Time Zones

Greenwich Mean Time (GMT) or Zulu Time (Z), Coordinated Universal Time (UTC), and Universal Time (UT) can all be interpreted as local time at 0 degrees longitude, the prime meridian (location of Greenwich, England). Worldwide air traffic control systems, international radio broadcasts, and numerous military activities all take place at this period. Instead of two twelve-hour time periods, UTC is set in zero to twenty-four hour time intervals (a.m. and p.m.). 

 

The term" a.m." and "p.m." correspond to ante meridiem, or "before noon," and "p.m." refers to post meridiem, or "after noon," respectively, with relation to the central meridian. The terms UT, UTC, GMT, and Z all refer to the same 24-hour time zone that helps to establish a standard time for international activities. For instance, all air travel adheres to the 24-hour time zone so that the pilots can schedule flights throughout the world and across time zones. 

 

The rate of rotation of the earth's axis is 15 degrees per hour, or once every twenty-four hours (15 x 24 = 360). To ensure that local times match to comparable hours of day and night, time zones are established approximately every 15 degrees of longitude. With this method, noon is often when the sun is overhead in every time zone that adheres to the 15-degree scheme. There are four primary time zones in the continental United States (see Figure 1.7, "Major Time Zones of the World," and Table 1.1, "Four Main Time Zones in the Continental United States and Their Central Meridians").

 

•    Eastern Standard Time zone          75 degrees W

•    Central Standard Time zone          90 degrees W

•    Mountain Standard Time zone     105 degrees W

•    Pacific Standard Time zone          120 degrees W

 

In relation to the 24-hour time clocks used by Zulu Time (Z), Greenwich Mean Time (GMT), and Universal Coordinated Time (UTC), the twenty-four time zones are based on the prime meridian. The distance from the prime meridian determines whether a local time zone is plus or minus.  In this diagram, 75 W is the central meridian for the Eastern Standard Time zone in the United States.

 

Since the Eastern Standard Time zone is roughly 75 degrees west of the prime meridian (UTC), it is five hours earlier than UTC. For instance, if it is noon in London, it is 7 a.m. in New York, and suppose it is 1 p.m. in New York It is ten o'clock morning in San Francisco, which is three times zones to the west. There are twenty-four time zones on Earth because there are twenty-four hours in a day. There are 15 degrees between each time zone. The 15-degree time zones have the drawback that they do not always adhere to national, regional, or local borders. 

 

As a result, time zones rarely have border lines that are precisely 15 degrees apart. In some circumstances, time zones zigzag to follow state lines or to maintain cities inside a single time zone in the United States, where the borders between the various time zones don't match the lines of longitude. Varied nations have different approaches to the issue. For instance, China has the same amount of land area as the United States yet only uses one time zone for the entire nation.