Calendars

Calendars

Babylonians | Greeks | Egyptians | Others | Calendars

The calendar is one of the most important creations of a society. An accurate means to tell the passage of time, record events, and to forecast events, necessitates the creation of a sense of time and a means to measure that time.

Throughout history, people have used the sun, moon, stars, and rivers as objects by which to mark the year, and the sun, moon, stars, and water again to mark the time of day. Modern society has improved upon that to use the pendulum, then springs, progressing to electricity, crystals, and now atoms with which to accurately measure time.

This page covers nearly all the cultures discussed in this section (Babylonians, Egyptians, etc.) that had a major calendar system. The reason for a whole separate page devoted to calendars is for the sake of the importance of the topic as well as an easy way to compare the various systems used.

The Babylonian year was based upon the "heliacal" rising of stars - when they rise in the region of the sun before sunrise which occurs on a yearly cycle (equivalent to the modern sense of Right Ascension (RA, α)). This dates to at least 2000 B.C., due to the language in which it is described (Accadian).

However, though this gives a year of 365 days, the ancient Babylonians are believed to have used a lunar calendar. Main evidence for this comes from the records of lunar eclipses usually occurring on day 14 of the month, and solar eclipses appeared mainly between days 28-30.

Originally, the calendar was changed very often in China. It was reasoned that since the emperor had a divine connection with the heavens, that the calendar should change after a major regime change or dynastic change. Between the Han and Ming dynasties (A.D. ~220-1368), there were about 40 different calendars.

The lunar year was used by the time of the Shang dynasty (~1400 B.C.). Since the mean lunar cycle when seen from Earth lasts 29.53 days, the calendar was made of alternating 29 and 30 days per month, with a year made of 12 months.

Unfortunately, the solar year is 365.2422 days, which is in no way a multiple of the 29.53-day lunar month. The solution to this lies with the Metonic cycle, which lasts 19 years. It is the time it takes the full moon that falls on a certain day of the year to fall on the same day of the year. The Metonic cycle is 6939.6 days, which is 235.0017* lunar cycles - a difference of only 1.2 hours if one rounds. The Chinese were the first to utilize the Metonic cycle, though the Babylonians also adopted it around the same time. The Greeks would not use it until around 432 B.C., a century later.

In order to synchronize these cycles, an extra month was periodically added into the year to keep the solar and lunar calendars in phase. This is known as "intercalation." By the Tang dynasty (A.D. 618-906), the lunar month had been refined to essentially the modern value, making the prediction of lunar eclipses pretty precise.

Another method of measuring days was the use of 24 fortnights. This was based upon a solar calendar where each fortnight corresponded to 15° of motion along the ecliptic. Each period lasted 15.218 days, so the year was made of 265.23 days. This was in use by the first century B.C. By our calendar, the year started on February 5 with the "beginning of Spring."

* Since 235 = 5 * 47, there is not a shorter period.

Early Egyptians used a lunar calendar, but unlike most civilizations, they began the lunar month in the morning when the old crescent of the waning moon could no longer be seen just before sunrise in the East in stead of with the first appearance of the new crescent in the West at sunset. Thus, the lunar year divided into 3 seasons with 4 months each.

But, at 2- or 3-year intervals, they had to add an extra 13^th month in order to account for the 11 day short lunar calendar in order to keep the seasons in place. Eventually, the first appearance just before sunrise in the eastern horizon of the star Sirius was used to regulate this extra month - whenever it happened in the last 11 days of the 12^th lunar month (the 4^th month of the 3^rd season), the extra month was added to the year.

By the third millennia B.C., the Egyptians had realized that the year needed to have 365 days, and these were divided into three seasons of four 30-day months each, with an additional 5 "days upon the year." This was termed a "civil" year.

This calendar remained in use without alteration to the time of Augustus when a 6^th additional day every four years was introduced, and it was termed a "natural" year. The Egyptians are credited with the discovery of the natural year, and it formed the basis of the later Julian and Gregorian calendars.

The Meso-Americans had developed a calendar based upon Venus. Five Venus years was equivalent to eight solar years. This calendar was accurate to 1 in 500 days, and was recyclable with minor corrections.

The Mayan calendar was based upon several things. The Tzolkin was a 260-day sacred round, made of 13 numerals and 20 day names. There is a variant of this that is still used in the Guatemalan highlands for divination.

Another Mayan calendar was the solar calendar, divided into 12 groups of 30, plus an additional 5. This cycled around a period of 52 years that returns the tzolkin date to the solar date.