Globe is the true model of the earth. The full form of Globe is ‘The Global Learning and Observation to Benefit the Environment’. It can be rotated. On the globe, countries, continents and oceans are shown in their correct size. A needle is fixed through the globe in a titled manner is called its axis. Two points, through which the needle passes are two poles – North Pole and South Pole. Like earth, it can move from west to east. An imaginary line running on the globe divides it into two equal parts. This line is known as equator. Both, Northern and Southern are equal halves. All parallel circles from the equator up to the poles are called ‘Parallel of Latitudes’. Latitudes are measured in degrees. The equator represents the zero degree latitude. The distance from the equator to the poles is one-fourth of a circle round the earth.
Thus 90º North latitude marks the North Pole and 90º south latitude marks South Pole As such all parallels north of the equator are called ‘north latitudes’ and parallels south of the equator are called ‘south latitudes’.
Besides the equator (0°), the North Pole (90°N) and the South Pole (90°S). There are four important parallels of latitudes –
The mid – day sun is exactly overhead at least once a year on all latitudes in between Tropic of cancer and the Tropic of Capricorn. This area therefore 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 rays goes on decreasing towards the poles. That’s why they have moderate temperature. Therefore these are called Temperate Zones.
Areas lying between the Arctic Circle and the North Pole and the Antarctic Circle and the South Pole. Here the sun does not rise much above the horizon. Therefore its rays are always slanting and provide less heat. So these areas are very cold. Therefore it is called ‘Frigid Zones’.
The lines running from the North Pole to the South Pole are called the meridians of longitudes and distances between them are measured in ‘degrees of longitudes’. Each degree is divided into minutes and minutes into seconds. The distance between the meridians decreases steadily poleward until is become zero, 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, where the British Royal Observatory is located. This meridian is called the ‘Prime Meridian’. Its value is 0° longitude and from it we count 180° eastward as well as 180° westward.
When the Prime Meridian of Greenwich has the sun at the highest point in the sky, all the places along this meridian will have mid-day or noon.
As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich Time and those to the west will be behind it. The rate of difference can be calculated as follows. The earth rotates 360° in about 24 hours. 360°/24h=15° in one hour, therefore 1 degree in 4 minutes. Thus, when it is 12 noon at Greenwich, the time at 15° east of Greenwich will be 15*4=60 minutes. i.e., 1hour ahead of Greenwich line, which means 1 P.M. But at 15° west of Greenwich time by one hour. It will be 11 A.M.
At Any Place, the watch can be adjusted to read 12 o’clock, when the sun is the highest point in the sky.
It is necessary to adopt the local time of some central meridian of a country as the standard time for the country, so that the time should be same in the whole country. In India, the Longitude of 821/2°E (+5:30) is treated as the standard meridian. It is known as the ‘Indian Standard Time (IST).
(1°=4 minutes, 821/2*4=330, 330/60=51/2, In time format it is +5:30)
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