Timing

In order to understand time, the concepts of date, interval and synchronization must be first understood. “Time” can mean either date or time interval (i.e., duration). An example of date is November 15, 1996, 15:35:14 PST (Pacific Standard Time), where 15:35:14 indicates time of day in hours, minutes and seconds. An example of time interval is the amount of time required to fly between two cities, say 3h: 51m: 12s. This latter example gives no indication of when (i.e., the date) the flight occurred, only that it lasted 3 hours, 51 minutes, 12 seconds. Note that a notation of hours, minutes and seconds can indicate either time of day or duration. Synchronization is the third important time concept. For example, it is not normally crucial for an orchestra to begin its concert at a precise hour, minute or second of the day, but it is essential that all members of the orchestra begin at the same instant and that they stay at the same tempo. A gasoline engine’s timing must be correct within a thousandth of a second or so, otherwise the sparks will not fire the fuel at just the right time to provide power to the pistons. Beyond the casual user, many electronic navigation systems, computer networks and even television receivers require synchronization to transmitted signals with accuracy of a millionth of a second or better. A time scale is a system of assigning dates to events. The sun’s apparent motion in the sky provides one of the most familiar time scales, but it is certainly not the only one. In order to completely specify a solar date, you must count days (i.e., make a calendar) from some beginning date to which all parties have agreed. In addition (depending on accuracy needs), you must measure the fractions of days (commonly, hours, minutes and seconds). That is, you must count cycles (and fractions of cycles) of the sun’s daily apparent motion around the earth. Time derived from the sun’s apparent position is called apparent solar time. A sundial indicates the fractions of cycles (i.e., time of day) directly. Calendars, like the Gregorian Calendar which we commonly use, aid in counting the days and naming them. Another system, used by astronomers, is called Julian Day. It numbers the days that have occurred since noon January 1, 4713 B.C. In this system (which is not related to the Julian calendar) noon January 1, 2000 began day 2,451,910. This time scale is useful for calculating the number of days between two events.