(i) In India, 82°30' E is taken as a standard meridian because for every 1° longitude time difference is 4 minutes. India's total longitudinal extent is 30°(97° E - 67° E = 300E). So, total time difference is 30 x 4 = 120 minutes or 2 hours, which means the time difference between Gujarat and Arunachal Pradesh would be 2 hrs. To avoid such confusion and to have same time all over the country, we have taken a standard meridian.
(ii) Indian Standard Time (IST) is 5:30 hours (5 hours 30 minutes) ahead of Greenwich Mean Time.

The difference between parallels of latitudes and meridians of longitudes are given below:

It is necessary to have a standard time because:
(i) Different meridians have different time, which makes it difficult to prepare a time table for trains and flights.
(ii) It helps to maintain uniformity of time throughout the country.
Indian Standard Time: In India, the longitude of 82° 30'E is treated as the standard meridian and the local time at this meridian is taken as the standard time for the whole country. It is known as Indian Standard Time (IST).

The International Date Line (IDL) is an imaginary line on the surface of the Earth opposite the Prime Meridian, where the date changes as one travel across the East or West. Roughly along 180° longitude, it almost corresponds to the time zone boundary separating -12 and +12 hours, Greenwich Mean Time (GMT). Crossing the IDL, travelling east results in a day or 24 hours being added and crossing west results in a day being subtracted. The exact number of hours depends on the time zones.

In the figure, three places namely a, b and c are marked. Let us assume that the time difference between' a' 'b' and 'c' is 1 hour each. If the time at a place 'b' is 6 in the morning then the time at 'a' would be 5 in the morning and time at 'c' would be 7 in the morning. The time at place' a' is 1 hour less because it is to the West of 'b' and time at 'c' is 1 hour more because it is to the East of 'b'. This is due to the fact that the Earth rotates from West to East. As the sun rises in the East so place 'c' has 7 am, place 'b' has 6 am and place' a' has 5 am. This shows that time increases as we go towards east and it decreases as we go towards west.

Besides the equator (0°), the North Pole (900N) and the South Pole (90° S) are four important parallels of latitude.
(i) Tropic of Cancer: It is at an angular distance of \(23{1\over 2}°\) from the Equator. It is important because 2 on 21^st June, the Sun is directly over it.
(ii) Tropic of Capricorn: It is at an angular distance of 23°,30 S from the Equator. On 22^nd December, the sun is over this latitude.
(iii) Arctic Circle: It is at a distance of 66W N of the Equator. On 21^st June, there is 24 hours daylight on the parallel.
(iv) Antarctic Circle: It is at a distance of 661/2° S of the equator. On 22nd December, there is 24 hours day-light on this parallel.