### Theory:

Standard Time
• 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.
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.