The calendar is a system of the measuring of time, by dividing it into days, weeks, months and years.
Mans first awareness of recurrent phenomena marking divisions of time was dictated by heat and cold variation. By observations of the stars or constellations, divisions of the year can be divided into four seasons.
The ancient calendars, based on lunar months, eventually failed to agree with the seasons. To reconcile lunar months with the solar year, a month occasionally had to be added.
The Primitive Calendar
The primitive people had probably, like us, noticed that the seasons varies from the sprout of the new plants every spring, and the ripening of seeds in the autumn, but in substance they used the shorter, more observable measuring-rod to determine the duration.
The most obvious of all time units is of course the day, since its so clearly divided through the variation of dark and light. The contras between day and night is so big that it determined the life-style of almost all animals and even of the human. Continually the day became the basic time unit. All that the primitive person needed to count the course of time, was a tree on which he could mark with a line for every sunrise.
The visible changing shape of the moon was noticed
indefinitely during the dark nights of the primitive civilization. A regular cycle
repeated itself every 29.5 days. This inspired the month of the later calendars.
The Egyptian Calendar
The old Egyptians, who reached high levels of supremacy, were practical people, interested in astronomy and science, using it to their advantage. They used astronomical observations to help them with the organizing of their calendar and to determine their religious feasts.
Their calendar was rather interesting. The year started on the day when the Nile was in flood. This happened quite often when the sun lined up with the star Sirius. Their year was divided into 12 months of 30 days each, which means 360 days. Every month was then divided in three weeks of ten days each. Because the year of the season had 365,25 days, they had to include five days extra in any week or month that werent taken into account.
The Roman Calendar
The Roman calendar had 10 months with 304 days in a year, and it was introduced about the 7th century BC. It began with March, but later in the 7th century BC, two more months were added, January and February. Because the months were only 29 or 30 days long, an extra month had to be inserted every second year.
The late Roman republicans used various lunar-solar calendars that were supposedly based only on observation, but were in fact influenced by political considerations. Julius Caesar recognized the need for a stable, predictable calendar, because the Roman calendar had several errors by months. He formed one with the help of an astronomer, Sosigenes. The year 46 BC was given 445 days, to compensate for past errors. Every common year thereafter was to have 365 days and every fourth year, starting with 45 BC, was to have a leap year of 366 days, and February, which had 28 days, was extended one day extra in that year. The rule was not correctly applied, but the calendar was corrected by Augustus Caesar by AD 8.
The method of counting backwards from three dates:
The Calends, or first of the month;
The Ides, or middle of the month falling on the 13th of some months and the 15th of others and
The Nones, or 9th day before the ides.
By this they could designate the days of the month.
The Gregorian Calendar
Pope Gregory XIII had called in the help of the astronomer Clavius, who showed him there upon that the Julian year was a bit too long. On Claviuss advise, the Pope ordained that the day after 4 October 1582, would not be the fifth, but be 15 October, and from that time on the centuries that can be divided by 400, would be leap years. Otherwise, 1700, 1800, 1900, 2100, 2200, etc., will not be leap years,while 1600, 2000, 2400, etc., will be leap years.Being calculated without reference to the moon, the Gregorian calendar is a solar calendar. Based on the Sun and the Moon, the Gregorian calendar includes rules for determining the dates of religious holidays.
The difference in the structure of calendars can
cause various confusions. To convert a date from one type of calendar to another, visit http://www.genealogy.org/~scottlee/calconvert.cgi
for a conveter.