- When the sun is overhead at noon, Local time is calculated.
- In some cases, a country may have many lines of longitude passing through it.
- Keeping a particular meridian as a standard meridian, the standard time of a country or a part of it is calculated.
- Many countries do not observe a common time.
Some countries use Daylight Save Time (DST).
More than \(60\)% of the countries use Standard time.
- The meridians are selected in multiples of \(15°\)or \(7 ½°\).
- It is done in such a way that the variation of standard time from the Greenwich is expressed either as \(1\)hour or \(½\) an hour.
Indian Standard Time
- The longitudinal extent of India is from \(68°7’\) E to \(97°25’\) E.
- Number of longitudinal lines passing through India: \(29\)
- It is not feasible or advisable to have \(29\) standard times for the country.
- So, \(82½°\) E line of longitude is observed as the Prime Meridian to calculate the Indian Standard Time (IST).
- The \(82½°\)E line of longitude passes through Mirzapur near Allahabad in Uttar Pradesh.
- This is located at an equal distance from Ghuar Mota in Gujarat and Kibithu in Arunachal Pradesh.
The standard meridian of India passes through: Uttar Pradesh, Madhya Pradesh, Chhatisgarh, Orissa and Andhra Pradesh.
- The world has \(24\) time zones.
- Some countries have a great longitudinal extent.
- So they have more than one standard time.
- Example: Russia has \(7\) time zones.