Time Zones and Universal Time

Fred Espenak


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 (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 (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.

Most astronomical predictions are usually presented in terms of Universal Time. In order to convert such 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 standard time.

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 standard time. So the local time in Toledo is 16:25 (=4:25pm).

World Time Zones

World Time Zones
World Time Zones (click to see larger image)

Time zones for countries around the world can be determined using the map above. You also need to know if Daylight Saving Time is practiced during that time of year.

Coordinated Universal Time

Coordinated Universal Time (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). UTC is the time used by GPSs and is the standard reference for Time Zones around the world.

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:

Refresh or Reload page

U.S. Naval Observatory Master Clock
Reload Frame to Update Clock

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

Time Zone Links

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