The meridian of the Greenwich Observatory was chosen as the zero point for the world’s time zones. The decision was made in part because of Greenwich’s historical association with timekeeping, and the fact that the United Kingdom still dominated maritime commerce. Not all of the 35 delegates were happy. In particular, the French insisted the prime meridian not be tied to any one nation and should be neutral. French clocks continued to use time issued from the Paris Observatory. They remained 9 minutes and 21 seconds ahead of Greenwich Mean Time (GMT) until March 10, 1911. Apparently, even time can be political.
Counting the days
Astronomers thrive on precision, and calendar dates can often be cumbersome and confusing. To be more precise, observations and events are often recorded by their Julian Date. The Julian period was the brainchild of the 16th century historian Joseph Justus Scaliger and begins on Jan. 1, 4713 b.c. This date is one where several cycles coincide: the 28-year solar cycle in the Julian calendar, after which the days of the year fall on the same days of the week; the 19-year Metonic cycle, when lunar phases recur on the same days of the year; and the 15-year indiction cycle, the tax cycle of the Roman Empire, which was another method for recording dates.
When John Herschel adapted Scaliger’s idea for astronomical use in 1849, he chose noon as the zero hour for the current Julian Period, thus avoiding a date change during nighttime observations. Julian Date is then simply the number of days that have passed since noon on Jan. 1, 4713 B.C.
In 1957, the Smithsonian Astrophysical Observatory created a Modified Julian Date, which begins at midnight GMT, Nov. 17, 1858. This made the day count considerably smaller and more manageable for early computers.
Time and astronomy are rooted in the way we order our lives. Ancient sky watchers looked to the sky to bring order to their world, and we still use astronomical cycles to set the very patterns of our lives. Astronomers have given these patterns order and precision in an effort to answer that age-old question: “What time is it?”