- When the sun is overhead at noon Local time is calculated.
- Through a country many longitudinal lines may pass through.
- 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 a 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.