Time Zones and Universal Time

Time can be measured in a number of ways. For instance, we can measure the passage of time via the orbital motion of Earth and other planets in the solar system (Dynamical Time). Or we can measure time based on the rotation of Earth on its axis with respect to the stars (Universal Time). Finally, we can measure time through the oscillations of atoms (International Atomic Time).

Universal Time or UT is the precise measure of time used as the basis for all civil time-keeping. Although their exact definitions differ, most readers can assume that Universal Time is equivalent to Greenwich Mean Time or GMT. Universal Time is actually based on the mean sidereal time as measured in Greenwich, England. It's also approximately equal to mean solar time from Greenwich.

Like most other astronomical calculations, eclipse predictions are usually presented in terms of Universal Time. In order to convert eclipse predictions from UT to local time, you need to know what time zone you are in. For North Americans, the conversion from UT to local time is as follows:

                Atlantic Standard Time (AST) =  UT - 4 hours
                Eastern Standard Time (EST)  =  UT - 5 hours
                Central Standard Time (CST)  =  UT - 6 hours
                Mountain Standard Time (MST) =  UT - 7 hours
                Pacific Standard Time (PST)  =  UT - 8 hours

If Daylight Saving Time is in effect in the time zone, you must ADD one hour to the above standard times.

For example, let's assume that an eclipse begins in Toledo, Ohio on June 20 at 20:25 UT. Toledo is in the Eastern Standard Time zone, so:

                     Local Time = 20:25 - 5 hours
                                = 15:25 (= 3:25 pm)
But since Toledo observes Daylight Saving Time in June, we must ADD one more hour to the above time. So the eclipse will begin at 16:25 (=4:25pm) local time.

Time zones for countries around the world can be determined with a special Time Zone Map courtesy of HM Nautical Almanac Office, Royal Greenwich Observatory. Just remember that you'll need to check with your travel agent or with a guide book to find out if Daylight Saving Time is practiced during that time of year.

Coordinated Universal Time (or UTC) is based on atomic time. It is synchronized and adjusted to stay within 0.9 seconds of Universal Time (UT). Occasionally, a "leap second" is added to UTC in order keep it in sync with UT (which varies due to Earth's rotation).

The official source of time used in the United States is the Time Service Department of the U.S. Naval Observatory. The current UTC is shown below:

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U.S. Naval Observatory Master Clock
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You can use the time on the U.S.N.O. Master Clock to determine how many hours different your local time is from Universal Time. Once noted, use this number to convert all eclipse prediction times from UT to your own local time.

For example, if you are in the Eastern Standard Time zone, you will see that your local time is 5 hours earlier than UT. In order to convert any eclipse predictions from UT to local time (i.e. - EST), you must subtract 5 hours from UT:

                          Local Time = UT - 5 hours

For more information on time reckoning and time zones, check out the following sites:

2010 Jun 24