Question

what is the formula for taking out standard meridian of a country ? Plz tell

Answer 1

The international convention says that longitudinal difference between two standard meridians should be a multiple of 7.5 degrees, so that time difference is always in half hours or full hours (7.5 deg * 4 min/deg = 30 minutes).

To make a preliminary guess about the standard meridian of a country, take the westernmost longitude and the easternmost longitude of that country, take the mean, and find the nearest multiple of 7.5 degree. However, if the country has a large longitudinal expanse, there may be more than one time zone (and hence multiple standard meridians). Location of standard meridian can also be affected by political considerations, demographics, etc. (Nepal and Chatham Islands are the only two exceptions to the 7.5 degree rule, as they have time zones of UTC +05:45 and UTC +12:45.)

Here's an example-

India
Longitudinal extent: 68° 7′ 53″ E to 97° 24′ 47″         E.
Mean: 82°46’20”
Nearest multiple of 7.5°: 82.5°
First guess of standard meridian: 82° 30′ E
This is the actual standard meridian of India.
Time zone: 82.5 × 4 minutes ahead of GMT = 5.5 hours ahead of GMT.
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