Latitude and Longitude
Maps are flat models of three-dimensional objects. For thousands of years people have used maps to define borders and to find places. The map at the beginning of this chapter was made in 1570. What do you notice about the size and shape of the continents? Today, more information is available to create more accurate maps. The science of mapmaking is called cartography.
Cartographers use an imaginary grid of parallel lines to locate exact points on Earth. In this grid, the equator horizontally circles Earth halfway between the north and south poles. The equator separates Earth into two equal halves called the northern hemisphere and the southern hemisphere.
Lines on a map running parallel to the equator are called lines of latitude. Latitude is the distance in degrees north or south of the equator as shown in belo picture. The equator, which serves as the reference point for latitude, is numbered 0° latitude. The poles are each numbered 90° latitude. Latitude is thus measured from 0° at the equator to 90° at the poles.
Locations north of the equator are referred to by degrees north latitude (N). Locations south of the equator are referred to by degrees south latitude (S). For example, Syracuse, New York, is located at 43° N, and Christchurch, New Zealand, is located at 43° S.
Degrees of latitude
Each degree of latitude is equivalent to about 111 km on Earth’s surface. How did cartographers determine this distance? Earth is a sphere and can be divided into 360°. The circumference of Earth is about 40,000 km. To find the distance of each degree of latitude, cartographers divided 40,000 km by 360°.
To locate positions on Earth more precisely, cartographers break down degrees of latitude into 60 smaller units, called minutes. The symbol for a minute is ?. The actual distance on Earth’s surface of each minute of latitude is 1.85 km, which is obtained by dividing 111 km by 60?.
A minute of latitude can be further divided into seconds, which are represented by the symbol ?. Longitude is also divided into degrees, minutes, and seconds.
To locate positions in east and west directions, cartographers use lines of longitude, also known as meridians. As shown in Figure 2.2, longitude is the distance in degrees east or west of the prime meridian, which is the reference point for longitude.
The prime meridian represents 0° longitude. In 1884, astronomers decided that the prime meridian should go through Greenwich, England, home of the Royal Naval Observatory. Points west of the prime meridian are numbered from 0° to 180° west longitude (W); points east of the prime meridian are numbered from 0° to 180° east longitude (E).
Unlike lines of latitude, lines of longitude are not parallel. Instead, they are large semicircles that extend vertically from pole to pole. For instance, the prime meridian runs from the north pole through Greenwich, England, to the south pole. The line of longitude on the opposite side of Earth from the prime meridian is the 180° meridian. There, east lines of longitude meet west lines of longitude. This meridian is also known as the International Date Line, and will be discussed later in this section.
Degrees of longitude
Degrees of latitude cover relatively consistent distances. The distances covered by degrees of longitude, however, vary with location. As shown in in the second picture, lines of longitude converge at the poles into a point. Thus, one degree of longitude varies from about 111 km at the equator to 0 km at the poles.
Using coordinates Both latitude and longitude are needed to locate positions on Earth precisely. For example, it is not sufficient to say that Charlotte, North Carolina, is located at 35°14´ N because that measurement includes any place on Earth located along the 35°14´ line of north latitude. The same is true of the longitude of Charlotte; 80°50´ W could be any point along that longitude from pole to pole. To locate Charlotte, use its complete coordinates — latitude and longitude — as shown in the below picture.
Earth is divided into 24 time zones. Why 24? Earth takes about 24 hours to rotate once on its axis. Thus, there are 24 times zones, each representing a different hour. Because Earth is constantly spinning, time is always changing. Each time zone is 15° wide, corresponding roughly to lines of longitude. To avoid confusion, however, time zone boundaries have been adjusted in local areas so that cities and towns are not split into different time zones.