Why Do We Have Time Zones?
Time is not same everywhere. When it is noon in New York, for example, it is still 9 a.m. in San Francisco. The sun seems to move across the sky from east to west as the Earth turns on its axis.
As the sun “moves,” the hour moves. If every city and town set its clocks by sun time, few places would be on the same time. To establish a regular sequence of time changes around the world, the earth has been divided into 24 standard time zones. All the clocks in a time zone are set at the same time. All those in the time zone to the east are one hour later.
If we had one single time zone for Earth, noon would be the middle of the day in some places, but it would be morning, evening, and the middle of the night in others. Since different parts of Earth enter and exit daylight at different times, we need different time zones.
In the late 1800s, a group of scientists figured out a way to divide the world into different time zones. In order to build the time zone map, they studied Earth’s movements.
As Earth rotates on its axis, it moves about 15 degrees every 60 minutes. After 24 hours, it has completed a full circle rotation of 360 degrees. The scientists used this information to divide the planet into 24 sections or time zones. Each time zone is 15 degrees of longitude wide.
Distance between the zones is greatest at the equator and shrinks to zero at the poles, due to the curvature of Earth. Since the equator is approximately 24,902 miles long, the distance between time zones at the equator is approximately 1,038 miles.
The imaginary dividing lines begin at Greenwich, a suburb of London. The primary dividing line of longitude is called the prime meridian. Longitude is the angular distance between a point on any meridian and the prime meridian at Greenwich.
The time at Greenwich is called Greenwich Mean Time (GMT). As you move west from Greenwich, every 15-degree section or time zone is an hour earlier than GMT, while each time zone to the east is an hour later.
Having different time zones means that no matter where you live on the planet, your noon is the middle of the day when the sun is highest, while midnight is the middle of the night.