Gregorian Calendar

In 1582 Pope Gregory XIII introduced a dramatic calen drical reform to overcome the problem with the existing Julian calendar: it was getting gradually further and further out of step with the seasonal ("Tropical") year. The problem would not have occurred if the mean length of the year were exactly 365 days and 6 hours, but it is in fact somewhat shorter—365 days, 5 hours, 48 minutes, and 46 seconds. By the sixteenth century, the Julian calendar was running ten days behind the true solar year.

The solution to the problem was two-pronged. First, the pope issued a decree that the day following October 4, 1582, would be October 15. This brought the calendar back in step with the seasonal year. Second, steps were taken to prevent a significant error from accumulating in the future. A scheme was introduced whereby in each subsequent period of four hundred years, three years that would have been leap years under the "every fourth year" rule now would not be. This makes the mean length of the calendrical year 365 days, 5 hours, 49 minutes, and 20 seconds, which is only 34 seconds longer than the true (Tropical) year and means that the accumulated error will not reach a full day again for some 2,500 years. The specific scheme adopted was that century years not divisible by four hundred would not now be leap years, that is, 1700, 1800, 1900, 2100, etc.

Calendrical reform took place immediately in Catholic countries, but not surprisingly, in Protestant countries the pope's decree was ignored. The Gregorian calendar was not adopted in Britain and its colonies until 1752, by which time the error had increased to eleven days, and in Russia until the Bolshevik revolution of 1917. In Ethiopia, the Julian calendar still remains in use, and the timing of Easter for Orthodox Christians in eastern Europe still follows the Julian calendar.

Although it bears no direct relationship to ancient calendars, the Gregorian calendar is related closely to the seasonal year and so provides a convenient frame of reference when discussing the timing of seasonal activities, astronomical observations, or the dates when the sun rose or set in line with temples or buildings. If we are interested in whether the orientation of certain temples or buildings had a calendrical significance, we do have an advantage where we know something of the calendar involved. An obvious case in point is the orientation of medieval churches, where we must be especially careful to distinguish between the Gregorian calendar as our point of reference and the Julian calendar that was being used by the builders.

See also:

Church Orientations; Julian Calendar.

References and further reading

Aveni, Anthony. Empires of Time: Calendars, Clocks and Cultures, 116-118.

New York: Basic Books, 1989.

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