Coordinated Universal Time

Most satellite images will have times expressed in UTC, GMT or Z time. These stand for Greenwich Mean Time, Coordinated Universal Time and Zulu time respectively, a way to tell time worldwide.

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 (Encyclopedia Britannica). 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.

There are six time zones in the United States. The following table shows how many hours to subtract from UTC to convert to local time depending on what time zone you are in.

Close Window