Theory:

Standard Time
  • 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.
Important!
The standard meridian of India passes through: Uttar Pradesh, Madhya Pradesh, Chhatisgarh, Orissa and Andhra Pradesh.
Time Zones
  • 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.