TIME ZONES AND UNIVERSAL TIME 

From NASA/GODDARD SPACE FLIGHT CENTER
by 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 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