Earth’s Rotation and Time Zones
Every day in the United States, the Sun shines frst on East Coast cities such as New York and Miami. It takes about three more hours for Earth to rotate enough to move West Coast cities like Seattle and Los Angeles into the sunlight. That’s why sunrise on the East Coast is about three hours earlier than sunrise on the West Coast.
Until the mid-1800s, cities and towns kept their own time. For each of them noon was the exact time when the Sun reached its highest point in the sky. This meant that when the clocks in one town read exactly 12:00, the clocks in a town 50 miles to the east might have read 12:03. As train travel became common, these time differences began to confuse everyone. With each town using its own local time, scheduling arrival and departure times was very complicated. And the train engineers and conductors had to keep resetting their watches to the correct local time. Something had to be done to standardize the time of day.
First, each railroad company used its own standard time for its train schedules. That meant its time didn’t match local times or times for other railroads’ trains. Passengers kept missing their trains, and trains crashed into each other, too. The answer to this problem was time zones.
A time zone is an area of the world where all clocks are set to the same time. The maps in Figure 4 show the time zones for the whole world and a close-up view of the U.S. time zones. Earth is divided into 24 major time zones, because it takes 24 hours for Earth to make a complete rotation around its axis. In 1883, all the railroads divided the United States into four time zones to make a single system of standard time that was known for many years as “railroad time.” When it is noon everywhere in the Eastern Time Zone, it is only 9 a.m. everywhere in the Pacifc Time Zone. It’s even earlier in most of Alaska and Hawaii and midnight in some parts of Asia.
FIGURE 4: TIME ZONE MAPS The map at the top shows time zones of the world. The map below it shows those of the United States.
