Calendars

Throughout history, people have used various systems to organize the year into units such as days, weeks, and months. One of the most significant reforms of the calendar system occurred during the Renaissance. It resulted in the creation of the Gregorian calendar, which still serves as the worldwide standard today.

The Julian Calendar. Calendars are based on ASTRONOMY. The rising and setting of the Sun determine the length of a day. Each day the Sun rises to a slightly different point in the sky than it did the day before. The time it takes the Sun to return to a specific noontime height is equal to one year. Originally, the month also related to astronomy—it depended on the phases of the Moon. However, calendars did away with this relationship around 2,000 years ago.

The main difficulty in creating a calendar is that the year contains approximately 365.25 days. The odd quarter day means that calendars based on whole days sooner or later fall out of step with the solar year. In 45 B.C., the Roman leader Julius Caesar introduced the Julian calendar. Adopted in A.D. 8, the calendar solved the problem of the odd quarter day by adding an entire extra day to the calendar every fourth year, or leap year.

The early Christian church used the Julian calendar to determine the dates of many religious holidays. Easter, however, had no fixed date in the calendar. In the late 400s a church scholar named Dionysius Exiguus developed a method for determining the date of Easter based on the phases of the Moon. He also began the practice of dating the modern or Christian era from the birth of Christ, although he erred by four to seven years in calculating the date of that event.

The Julian calendar made January 1 the official start of the year. However, countries that used the calendar chose a variety of dates to mark the new year. In Florence, Italy, the year began on March 25, while in nearby Pisa it began on March 24. In England the start of a new year fell on Christmas Day, December 25, until the 1300s, when it shifted to March 25. Not until the 1700s did January 1 become the accepted start of a new year in England.

The Gregorian Reform. The Julian calendar was an improvement on older calendars, but it was based on a length for the year that was 11 minutes and 14 seconds too long. As a result, the vernal equinox, one of the two times each year that the day and night are of equal length, moved ahead about one calendar day every 314 years. By the 1500s the astronomical or seasonal equinox was ten days ahead of the calendar's equinox, which fell on March 21.

The fact that the calendar was out of step with nature concerned church officials because the vernal equinox determined when Easter was celebrated. In the 1570s Pope Gregory XIII appointed a commission to study the matter and suggest reforms. The result was the Gregorian calendar, put into use in 1582. Ten days were dropped from the calendar to bring the vernal equinox back to where it had been in Julius Caesar's time. This meant that in 1582, October 15 directly followed October 4. The Gregorian calendar was based on a year of 365.2425 days. It continued to use leap years but dropped three of them from the calendar in each 400-year period.

Most Catholic countries immediately accepted the Gregorian calendar. But because the calendar came from Rome, center of the Catholic world, most Protestant countries resisted it for political and religious reasons. Eventually, however, they accepted the reform. England introduced the Gregorian calendar in 1752 and, at the same time, changed the beginning of the year to January 1. Simple, easy to use, and reasonably accurate, the Gregorian calendar is still the standard—although astronomers now know that it is off by about one day for every 2,500 years.