Imagine shining a flashlight at a globe. Only part of the globe would receive light, while the opposite side of the globe would be dark. As Earth rotates, different parts of Earth receive sunlight or darkness, giving us day and night.
As your location on Earth rotates into sunlight, you see the Sun rise. When your location rotates out of sunlight, you see the Sun set.
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.