Readers who are not familiar with advanced amateur work may be surprised at the wide range of astronomical activities that are timedependent. Not only eclipse events, occultations of stars and movements of planetary satellites, but even the precise shapes of markings on the planets are phenomena whose measurement requires techniques that involve the clock.

Universal Time

For purposes of worldwide uniformity, all information provided in astronomical tables, yearbooks and periodicals and also all reported observations are recorded in Universal Time (UT). Unlike civil time, which varies from one time zone to the next, UT is independent of the observer's geographic location, and is equivalent to the time at longitude o°. Thus, to convert the time of an event from UT to local civil time, we must subtract one hour for each time zone by which we are removed (westward) from the zero-degree or Greenwich zone. Because UT is expressed in the 24-hour system, the civil 1.00pm is UT 13.00, 2.30pm is 14.30 and so forth.

The process of converting time will often result in a situation that confuses some users of astronomical tables (including editors of newspapers, etc., who attempt to record the local data of a phenomenon such as the full Moon). An event that occurs at one given UT date may, in one's own time zone, occur at a civil time that falls in the previous calendar day.