If the whole world was flat, the scheme of rectangular coordinates would serve all map purposes. The earth is not a perfect sphere. It is slightly flattened at the North and the South Poles and bulge in the middle. It is difficult to describe the location of a point on a sphere like the earth.
A globe begins nowhere and ends nowhere, but it moves. The Earth turns on its axis, it has poles, they are definite points.
The most important point to locate other points and lines of reference is North Pole. The opposite end of the imaginary axis is called South Pole. A line circular in nature which is equidistant from the two poles (midway between the poles) is called Equator. The northern half of the earth is known as the Northern Hemisphere and the southern half is known as the Southern Hemisphere. They are both equal halves.
Poles and Equator are needed for a frame of reference to build a system of coordinates. All parallel circles from the equator up to the poles are called parallels of latitudes. Latitudes are measured in degrees.
Parallels of Latitudes
Equator is the largest of a series of circles. Equator is called the parallel of 0°. Since the distance from the equator to either of the poles is one-fourth of a circle round the earth, it measures ¼th of 360 degrees, i.e. 90°. So, the pole is a parallel of 90°.
All parallels north of the equator are called north latitudes (N). Similarly, all parallels south of the equator are called south latitudes (S).
Besides the equator (0°), the North Pole (90° N) and the South Pole (90° S), there are four important parallels of latitudes:
- Tropic of Cancer: 23½° N
- Tropic of Capricorn: 23½° S
- Arctic Circle: 66½° N
- Antarctic Circle: 66½° S
Heat Zones of the Earth
The mid-day sun is exactly overhead at least once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. Therefore, this area receives the maximum heat and is called the Torrid Zone.
The mid-day sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn. The angle of the sun’s rays goes on decreasing towards the poles. The areas bounded by the Tropic of Cancer and the Arctic Circle in the Northern Hemisphere, and the Tropic of Capricorn and the Antarctic Circle in the Southern Hemisphere, have moderate temperatures. These are called Temperate Zones.
Areas lying between the Arctic Circle and the North Pole in the Northern Hemisphere and the Antarctic Circle and the South Pole in the Southern Hemisphere, are very cold. It is because here the sun does not rise much above the horizon. Therefore, its rays are always slanting and provide less heat. These are called Frigid Zones.
Meridians of Longitude
On the globe there is an another set of lines. They are semi-circles which swing from pole to pole. These lines are Meridians, joining north and south poles. The distances between them are measured in degrees of longitude. Each degree is further divided into minutes, and minutes into seconds. 60' (60 minutes) make up a degree and 60" (60 seconds) make up a minute. So 30' means half a degree and 40" means two-thirds of a minute.
Meridians of Longitude are semicircles and the distance between them decreases steadily pole-wards until it becomes zero at the poles, where all the meridians meet.
Unlike parallels of latitude, all meridians are of equal length. Thus, it was difficult to number the meridians. Hence, all countries decided that the count should begin from the meridian which passed through Greenwich (UK), where the British Royal Observatory is located. This meridian is called the Prime Meridian or Zero Degree Meridian.
The meridian passing through Greenwich has been adopted internationally as the Standard meridian. This meridian divides the sphere into two hemispheres, one the Eastern Hemisphere (E) and the other the Western Hemisphere (W). The longitude is measured from 0° to 180° either towards the west or towards the east.
All meridians intersects parallels at right angles.