map coordinates for a location in Venezuela.
Most locations do not lie directly on either a line of latitude or longitude. A DMS location uses degrees (°), minutes (‘), and seconds (“). The degrees and minutes values are whole numbers, while seconds are a decimal value. For example, writing the coordinates 10°00’00.0”N, 70°00’00.0”W means exactly the same thing as 10°N, 70°S.
Another method uses degrees and decimal minutes and omits the north/south and east/west part of the address. This is the Decimal Degrees or DD format. In this system, each line of latitude or longitude representing 1° gets divided into 60 minutes and each minute gets expressed as a decimal. A positive number means the location is north of the Equator, while a negative numbers means a location is south of the Equator. A positive longitude value is east of the Prime Meridian, while a negative number represents a location west of the Prime Meridian. For example, the location 41.40338, 2.17403 is in Barcelona, Spain. Even though north/south and east/west are not expressly specified, you know the location is north of the Equator because the first number is positive. You know the location is slightly east of Greenwich because the second number is positive.
History
Eratosthenes of Cyrene gets credit for inventing the geographic coordinate system, which he described in his Geography text in the 3rd century BC. Marinus of Tyre plotted a map of the known world using latitude and longitude in the 1st or 2nd century. Ptolemy’s Geography (2nd century) used the Equator for measuring latitude and a prime meridian off the coast of western Africa. The Prime Meridian was placed at the Royal Observatory in Greenwich, England following an international vote in 1884.
References
- Chang, Kang-tsung (2016). Introduction to Geographic Information Systems (9th ed.). McGraw-Hill. ISBN 978-1-259-92964-9.
- Evans, James (1998). The History and Practice of Ancient Astronomy. Oxford, England: Oxford University Press. ISBN 9780199874453.
- McPhail, Cameron (2011). Reconstructing Eratosthenes’ Map of the World. Dunedin, New Zealand: University of Otago.
- Mittenhuber, Florian (2010). “The Tradition of Texts and Maps in Ptolemy’s Geography”. In Jones, Alexander (ed.). Ptolemy in Perspective: Use and Criticism of his Work from Antiquity to the Nineteenth Century. Archimedes. Vol. 23. Dordrecht: Springer. ISBN 978-90-481-2787-0. doi:10.1007/978-90-481-2788-7_4
- Torge, W. (2001). Geodesy (3rd ed.). De Gruyter. ISBN 3-11-017072-8.