A grid of lines called latitude and longitude can be used to describe the location of any place on Earth. These lines appear on maps, charts, and globes. Lines of latitude are used to describe positions north or south of a line called the equator. Lines of longitude describe positions east or west of a line called the Greenwich, or prime, meridian. Measuring latitude and longitudeThe Earth is a spinning sphere, or ball. The center of the spin is a line called the Earth's axis. The axis meets the Earth's surface at two points—the North Pole and the South Pole. The poles are the starting points for the grid system of latitude and longitude. Exactly halfway between the two poles is the equator. This line runs east-west around the Earth, dividing it into two equal parts called hemispheres. The region between the equator and the North Pole is called the Northern Hemisphere. The region between the equator and the South Pole is called the Southern Hemisphere. LatitudeLines of latitude circle the Earth parallel to the equator. For this reason they are also called parallels. These lines indicate distances between the equator and the poles. The basic unit is the degree (°). The circumference of the Earth is essentially a circle, and like all circles it has a total of 360°. Lines of latitude north and south of the equator are numbered to 90° because the distance from the equator to each pole is one fourth of a circle, or one fourth of 360°. The equator is therefore 0°, the North Pole is 90° north, and the South Pole is 90° south. Lines of latitude vary in length, becoming shorter as they approach the poles. The 60th parallels (60° north or south), for example, are half the length of the equator. LongitudeMany places around the world share the same latitude because they are the same distance north or south of the equator. That is why the measurement of longitude is also necessary. Lines of longitude, like lines of latitude, are evenly spaced. However, they do not run parallel to each other. Instead, they run between the North Pole and the South Pole, each making half a circle. These lines, called meridians, cross the lines of latitude to form the grid. The north-south line that marks 0° longitude passes through Greenwich, England. This is the Greenwich, or prime, meridian. Longitude is measured in degrees east and west of the Greenwich meridian. The region to the east of the Greenwich meridian is called the Eastern Hemisphere. The first meridian on this side is called 1° E. The region to the west of the Greenwich meridian is called the Western Hemisphere. The first meridian on this side is called 1° W. The Eastern and Western hemispheres are both divided into 180 degrees of longitude. At the 180th meridian, they meet again. Together the 180th meridian and the prime meridian form a great circle that goes around the Earth through the poles. How the two work together - Lines of latitude and longitude form a grid pattern on a globe. The lines mark the unique location …
Latitude and longitude together can describe the exact location of any place on Earth. For example, the city of Sydney, Australia, lies at a latitude of 34 degrees south of the equator and at a longitude of 151 degrees east of the Greenwich meridian. Therefore it is in the Southern and Eastern hemispheres. The city's position can be written as 34° S, 151° E. Washington, D.C., lies 39 degrees north of the equator and 77 degrees west of the Greenwich meridian. It is in the Northern and Western hemispheres. Its position is 39° N, 77° W. Measurements of latitude and longitude can be made more precise by breaking down degrees into smaller units. Each degree of latitude or longitude can be divided into 60 minutes (′). Even more accurate measurements can be made by dividing a minute into 60 seconds (″). Using latitude and longitude for navigationLatitude and longitude are especially useful for navigation while sailing or flying because there are often no landmarks to go by. Finding latitude and longitude begins with observing heavenly bodies such as the sun, moon, or stars. Using an instrument called a sextant, a navigator can measure the position of these bodies in the sky. The latitude can be determined by comparing their positions to information in an almanac, which is a book of data about the movement of heavenly bodies. To find longitude, a navigator needs more information than that. In particular, it is necessary to know the exact time. Exact navigation at sea was not possible before the invention of a clock that could keep good time on a ship traveling through all kinds of weather. Such a clock, called a chronometer, was first built in 1735 by the English clockmaker John Harrison. Once sailors had a way to tell time accurately, they were able to determine longitude by comparing local time with Greenwich mean time (GMT), or the time at the Greenwich meridian. Shipboard chronometers are set to show GMT. Because of the speed and direction of the Earth's rotation, navigators know that local time at a given place will be one hour behind GMT for every 15 degrees west of the prime meridian and one hour ahead of GMT for every 15 degrees east of the prime meridian. For example, if a ship's chronometer reads 4:00 AM and the ship's local time is 10:00 AM, the ship is 90 degrees east of Greenwich, or at 90° E. Modern navigation systems use radio signals from land stations or from artificial Earth satellites. Important latitudes and longitudesLatitudesApart from the equator, there are four lines of latitude that mark off natural zones. They are the Tropic of Cancer (23°30′ N), the Tropic of Capricorn (23°30′ S), the Arctic Circle (66°30′ N), and the Antarctic Circle (66°30′ S). The area between the tropics of Cancer and Capricorn includes the equator and is called the tropical zone or the tropics. It is known for its generally hot weather. Between the Arctic Circle and the North Pole, and between the Antarctic Circle and the South Pole, there is at least one day in summer during which the sun never sets and at least one day in winter during which the sun never rises. These regions include the poles and are called the polar regions. They are known for their generally cold weather. The region between the Arctic Circle and the Tropic of Cancer is called the north temperate zone. The region between the Tropic of Capricorn and the Antarctic Circle is called the south temperate zone. Climates in these regions are usually marked by four seasons. LongitudesThe equator makes a natural 0° parallel of latitude. However, there is no natural 0° meridian of longitude. Mapmakers traditionally used the meridian that passed through their city or through the site of their national observatory. To standardize maps, a worldwide prime meridian was needed. Greenwich, then the site of Great Britain's official observatory, was chosen in 1884 for two reasons. First, Britain was a country whose importance was felt around the world. And second, putting the prime meridian there meant that the 180° meridian would pass mostly through the middle of the Pacific Ocean. The second point was important because at about 180° longitude is the International Date Line. The International Date Line is where each calendar day starts and ends. At any hour of the day, a traveler stepping across the line from the east steps forward a day. Turning back, the traveler returns to the previous day. The International Date Line would cause much confusion if it ran through places where many people live. Therefore the line moves east or west of the meridian in certain areas to avoid cutting through island groups. This allows all the islanders to use the same date. |