- Axis (अक्ष) is an imaginary line on which the Earth moves around from West to East. Two points on the Earth through which this axis passes are two poles – North Pole and South Pole.
- Another imaginary line called Equator (भूमध्य रेखा) running on the Earth divides it into two equal parts. 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. All parallel circles from the equator up to the poles are called parallels of latitudes (अक्षांश). Latitudes are measured in degrees.
- Equator represents the 0° latitude. Since the distance from the equator to either of the poles is one-fourth of a circle round the earth, it will measure ¼th of 360 degrees, i.e. 90°. Thus, 90 degrees north latitude marks the North Pole and 90 degrees south latitude marks the South Pole.
- All parallels north of the equator are called north latitudes. Similarly, all parallels south of the equator are called south latitudes. The value of each latitude is, therefore, followed by either the word north (N) or south (S).
- Besides the equator there are four important parallels of latitudes:
(ii) Tropic of
Capricorn (23½° S) [मकर रेखा] in the Southern
Hemisphere,
(iii) Arctic Circle at 66½°
N [उत्तरी ध्रुववृत्त] of the equator,
(iv) Antarctic
Circle at 66½° S [दक्षिण ध्रुववृत्त] of the equator.
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. 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’s rays goes on decreasing towards the poles. As such, 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. and are therefore 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, therefore, called Frigid Zones (very cold).
Longitude (देशान्तर)
- The line of reference running from the North Pole to the South Pole is called the meridians of longitude. The distances between them are measured in ‘degrees of longitude.’ Each degree is further divided into minutes, and minutes into seconds.
- Meridians of longitude are semi-circles and the distance between them decreases steadily polewards until it becomes zero at the poles, where all the meridians meet. Unlike parallels of latitude, all meridians are of equal length.
- 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.
- The Prime Meridian and 180° meridian divide the earth into two equal halves: the Eastern Hemisphere (E) and the Western Hemisphere (W). It is, however, interesting to note that 180° East and 180° West meridians are on the same line.
Time
- 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 earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes. Thus, when it is 12 noon at Greenwich, the time at 15° east of Greenwich will be 15 × 4 = 60 minutes, i.e., 1 hour ahead of Greenwich time, which means 1 p.m. But at 15° west of Greenwich, the time will be behind Greenwich time by one hour, i.e., it will be 11.00 a.m. Similarly, at 180°, it will be midnight when it is 12 noon at Greenwich. The earth has been divided into twenty-four time zones of one hour each. Each zone thus covers 15° of longitude.
- Standard Time - In India, for instance, there will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam. It is, therefore, necessary to adopt the local time of some central meridian of a country as the standard time for the country.
- In India, the longitude of 82½° E (82° 30'E) is treated as the standard meridian. The local time at this meridian is taken as the standard time for the whole country. It is known as the Indian Standard Time (IST).
- India located east of Greenwich at 82°30'E is 5 hours and 30 minutes ahead of GMT (GMT+5:30). So when it is 7:30 p.m. in India it will be 2:00 p.m. noon in London.
- Some countries have a great longitudinal extent and so they have adopted more than one standard time. For example, in Russia, there are as many as 11 standard times.
Facts
- At any place, a watch can be adjusted to read 12 o’clock when the sun is at the highest point in the sky, i.e., when it is midday. The time shown by such a watch will give the local time for that place.
- Large countries that stretch extensively from east to west do not have a single Standard Time for the whole country. The USA has 7, and Canada has 6 time zones.
Source: NCERT
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