The time line

• 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 of 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 the Greenwich Time (London).