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In the Mediterranean area such indications change rapidly, and Greek poet ) mentions a wide variety: the cry of migrating cranes, which indicated a time for plowing and sowing; the time when snails climb up plants, after which digging in vineyards should cease; and so on.An unwitting approximation to the tropical year may also be obtained by intercalation, using a simple lunar calendar and observations of animal behaviour.These might consist of noting the varying length of the midday shadow cast by a stick thrust vertically into the ground or follow the far more sophisticated procedure of deducing from nocturnal observations the Sun’s position against the background of the stars.In either case the result was a year of 365 days, a period incompatible with the 29 -day lunation.The menstrual period of women and also to the duration of cyclic behaviour in some marine creatures.
A calendar is convenient for regulating civil life and religious observances and for historical and scientific purposes.
The first practical calendar to evolve from these requirements was the Egyptian, and it was this that the Romans developed into the Julian calendar that served western Europe for more than 1,500 years.
The Gregorian calendar was a further improvement and has been almost universally adopted because it satisfactorily draws into one system the dating of religious festivals based on the phases of the , a “book of hours” containing prayers to be recited.
Once the day is divided into parts, the next task is to gather numbers of days into groups.
Among primitive peoples, it was common to count moons (months) rather than days, but later a period shorter than the month was thought more convenient, and an interval between market days was adopted.