# 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:

U.S. Naval Observatory Master Clock