(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:

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.

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.

(i) 23°30' N is value of the Tropic of Cancer.
(ii) Frigid zone is situated between 66030' and poles.

We can determine the position of a place with the help of information about its latitude and longitude. We can determine it by taking the help of a grid in following manner.
First of all, find the point of reference where the two lines cut each other. e.g. if we have to find out the location of point b in grid (as shown along side in figure), then:
The location of point b will be D^o N latitude and 1^o E longitude. Vertical lines represent East longitudes and horizontal lines as North Latitudes.

The rate of difference of time can be calculated as follows:
The Earth rotates 360^o in about 24 hours, i.e. 360/24 = 15^o/ hour
15^o in1 hour means, 60 minutes/15 = 4 minutes for 1^o.
For example (i) When it is 12 noon at Greenwich, what time will be at 15^o East of Greenwich?
(i) The time at 15^o East of Greenwich (when it is 12 noon at Greenwich) will be 15x4 =60 minutes or 1 hour ahead of Greenwich, it means 1 p.m.
(ii) When it is 12 noon at Greenwich, what time wil be at 15^o West of Greenwich? The time at 15^o West of Greenwich (when it is 12 noon at Greenwich) will be 15x4 =60 minutes or 1 hour behind, it means 11 a.m.
The time at 180^o will be 12 0' clock of mid-night when it is 12 noon at Greenwich.

Local time is based on the local meridian passing through that place. The places lying North and South on the same meridian will have the same local time and the places lying East or West of that meridian will have different local times. It is important because the time for any country is measured by the local time. The local time of places on different meridians are different. Therefore, it will be difficult to prepare time-table for trains which cross several longitudes.
For example There will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam. Therefore, it is necessary to adopt the local time of some central meridian of a country as the standard time for the country.

The three heat zones of the Earth are as follows:
Torrid zone The mid-day sun is atleast once in a year exactly overhead on all latitudes in area between the tropic of Cancer and tropic of Capricorn.
Temperate zone The areas between the Arctic circle and tropic of Cancer in Northern hemisphere, and the Antarctic circle and tropic of Capricorn in the Southern hemisphere are the regions of moderate - temperatures. These regions are called Temperate zones.
Frigid zones The areas lying between the Arctic circle and the North pole in the Northern hemisphere, and the circle of Antarctic and the South pole in the Southern hemisphere remain very cold because the sun provides less heat due to its slanting rays. These regions are called Frigid zones.