NOTE: This Supplemental Information is a repeat of that which appeared in last week's Supplemental Information file.
Scientists who study the Earth system have found that a common time-keeping scheme is needed in their monitoring efforts and for simultaneous comparisons of conditions in different locations. For example, a weather map portrays the state of the atmosphere over a particular geographical area based on weather observations that are taken at essentially the same time. Meteorologists use "Z" time.
For centuries, people measured their activities by the daily motions of the sun. Local noon was a convenient reference, marking the time when the sun is at its highest point in the sky locally. Consequently, places located even several miles to the east or west would have different local times. With more rapid long distance transportation and communications made possible by railroads and telegraphy after the American Civil War, travel east or west meant that a person's local time continually changed. To reduce the great number of locally observed times, the railroads argued for the simplified standardized time keeping scheme we currently use. As a result, civil time zones were instituted in the U.S. and Canada beginning in November 1883 to standardize time keeping. The concept of international time zones was officially adopted in November 1884 at the International Meridian Conference in Washington DC. Because The Old Royal Observatory in Greenwich, England had the best early astronomical time determinations, the meridian of longitude passing through this observatory was designated as the Prime Meridian (zero-degrees longitude) and serves as the worldwide standard for time keeping.
Because collection and exchange of geophysical information are of international concern, use of a single worldwide time system was needed so that all observers around the world could take measurements simultaneously providing, for example, a "snapshot" of the weather. For more than 50 years, the times for essentially all meteorological reports have been given according to Greenwich Mean Time (GMT), a time system based upon the daily rotation of the Earth with respect to a "mean sun." Often, the single letter "Z" (phonetically pronounced "Zulu") is used because this letter is used to identify the Greenwich time zone (centered on the Greenwich Prime Meridian). Currently, the conventional practice is to use the more precise Coordinated Universal Time or Temps Universel Coordinné system (UTC), which is based on an atomic clock and time reckoned according to the stars (known as mean sidereal time). For practical purposes, UTC and GMT systems are essentially equivalent.
Because Earth rotates on its axis with respect to the sun once every 24 hours, ideally we should have 24 major civil time zones of equal width. The 360 degrees of rotation divided by 24 give 15 degrees of width to each time zone. The central meridian of the time zone is then defined as a longitude evenly divisible by 15. If you were located in the U.S. Central Time Zone where Central Standard Time (CST) is observed, you would be near 90 degrees west longitude (6 times 15). At any place within this zone, the time would be 6 hours different from the time at Greenwich, England. Earth rotates eastward so that Greenwich is ahead of CST by 6 hours. For example, when it is noon at Greenwich (1200 UTC), it is 6 a.m. in the Central Time Zone. To reduce confusion, all times should be expressed in the 24-hour format, so that 8:45 a.m. corresponds to 0845 and 1:15 p.m. corresponds to 1315.
Modifications of the boundaries between time zones were made to accommodate political boundaries of various countries. Some countries adhere to a local civil time that may differ by one half hour from that of the central meridian. For the location of the four time zones in the continental United States, consult http://www.time.gov/. While most of the United States observes Daylight Saving Time during the summer (second Sunday in March to the first Sunday in November), UTC remains fixed and does not adhere to a "summer schedule." Therefore, you will have to adjust the time by one hour for summer. As an example, during the summer, residents of the U.S. Eastern Time Zone lag Greenwich time by only 4 hours, with 0800 EDT=0700 EST=1200 UTC.
Suppose that you would like to know the current time as maintained by the Master Clock at the U.S. Naval Observatory in Washington, DC. Get your clocks or watches ready and then access the current time from the Time Service Department at http://tycho.usno.navy.mil/. The site http://www.time.gov/ also provides an accurate time check.
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2008, The American Meteorological Society.