What Time Is It?
ES0405  Standard Time Zones

In a world of global travel, it is not convenient for every single place on Earth to keep its own unique time. Thus, to make keeping track of time easier, Standard Time Zones were established: the time is always the same throughout each time zone. Earth is divided into twenty-four time zones of approximately equal width. The time in each successive time zone is one hour different from the times in neighboring time zones.

To establish time zones, Earth's rotation rate of 360 degrees of longitude per day was divided by 24 hours. The result shows that Earth turns 15 degrees of longitude per hour. Adopting time zones that are exactly 15 degrees wide would result in 24 equally spaced time zones around the world. For convenience, however, time zone boundaries often follow political boundaries instead of longitude lines, resulting in the irregularly shaped time zones shown below.

The clock in the animation below shows the time (to the nearest hour) in the time zone that includes the prime meridian. The time in this time zone is known as Universal Time (UT). Universal Time is commonly used for indicating timing of global and astronomical events.

  !   Click the map to start the animation. Click the next or back arrows to watch the animation one frame at a time.

Images captured using Earth and Moon Viewer by John Walker
and with satellite imagery provided by the Living Earth, Inc.

The clock in the animation below shows the time (to the nearest hour) in the time zone that includes the prime meridian. The time in this time zone is known as Universal Time (UT). Universal Time is commonly used for indicating timing of global and astronomical events. In Universal Time, the hours of the day are numbered from zero to 23, so the notations of a.m. and p.m. are not used. For example, 7:00 p.m. would be 19:00 in this system.

10. Write a simple equation to convert Universal Time to local time for the time zone where you live.


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