Because astronomers have divided the globe into 24 time zones, using one clock time worldwide is impractical. That’s why the times of astronomical events are often given in Universal Time (UT). This practice started in 1928, when the International Astronomical Union recommended that title for the time used in the compilation of astronomical almanacs. UT is the time kept on the 0° longitude meridian, the one running through the old Greenwich Observatory in England. This meridian is five hours ahead of U.S. Eastern Standard Time. Times given in UT are given in terms of a 24-hour clock. So, 14:42 is 2:02 p.m., and 21:47 is 9:17 p.m.
Different meanings of time
When a precision of one second or better is needed, however, astronomers must be more specific about the meaning of UT. For that purpose, different designations of UT have been adopted. In astronomical and navigational usage, UT often refers to a specific time called UT1, which is a measure of the rotation angle of Earth as observed astronomically. It is affected by small variations in the way our planet spins, and can differ slightly from the time on the Greenwich meridian. Times labeled UT in astronomical almanacs such as those produced by the U.S. Naval Observatory are UT1.
In common usage, UT refers to a time scale called Coordinated Universal Time (abbreviated UTC), which is the basis for the system of time used worldwide. This time scale is determined using highly precise atomic clocks, which are accurate to approximately a nanosecond (billionth of a second) per day.
UTC is the time distributed by standard radio stations that broadcast time, such as WWV and WWVH. It can also be obtained readily from Global Positioning System (GPS) satellites. UTC is the basis for civil standard time in the U.S. and its territories. Standard time within U.S. time zones is an integral number of hours offset from UTC.
Greenwich Mean Time
Greenwich Mean Time (GMT) is a time scale based on the apparent motion of the “mean” Sun with respect to the meridian through the Old Greenwich Observatory (0° longitude). The mean Sun is used because time based on the apparent motion of the Sun doesn’t “tick” at a constant rate.
Earth’s orbit is slightly eccentric and its plane is inclined with respect to the equator about 23½°, so at different times of the year the Sun appears to move faster or slower in the sky. That’s why a sundial can be wrong (if it’s supposed to be telling mean time) by up to 16 minutes. So if the mean (i.e. corrected) Sun is directly over the meridian through Greenwich, it is exactly 12 noon GMT. Mean time on selected meridians 15° apart is generally known as standard time. For example, Eastern Standard Time (EST) is the mean solar time of the meridian at 75° West.
If you’re an amateur astronomer and only keep a personal log of your observations, using your local time is fine. But if you’re someone who, for example, shares images on the Internet, consider using Universal Time. It will make your data a bit friendlier to others on our spherical world.
