The Earth is an Oblate Spheroid. Therefore it is difficult to locate places on it unless a mathematical system is used. There are two reference points on the Earth- the North Pole and South Pole. With the help of these two points it has been possible to draw the Equator, as it lies exactly midway between the poles. In order to locate places accurately, a network of lines are drawn o the globe. The horizontal lines are the lines of latitudes and the vertical ones are the lines of longitudes. These lines intersect each other at right angles and create a network called a grid or graticule. The graticule helps us to locate places on the surface of the Earth accurately.
Our earth spins continuously on an imaginary axis which passes through the Centre of the earth. Northern tip of the axis is North Pole and the Southern tip of the axis is South Pole. A largest possible circle that can be drawn on a globe is Equator. It divides the globe into two equal halves. The northern half is known as Northern hemisphere and the southern half is Southern hemisphere. Since the distance from the equator to either of the poles is 1/4th of a circle round the earth, it will measure ¼ of 3600 i.e.900. Thus 900 North latitude marks the North Pole and the 900 South latitude marks the South Pole.
Parallel to the equator one set of imaginary lines are drawn which encircle the earth and run in east west directions. These are called latitudes. Since all these lines are parallel to the equator as well as one another, they are called parallels of latitudes. Latitude is the angular distance of a place from the equator, lying either in the north or in the south directions. It is measured in degrees from the equator toward either pole. One degree (0) is divide into sixty equal parts and each unit is called a minute (’).
A minute is further divided into sixty equal parts and each unit is called a second (”).
From the equator to the poles, parallels of latitudes become smaller. It becomes a point at the pole. There are 90 latitudes at an interval of 10 in the northern and southern direction of the equator. The value of the equator is zero. All points north of the equator are called ‘north latitudes’ and all points south of the equator are called south latitudes. Therefore, the value of each latitude is, followed by letters ‘N’ or ‘S’.
Besides the equator (00), the North Pole (900 N), the South Pole (900 S), we have four important parallels of latitude. They are:
• The tropic of Cancer (230 30’ N)
• The tropic of Capricorn (230 30’ S)
• The Arctic Circle (660 30’ N)
• The Antarctic Circle (660 30’ S)
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 gets maximum heat and is called the Torrid Zone. On 21st June the Sun is directly over the Tropic of cancer. On 22nd Dec the Sun is directly over the Tropic of Capricorn. These two latitudes form the outer limit of the Torrid Zone. It is the hottest part of the world. Most of the deserts are located here.
The midday 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 temperature i.e. neither very hot nor very cold. So this is Temperate Zone.
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 the sun does not shine much above the Horizon. Therefore its rays are always slanting. So this is Frigid Zone.
A set of imaginary lines which divide the earth into eastern hemisphere and western hemisphere run over the earth in north south directions are called “meridians of longitudes”. These lines are not parallel to one another. They all converge at poles. The distances between them are measured in ‘degrees of longitude’.
They form semi-circles. They are also drawn at an interval of 10. These meridians of longitude and parallels of latitude form a network which is called grid. Unlike the parallels of latitudes, the meridians of longitudes are equal in length.
As one go from the equator to the poles the distance between two meridians decreases. There was an agreement world over that the longitude passing through Greenwich Observatory near the city of London will be considered as the Prime Meridian. It is considered to be 00 Longitude and from it we count 1800 eastward as well as 1800 westward. It is interesting that 1800 east and 1800 west meridians are the same line. To avoid the confusion letters ‘E’ and ‘W’ are written with the values of meridians for the eastern hemisphere and western hemisphere respectively.
The earth takes about 24 hours to complete a rotation. This period of rotation is known as the earth-day. This means in 24 hours the earth completes 360 degree. Therefore, it takes about one hour to complete each 150 or 4 minutes for each degree. Accordingly, the earth has been divided into 24 time zones of one hour each. As the earth rotates from west to east, day starts at different times in different places. Places east of the Greenwich meridian experience sunrise earlier than the place lying west of the prime meridian. All the places situated along the same meridian will have the same local time.
• In India the longitudinal span is from 680 7’ to 970 25’ E. Therefore, it was felt necessary to adopt the local time of central meridian of our country as the standard time for the country as a whole.
• In India 820 30’ E has been considered for this purpose. It is called the standard meridian. This is known as Indian Standard Time (IST).
• The Greenwich Mean Time is 5 hours and 30 minutes behind the Indian standard time. In global context Greenwich (00) time is followed which is called the Greenwich Mean Time (GMT).