When describing location, it is common to mention the city, state, or country as a location descriptor. It is also common to talk about landmarks that may be nearby. Another way to describe location is to use reference lines to describe coordinates, or absolute position, on the globe.
Two types of imaginary reference lines are used to locate positions or points and to make accurate globes and maps. These lines are called parallels of latitude and meridians of longitude. Two of these imaginary reference lines, the equator and the prime meridian, are called primary reference lines because they are where we start the numbering system.
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Equator, Hemispheres, Axis, and Directions
The earth rotates daily about its axis. The north and south poles are the two imaginary points where the axis would enter and exit from the earth if the axis were a pole or a line (see Fig. 1.9). The equatoris the imaginary primary reference line drawn around the earth halfway between the north and south poles. The half of the earth to the north of the equator is the northern hemisphere; the half to the south is the southern hemisphere (Fig. 1.9). (The prefix hemi- means “half”; thus, hemisphere means “half-sphere.”) The poles determine north and south directions. Movement toward the North Pole is northerly in direction. Movement toward the South Pole is southerly in direction.
Parallels of Latitude
Latitude is measured in degrees (°)—from 0˚ to 90˚—north or south of the equator. Degrees of latitude are measured from an imaginary point at the center of the earth. If the earth was cut in half, this imaginary point would be intersected by a line drawn from the North Pole to the South Pole and by a line drawn from the equator on one side of the earth to the equator on the other (Fig. 1.10 A). A radius is a line drawn from the edge of a circle to its center. The angle between the radius lines drawn from the equator and from the north pole (or south pole) forms a right angle, which is 90°.
The equator is at 0°, and both of the earth’s poles are at 90° from the equator. Latitude is determined by the angle between a point on the earth’s surface and the equator. To calculate the angle, draw a line from the point to the center of the earth and a line from the equator to the center of the earth (Fig. 1.10 A).
Parallels of latitude are imaginary reference lines that form complete circles around the earth parallel to the equator and parallel to each other. Every point on a parallel of latitude is the same distance from the equator, and thus the angle formed between the equator and the latitude line is constant. This is shown in Fig. 1.10 B for the latitude lines 30° and 60° north.
Parallels of latitude are circles of different sizes (see Fig. 1.11). The largest parallel is at the equator, and the parallels decrease in size towards the poles. Except for positions located right on the equator (0°), parallels of latitude are described by the number of degrees that they are north (N) or south (S) of the equator. The greater the distance from the equator, either north or south, the higher the latitude. Honolulu, Hawai‘i, for example, is on the 21° N parallel. Sydney, Australia, is on the 34° S parallel.
Meridians of Longitude
Meridians of longitude are imaginary half-circles running from the North Pole to the South Pole. They are sometimes called lines of longitude. Unlike parallels of latitude that are different sizes, all lines of longitude are the same length. Since every meridian must cross the equator, and since the equator is a circle, the equatorial circle can be divided into 360°. These divisions of the equatorial circle are used to label the meridians.
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By international agreement, the 0˚ meridian (also called the prime meridian) is drawn through Greenwich, England. Meridians are numbered east and west from the prime meridian (Fig. 1.12 A).
Longitude is the distance east or west of the prime meridian, and longitude is measured in degrees from 0˚ to 180˚ (Fig. 1.12 B). Places to the east of the prime meridian have east longitude. Rome, Italy, for example, is located on the 12˚ E meridian, whereas Washington DC, USA, is located on the 77˚ W meridian.
East and west longitude meet at the 180˚ meridian, which runs through the Pacific ocean basin (Fig. 1.13). Therefore, most of the United States (including Hawai‘i) lies in the western hemisphere. Only a small portion of Alaska (part of the Aleutian Islands) crosses the 180˚ meridian into the eastern hemisphere. The complete circle around the earth made by the prime meridian (0˚) and the 180˚ meridian divide the earth into eastern and western hemispheres (see Figs. 1.12 and 1.13).
International Date Line
The international date line is an imaginary line running mostly along the 180˚ meridian (see Fig. 1.14). The international date line determines where on earth the date changes. For example, at the same moment the time is 6:00 am on July 1st in Bangladesh, the time is 6:00 pm on June 30th in Mexico and midnight on June 30th in England (see Fig. 1.15 A).
Places located immediately to the right and left of the date line are 24 hours apart. This means that on the left side of the international date line in Tonga, when the time is noon on Monday, July 1st, on the right side of the date line in Sāmoa, the time is noon on Sunday, June 30th (see Fig. 1.15 B).
Travelers who cross the dateline heading west lose a day, but travelers who cross the dateline going east gain a day. When traveling east across the dateline, it is actually possible to arrive at your destination earlier than when you left!
For practical purposes, the international date line has been adjusted to allow certain land areas to remain together in the same day and time zones. For example, the extreme eastern tip of Russia, which juts into the Bering Strait, was kept in the easternmost time zone, whereas the U.S.-owned Aleutian Islands were kept as part of the westernmost time zone (see Fig. 1.15 B).
In another example, the country of Kiribati (pronounced KIRR-i-bas) drastically changed the date line in 1995 so that the entire country could be on the same day at the same time. Before this, the western part of Kiribati, where the capital lies, would be 22 hours ahead of the eastern portion of the county. Now eastern Kiribati and Hawai‘i, which are located close to the same longitude, are a whole day apart (see Fig. 1.16).
Location
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Lines of latitude and longitude form an imaginary global grid system, shown in Fig. 1.17. Any point on the globe can be located exactly by specifying its latitude and longitude. This system is essential for ships at sea that cannot locate their positions using landmarks or coastal navigational aids such as buoys or channel markers. This system is just as useful for people on land when hiking, driving, or surveying an environment.
To locate a point on a globe exactly, degrees of latitude and longitude are further subdivided into minutes and seconds. In latitude and longitude measurements, minutes and seconds do not refer to time. Instead, they refer to parts of an angle. But, like with time, there are 60 minutes in a degree (just as there are 60 minutes in an hour). Similarly, there are 60 seconds in a minute of time and 60 seconds in a minute of longitude or latitude.
1 degree (1°) = 60 minutes (60’)
1 minute (1’) = 60 seconds (60”)
The latitude and longitude readings of a place are called its spherical coordinates. For example, the coordinates of the location of the USS Arizona Memorial in Pearl Harbor (Fig. 1.18) are “latitude 21 degrees, 21 minutes, and 54 seconds north; longitude 157 degrees, 57 minutes, and zero seconds west.” This is written as “21° 21′ 54″ N, 157° 57′ 0″ W”.
Latitude and Longitude Use
If the latitude and longitude coordinates of a location are known, it can be pinpointed on a map or globe. Knowing the spherical coordinates of a location is useful for people when hiking, diving, or surveying an environment. Sophisticated navigational aids use latitude and longitude to give directions when driving and flying. The spherical coordinate system is essential for ships at sea that cannot locate their positions using landmarks or coastal navigation aids like buoys or channel markers.
Nautical Miles and Knots
In addition to using latitude and longitude to specify location, marine and air navigators also use the nautical mile as their unit of length or distance. A nautical mile is approximately one minute of latitude along a line of longitude, a distance of 1.85 kilometers. Navigators describe the speed of ships and airplanes in knots. Meteorologists also describe wind speeds in knots. One knot is equal to one nautical mile per hour.
1 nautical mile = 1.85 km
1 knot = 1 nautical mile/hour
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