What Do Latitude and Longitude Indicate?
IMAGINE TRYING TO DESCRIBE the location of an “X” on a featureless sphere. What system would you devise to convey the location? If the sphere did not have any markings or seams, we would need to first establish a frame of reference — a place on the sphere from which to reference the location of the X. For these reasons, we have devised systems of imaginary gridlines on the Earth. These are referenced as angles from the known points within or on the Earth. The most commonly used imaginary gridlines are latitude and longitude, which are displayed on many maps and are provided by the location capabilities of many cellular phones.
How Do We Represent Locations on a Globe?
If you were trying to convey the location of the X on the sphere, or the location of a city on our nearly spherical planet, a good place to begin visualizing the problem is to establish a framework of imaginary gridlines. Another important aspect is to consider how lines and planes interact with a sphere.
We could draw lines that circle the globe, each staying the same distance from the North or South Pole. The lines are parallel to one another and remain the same distance apart, and so are called parallels. In addition, these lines are parallel to imaginary cuts through the Earth, perpendicular to Earth’s spin axis (which goes through the North and South Poles). The parallel that is halfway between the North and South Pole is the equator.
If we traveled along one of these lines (i.e., along a parallel), we would stay at the same distance from the pole as we encircled the planet. In other words, our position in a north-south framework would not change.
Lines that encircle the globe from North Pole to South Pole are called meridians. Meridians do not stay the same distance apart and are not parallel. Instead, meridians are widest at the equator and converge toward each pole. A meridian would be the path you would travel if you took the most direct route from the North Pole to the South Pole, or from south to north.
The term meridian comes from a Latin term for midday because the Sun is along a meridian (i.e., is due south or north) at approximately noon. The terms A.M. (for before noon) and P.M. (for after noon) are also derived from this Latin term (e.g., post meridiem).
The intersection of a plane and a sphere is a curved, circular line that encircles the sphere. If the plane is constrained to pass through the center of the sphere, we call the resulting intersection a great circle. A great circle also represents the shortest distance between two points on a sphere and so is the path airlines travel over long distances.
A great circle divides the sphere into two equal halves. The equator is a great circle, separating the Earth into two hemispheres — the Northern Hemisphere north of the equator and the Southern Hemisphere south of the equator. A north-south oriented great circle is used to separate the Western Hemisphere, which includes North and South America, from the Eastern Hemisphere, which includes Europe, Asia, Africa, and Australia. Antarctica, over the South Pole, and the Arctic Ocean, over the North Pole, each straddle the great circle between the Eastern Hemisphere and Western Hemisphere.
An imaginary plane that intersects a sphere at any other angle, not going through Earth’s center, is called a small circle. As is obvious from the small circle illustrated in this figure, a small circle does not divide the globe into equal halves.
Note that all parallels, which are oriented east-west, are small circles, except the equator, which is a great circle (it divides the planet into two equal halves).
In contrast, each north-south meridian, when paired with its counterpart on the other side of the globe, forms a great circle. Any such pair of meridians divides the globe into two equal halves. When viewed together, parallels and meridians divide the planet into a grid of somewhat rectangular regions. Such regions encompass greater area near the equator than near the poles, due to the convergence of meridians toward the poles.
What Are Latitude and Longitude?
If you were a pilot flying from New York City to Moscow, Russia, how would you know which way to go? Our imaginary grid of parallels and meridians provides a precise way to indicate locations using latitude and longitude, which are expressed in degrees. Fractions of a degree are expressed as decimal degrees (e.g., 9.73°) or as minutes and seconds, where there are 60 minutes (indicated by ') in a degree and 60 seconds (“) in a minute (e.g., 9° 43' 48”).
This map illustrates the nature of the problem. If we want to navigate from New York to Moscow, we can tell from this map that we need to go a long way to the east and some amount to the north. These directions, although accurate, would not be good enough to guide us to Moscow. We need to more precisely specify the locations of each place and then figure out the shortest flight path. Fortunately, we can find on the Internet that the location of New York City, as given by latitude and longitude, is 40.7142° N, 74.0064° W. The location of Moscow is 55.7517° N, 37.6178° E. Now if we only knew what
these numbers signify!
Before you go any further, note the yellow line on this map, which represents the shortest route between New York and Moscow. It does not look like the shortest route on this flat, two-dimensional map, but it is indeed the shortest route on the three-dimensional globe. What type of path do you think this flight route follows? It is a great circle.
The latitude of a location indicates its position north or south of the equator. Lines of latitude are parallels that encircle the globe east-west.
The angle created by drawing lines connecting the position of an object on the Earth’s surface to the center of the Earth, and then to the equator defines the number of degrees of latitude of the object’s position. In the Northern Hemisphere, latitude is expressed as degrees north. In the Southern Hemisphere, latitude is expressed as degrees south or as negative degrees.
Parallels of latitude run east-west around the Earth. The zero line of latitude is the equator, with the values increasing to 90 at the north and south poles. There are ten million meters from the equator to the North (or South) Pole, so one degree of latitude is approximately 111 km (69 miles).
The longitude of a location indicates its east-west position. Lines of longitude are meridians that encircle the globe north-south.
As a starting point, a zero-degree meridian is defined as the north-south line that passes through Greenwich, U.K. — this is called the Prime Meridian. The angle created by the object’s position, the center of the Earth, and the Prime Meridian defines that object’s longitude, given as degrees east or west of the Prime Meridian. Meridians west of the Prime Meridian often are expressed as negative degrees.
Meridians of longitude run north-south. They are widest at the equator (where a degree of longitude is also about 111 km) and converge at higher latitudes until they meet at the poles. Starting at the zero meridian through Greenwich, values increase to 180° E and 180° W at the International Date Line, an imaginary line that runs through the middle of Pacific Ocean (not shown; on the opposite side of the globe).
In addition to the equator, there are a few lines of latitude that are especially important. These include the Tropic of Cancer and Tropic of Capricorn, which are 23.5° north and south of the equator, respectively. Also important are the Arctic Circle and Antarctic Circle, which are 66.5° north and south of the equator (23.5° away from the corresponding pole). As discussed later, the 23.5° angle is how much the Earth’s axis is tilted with respect to the Sun.