CALENDAR IN HISTORY

from TIME ALMANAC

THE PURPOSE of the calendar is to reckon past or future time, to show how many days until a certain event takes place—the harvest or a religious festival—or how long since something important happened.

The earliest calendars must have been strongly influenced by the geographical location of the people who made them. In colder countries, the concept of the year was determined by the seasons, specifically by the end of winter. The solar year was closer to 365.25 days (365 d, 5 h, 48 min, 46 sec or 365.242199 d).

In warmer countries, where the seasons are less pronounced, the Moon became the basic unit for time reckoning. Most of the oldest calendars were lunar calendars, based on the time interval from one new moon to the next—a so-called lunation. The lunar year was based on the 29.5-day lunar cycle (29 d, 12 h, 44 min, 2.8 sec or 29.53059 d; 12 lunations: 354 d, 8 h, 48 min, 34 sec or 354.3672 d). But even in a warm climate there are annual events that pay no attention to the phases of the Moon. In some areas it was a rainy season; in Egypt it was the annual flooding of the Nile River.

Lunar Calendars

During antiquity the lunar calendar that best approximated a solar-year calendar was based on a 19-year period, with 7 of these 19 years having 13 months. In all, the period contained 235 months. Using the lunation value of 29.5 days, this made a total of 6,933 days, while 19 solar years added up to 6,940 days, a difference of just one week per period and about five weeks per century.

Even the 19-year period required adjustment, but it became the basis of the calendars of the ancient Chinese, Babylonians, Greeks, and Jews. This same calendar was also used by the Arabs, but Prophet Muhammad later forbade shifting from 12 months to 13 months, so that the Islamic calendar now has a lunar year of about 354 days. As a result, the months of the Islamic calendar, as well as the Islamic religious festivals, migrate through all the seasons of the year.

The Roman Calendar

When Rome emerged as a world power, the difficulties of making a calendar were well known, but the Romans complicated their lives because of their superstition that even numbers were unlucky. Hence their months were 30 days long, with the exception of February which had 25 days, added up to only 355 days. Therefore the Romans invented an extra month called Mercedonius and was added every third year.

Even with Mercedonius, the Roman calendar eventually became so far off that Julius Caesar, advised by the astronomer Sosigenes, ordered a sweeping reform: 46 B.C. was made 445 days long by imperial decree, bringing the calendar back in step with the seasons. Then the solar year (with the value of 365 days and 6 hours) was made the basis of the calendar. The months were 30 or 31 days in length, and to take care of the 6 hours, every fourth year was made a 366-day year. Moreover, Caesar decreed the year began with the first of January, not with the vernal equinox in late March.

This calendar was named the Julian calendar, after Julius Caesar, and it continues to be used by Eastern Orthodox churches for holiday calculations to this day. However, despite the correction, the Julian calendar is still 11.5 minutes longer than the actual solar year.

The Gregorian Reform

By the 16th century the Julian calendar had drifted behind the solar calendar by ten days. The immediate correction, advised by Christopher Clavius and ordered by Pope Gregory XIII, was that Thursday Oct. 4, 1582, was to be the last day of the Julian calendar. The next day would be Friday Oct. 15. For long-range accuracy, a formula suggested by Aloysius Giglio was adopted: every fourth year is a leap year unless it is a century year like 1700 or 1800. Century years can be leap years only when they are divisible by 400 (e.g., 1600 and 2000). This rule eliminates three leap years in four centuries, making the calendar sufficiently accurate. In spite of the revised leap year rule, an average calendar year is still about 26 seconds longer than the Earth’s orbital period. But this discrepancy will need 3,333 years to build up to a single day.

The Gregorian reform was not adopted throughout the West immediately. Most Catholic countries quickly changed to the pope’s new calendar in 1582. But Europe’s Protestant princes chose to ignore the papal bull and continued with the Julian calendar. It was not until 1700 that the Protestant rulers of Germany and the Netherlands changed to the new calendar. In Great Britain (and its colonies) the shift did not take place until 1752, and in Russia a revolution was needed to introduce the Gregorian calendar in 1918.**