Time zones have always been a subject of great interest to people. Let us learn more about them in this chapter.

You must be knowing that sunrise occurs at different instants of time for different places. New York is lagging behind us (Chennai) by 11 hours. If it is 12 noon in Chennai, it is 1 a.m in New York.

In 6th standard, your geography teacher told you that this is due to the spherical shape of the earth. She was right. But how is it so?

The earth being a sphere comprises of 360°. (360° / 24 = 15°). The earth has been divided into 24 zones (Because there are 24 hours in a day) depending upon the longitudes. Each time zone differs from the next by 1 hour. Each zone is 15° wide. The time zones have been named from 0 to 23.

All places between the longitudes 7 ½° west of Greenwich and 7 ½° east of Greenwich are in time zone 0. The time used in this zone is known as the Greenwich Mean Time or GMT.

If it is 12 noon at this time zone 0, that is referred to as 12 noon GMT.

The time zone +1 begins at the longitude 7 ½° east of Greenwich and extends to longitude 22 ½° east of Greenwich. We can proceed like this till we reach the time zone +12 which extends from 172 ½ ° east of Greenwich to 187 ½° east of Greenwich. Similarly, time zone -1 begins at longitude 7 ½° west of Greenwich and extends to longitude 22 ½° west of Greenwich. Similarly we can proceed upto time zone -12.

Did you realize that the time zone -12 and time zone +12 are one and the same?

The meridian corresponding to 180° east (which is the same as 180° west) is called the "International Dateline ". It is exactly in the middle of time zones +12 or time zone -12.