Rina Mohammed
Calendar is a system of measuring time for the needs of civil life, by dividing time into days, weeks, months, and years. It is based on the movements of the earth and the regular appearances of the sun and the moon. The cycle of the seasons and the waxing and waning of the moon were predicted to some extent and used in calendars in the old and new world. There are two types of calendars: one is based on the moon, called the lunisolar calendar, and the other is based on the sun, called the solar calendar. The solar year is the most widely used and it is used in our modern calendar. It contains 365 days, 5 hours, 4 minutes, and 45.5 seconds. The moon calendar is also widely used, but it has disadvantages in an agricultural community because a solar year does not contain an even number of moon cycles. A year of 12 lunar months would be 11 days short of a solar year.
The variations among many calendars in use from ancient to modern times have been caused by the inaccuracy of the earliest determinations of the duration of the year, together with the fact that a year cannot be divided evenly by any of the other time units. The earliest calendars based on the lunar months eventually failed to agree with the seasons.