- Parallels of longitude determine the time of a given place.
- Local time (of any place) is measured in reference to Greenwich Time (London), which changes at the rate of four min/degree of longitude.
- Greenwich Time Line is also known as “Prime Meridian” i.e. 00.
- International Standard Time is measured in reference to the Prime Meridian (or Greenwich).
- The Earth rotates through 3600 in 24 hours or 150 in 1 hour or 10 in 4 minutes. Therefore, the local time varies at the rate of 4 min/degree of longitude from the Greenwich Time.
- As shown in the map given above, Kolkata is approximately 900 east of Greenwich; so, Kolkata is (900 x 4 = 360 min) six hours ahead of Greenwich Time (London).
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