You have probably noticed that most satellite images will have times expressed in UTC or Z time. These stand for Coordinated Universal Time and Zulu time respectively, a way to tell time worldwide. In order for scientists to communicate about data, it is necessary to have a global standard for referencing time. A standard time was originally adopted to end the confusion caused by each community’s use of its own local time. Sir Sanford Fleming, a Canadian railway planner and engineer, developed the first plan for a worldwide standard time reference system in the late 1870’s in response to scheduling challenges created by railway routes that spanned multiple communities in the United States and Canada. Following Fleming’s initiative, delegates from 27 nations met in Washington, D.C. in 1884 and agreed on a system for worldwide standard time. The system consisted of 24 standard meridians of longitude 15 degrees apart from each other, beginning with the Prime Meridian through Greenwich, England. Each meridian is the center of each standard time zone. A convenient way to explain this concept to students is that there are 360 degrees in a full rotation which on earth is equal to 24 hours. 360 divided by 24 is 15, which is why we add one hour for every time zone, or 15 degrees longitude away from Greenwich, England, to convert to local time. Throughout each time zone, time remains the same and differs by an integral number of hours from the time in the Prime Meridian zone based on the number of time zones west of the Prime Meridian zone. Because we live in the U.S. and the earth rotates toward the east, local time in the U.S. is earlier than the local time at the Prime Meridian -- that is, the sun rises in England hours before it does in the U.S. For example, Wisconsin is six hours behind the Prime Meridian time zone in the winter, because it is six time zones west of the Prime Meridian zone. So 1600Z (or 4 pm in the Prime Meridian time zone) is actually 10 am in Wisconsin during the winter. In the summer Wisconsin is five hours behind the Prime Meridian zone because of Central Daylight Time (CDT) (the clock has been moved one hour ahead). Standard time is synonymous with Coordinated Universal Time (UTC), Zulu time (Z), and Greenwich Mean Time (GMT). The National Weather Service has used UTC as the standard of time since January 1, 1972 (Glossary of Meteorology and The Astronomical Almanac). Most satellite images will have times expressed in UTC.
There are six time zones in the United States. Wisconsin is in the Central Time zone.
The new day in Greenwich England (0000Z) is actually 6 pm in Wisconsin during Standard Time.
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