I.

Introduction

Calendar, system of measuring time for the needs of civil life, by dividing time into days, weeks, months, and years. Calendar divisions are based on the movements of the earth and the regular appearances of the sun and the moon. A day is the average time required for one rotation of the earth on its axis. The measurement of a year is based on one revolution of the earth around the sun and is called a seasonal, tropical, or solar year. A solar year contains 365 days, 5 hr, 48 min, and 45.5 sec. A month was originally calculated by ancient peoples as the time between two full moons, or the number of days required for the moon to circle the earth (29.5 days). This measurement, called a synodic, or lunar month, resulted in a lunar year of 354 days, 11‚ days shorter than a solar year. In modern calendars, however, the number of days in a month is not based on the phases of the moon. The length of the months is approximately one-twelfth of a year (28 to 31 days) and is adjusted to fit the 12 months into a solar year. For information concerning the names or arrangement of the months, see the articles on each of the 12 months. The week was derived from the Judeo-Christian tradition requiring rest from labor every seventh day. It is not based on a natural phenomenon. The Romans named the days of the week in honor of the sun, moon, and various planets.

The variations among the 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: days, weeks, or months. The earliest calendars based on lunar months eventually failed to agree with the seasons. A month occasionally had to be intercalated, or added, to reconcile lunar months with the solar year. A calendar that makes periodic adjustments of this kind is a lunisolar calendar.