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KNOWLEDGE HUB – Information You Should KNOW


Prime Meridian

  • The meridian which passed through Greenwich, where the British Royal Observatory is located. This meridian is considered as the Prime Meridian.
  • Its value is 0° longitude and from it we count 180° eastward as well as 180° westward. The Prime Meridian and 180° meridian divide the earth into two equal halves, the Eastern Hemisphere and the Western Hemisphere

Longitude and Time

  • The best means of measuring time is by the movement of the earth, the moon and the planets. The sun regularly rises and sets every day.
  • When the Prime Meridian of Greenwich has the sun at the highest point in the sky, all the places along this meridian will have mid-day or noon.
  • As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich time and those to the west will be behind it.
  • It can be calculated this way- The earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes. Thus, when it is 12 noon at Greenwich, the time at 15° east of Greenwich will be 15 × 4 = 60 minutes, i.e., 1 hour ahead of Greenwich time, But at 15° west of Greenwich, the time will be behind Greenwich time by one hour
  • In India the longitudinal span is from 680 7’ to 970 25’ E. Therefore, it was felt necessary to adopt the local time of central meridian of our country as the standard time for the country as a whole.
  • In India 820 30’ E has been considered for this purpose. It is called the standard meridian. This is known as Indian Standard Time (IST).
  • The Greenwich Mean Time is 5 hours and 30 minutes behind the Indian standard time. In global context Greenwich (00) time is followed which is called Greenwich Mean Time (GMT).


  • The local time of places which are on different meridians are bound to differ.
  • In India, for instance, there will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam.
  • In India, the longitude of 82½° E (82° 30’E) is treated as the standard meridian. The local time at this meridian is taken as the standard time for the whole country. It is known as the Indian Standard Time (IST).

International date line 

The International Date Line (IDL) is an imaginary line of demarcation on the surface of Earth that runs from the North Pole to the South Pole and demarcates the change of one calendar day to the next. It passes through the middle of the Pacific Ocean, roughly following the 180° line of longitude but deviating to pass around some territories and island groups.

The dateline runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half

The 180° meridian was selected as the International Date Line because it mostly runs through the sparsely populated Central Pacific Ocean. It was decided at the International Meridian Conference in 1884 in Washington, D.C. where 26 countries attended.


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