A long time ago (thousands of years, in some cases), when ancient societies recognized the need to record events and plan future happenings, calendars came into being. In colder climates, a calendar reflected the changing of the seasons and the movements of Earth around the Sun.
A solar, or sun-based, calendar, with some modifications made by different regions and religious groups, is in use in most of the world today. In warmer climates, where seasons passed without dramatic climate changes, calendars were based on the actions of the Moon. Such moon-based, or lunar, calendars still exist in a few places.
In the system of solar calendars, the length of a day is determined by the approximate amount of time it takes Earth to rotate once on its axis (about 24 hours). The length of a year is measured by the time it takes Earth to rotate around the Sun (365 days, 5 hours, 48 minutes, and 46 seconds).
In 45 B.C., Roman emperor Julius Caesar instituted what came to be known as the Julian calendar. The Julian calendar was based on a solar year, with a year consisting of 365 days, 6 hours. The year was divided into months that were either 30 or 31 days long (except for February, which has 28 days). Caesar also decreed that the year would begin with January 1; previously the year had begun on March 25, coinciding with the beginning of spring in the Northern Hemisphere.
It turned out that the Julian calendar (still in use in some parts of the world), in estimating that a year is 365 days, 6 hours, was off by almost 12 minutes. After several hundred years, those minutes added up, and the Julian calendar was about a week off course from the movements of Earth around the Sun. In 1582 another major calendar reform took place, this time instituted by Pope Gregory XIII. The Gregorian calendar, used in the United States and most other countries of the world today, made further adjustments to align it more closely to astronomical movements.
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