History of Calendar

The Sun, Moon, planets and stars

Human always had a need to measure the passage of time. Even ice-age hunters in Europe over 20 000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the moon. One purpose of Stonehenge, a prehistoric monument located in the English county of Wiltshire, was the determination of seasonal or celestial events, such as lunar eclipses or solstices


By MARIJA MITROVIĆ
from Belgrade, SERBIA


The calendar we use today has been developing for millenia. The best minds of countless generations have worked to make the calendar as accurate as possible, but even today the calendar grows more inaccurate with each passing year. The Sun, Moon, planets, and stars - have provided us a reference for measuring the passage of time throughout our existence. Ancient civilizations relied upon the apparent motion of these bodies through the sky to determine seasons, months, and years.

Human always had a need to measure the passage of time. Even ice-age hunters in Europe over 20 000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the moon. One purpose of Stonehenge, a prehistoric monument located in the English county of Wiltshire, was the determination of seasonal or celestial events, such as lunar eclipses or solstices.

Most of the oldest calendars were lunar calendars, based on the time interval from one new moon to the next-a so-called lunation. But there are annual events that pay no attention to the phases of the Moon. The calendar had to account for these yearly events as well. First solar calendar maid ancient Egyptians. Egyptian calendar had 12 months of 30 days each, for total 360 days per year. They calculated that the solar year was whole 365 days plus one quarter of day. In order to bring their year closer to solar one they added 5 extra days about 4000 B.C. These days of 13th month became a festival. Still this calendar doesn't account one quarter of the year, so in every four years difference between solar and Egyptian year was raised approximately by one day. This means that as the years passed, the Egyptian months fell out of sync with the seasons, so that the summer months eventually fell during winter. Only once every 1,460 years did their calendar year coincide precisely with the solar year.

The calendar used by the ancient Greeks was based on the Moon, and is known as the Metonic calendar. This calendar was based on the observations of Meton of Athens which showed that 235 lunar months made up almost exactly 19 solar years. This 19-year cycle became known as the Metonic cycle. However, given a nominal twelve-month year, an additional 7 lunar months needed to be added to synchronize the cycle. These were added in years 3, 5, 8, 11, 13, 16, and 19 of the cycle. This calendar was modified few times. First around 325 B.C., by Callippus, who noticed that 4 Metonic cycles were very close to 27,759 day. More accurate cycle is Hipparchic cycle which consists of 4 Callippus cycles less one day, and it is near 3760 months. All these systems were very complicated and were not widely used. A lunar-based calendar is still used by some religions. The Islamic calendar or Muslim calendar is a lunar calendar having 12 lunar months in a year of about 354 days. The Jewish calendar, coordinates all three of these astronomical phenomena: the rotation of the Earth about its axis (a day); the revolution of the moon about the Earth (a month); and the revolution of the Earth about the sun (a year) . Months are either 29 or 30 days, corresponding to the 29?-day lunar cycle. Years are either 12 or 13 months, corresponding to the 12.4 month solar cycle. Even the Christian year, although a purely solar year, is forced to take account of the moon for the fixing of the date of Easter.

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 29 or 31 days long, with the exception of February, which had 28 days. However, four months of 31 days, seven months of 29 days, and one month of 28 days added up to only 355 days. Therefore the Romans invented an extra month called Mercedonius of 22 or 23 days. It was added every second 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. The names of the months were taken from the Roman calendar. Although, the Romans began their year in March, Caesar decreed the year began with the first of January.

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, and after a number of centuries, even 11,5 minutes adds up. By the 15th century the Julian calendar had drifted behind the solar calendar by about a week, so that the vernal equinox was falling around March 12 instead of around March 20.

We use calendar known as Gregorian calendar, named by Pope Gregory VIII, who reformed Julian calendar in order to make it more accurate. The leap year system remained intact. However 3 leap years out of every 400 years are skipped. Every centennial year ending in 00 except for ones divisible by 400. Thus 2000 was a leap year, but 2100 will not be. Error of The Gregorian calendar is about one day per 3300 years.

Year 1923, Orthodox Churches decided to change the Julian calendar. The project was prepared by one of the greatest Serbian scientists, Milutin Milankovic. According to Milankovic's calendar, every fourth year remains leap, but as for the secular ones, leap are only those that divided by 900 give the remainder of 200 or 600. Therefore, years 2000 and 2400 will be leap-years, but this calendar will differ from the Gregorian year 2800 when a day of difference will occur. Milankovic's calendar is most accurate of all and is late one day just once every 28 800 years in relation to the real astronomic time. Unfortunately, this calendar was not adopted by all Orthodox churches, and Gregorian calendar became the main calendar.

The duration of solar year is 365 days 5 hours 48 minutes and 46 seconds. This is the reason why is not possibile to make calendar that will be fully consistent with solar year. The next best thing we can do is to correct calendar every few thousands years.


(Published: 10.01.2008.)

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