The word "calendar," which has been in use since the 13^th century, comes from the Latin word, kalendae, which is a moneylender's account book.

We may not think of our modern calendar as an account book, but it does provide us with an essential organizational system.How did the calendar evolve?

The Egyptians
Our modern solar calendar (based on the time it takes Earth to revolve once around the Sun, approximately 365.24219 days) appears to have originated in Egypt. The Egyptians based their calculations on the appearance of Sirius, the Dog Star, and the regular, seasonal flooding of the Nile River. Initially their 360-day calendar added five days at the end of the year (to reach 365 days). Around 237 B.C. the Egyptians changed this practice and introduced an extra day every four years.

The Julian Calendar
The Romans had a 355-day calendar, to which they added an extra 22- or 23-day month every second year. By the time Julius Caesar became emperor, the Roman calendar was grossly out of sync with the seasons.

Around 46 B.C. Caesar adopted the Egyptian calendar. Caesar wanted his Julian version of this calendar year to start on the spring equinox or the winter solstice. However, he bowed to political pressure and agreed to a January 1 start date to coincide with the day the Roman Senate convened.

Unfortunately, there were some basic misunderstandings in the use of the Julian calendar. For example, sometimes the leap year was added every third year instead of every fourth. Sometimes the length of the months would change, giving February 30 days instead of 29, or adding an extra day to a 30-day August. Augustus Caesar, Julius Caesar's successor, wanted the number of days in his month (August) to be on a par with the 31 days in July (the month honoring Julius Caesar). After several decades of use and misuse, the calendar again was out of sync with the seasons.

The Gregorian Calendar
A calendar—even one as widely used as the Julian calendar — out of sync with the seasons posed a problem for the Catholic Church in determining the date of Easter. The Church fixed the date of holiday on the Sunday after the first full moon after the vernal equinox. The shifting of the Julian calendar caused Easter to creep closer to the summer. In 1582 Pope Gregory XIII instituted historic changes to the calendar. These changes, known as the Gregorian Calendar Reform, accomplished the following:

• Removed 10 days from the calendar (one-time correction only).
• Changed the rule for leap years. A leap year was redefined as a year that is divisible by 4, but not divisible by 100, except when the year is divisible by 400. So 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not.
• Adopted rules for determining the date of Easter.
• Set January 1 as the first day of the year.
• Moved the extra day in a leap year to the day after February 28.

The Gregorian calendar is used around the world. Its system of 365.2425 days will remain accurate for approximately 4,000 years.

• Read about the history of the Julian and Gregorian calendar in Peter Mayer's article "The Julian and the Gregorian Calendars."
• The Riverdeep Today news article "Is It Really the New Millennium?" presents a discussion about when the millennium really begins.

 

Popular rhymes help us remember the number of days in a month: Fourth, eleventh, ninth, and sixth, Thirty days to each affix, Every other thirty-one Except the second month alone. Thirty days hath September, April, June, and November; All the rest have thirty-one Excepting February alone: Which hath but twenty-eight, in fine, Till leap year gives it twenty-nine.

How are calendars of other cultures structured?

Other Calendars
Many calendars associated with other cultures or religions are used to determine holy days or months, or seasonal festivals. The information below provides a brief overview of three different calendars in use today.

Hebrew	Islamic	Chinese
History: This lunisolar (combining lunar and solar) calendar dates back to the late 10th century B.C.	History: A relatively "modern" calendar, the Muslim Era's Year 1 dates from the year Muhammad emigrated from Mecca to Medina (the Hegira) in 622 A.D.	History: This lunisolar calendar dates back to the 14th century B.C. According to Chinese legend, the emperor Huangdi invented the calendar in 2637 B.C.
Length: Twelve lunar months of 29 or 30 days; total number of days: 354.	Length: Twelve strictly lunar months of 29 or 30 days; total number of days: 354.	Length: Twelve lunar months of 29 or 30 days; total number of days: 353-355.
Leap year: A 13th month of 30 days is added every 3, 6, 8, 11, 14, 17 and 19 years of a 19-year cycle (in order to accommodate the longer solar year, and to ensure that the Jewish holidays, which are related to the seasons, occur at the proper time of the year).	Leap year: No accommodation is made for leap years or leap days. Therefore the Muslim months and holy days do not occur at the same time each year (they "drift" by approximately 11 days annually).	Leap year: A 13th month is added to years if there are 12 new moons between the month of the winter solstice (the 11th month) of one year and the 11th month of the following year.
New month: Begins on a new moon. (A new moon is defined as the waxing crescent.) The day begins at sunset.	New month: Begins with the actual physical sighting of the new moon. (A new moon is defined as the waxing crescent.) Due to the weather conditions, location of the observer, and other factors, Muslim holidays may begin on different dates even in the same country. The day begins at sunset.	New month: Begins with the new moon. (A new moon is defined as completely dark--illuminated side facing the Sun, not Earth--not a waxing crescent.)
New year: Falls anytime between early September and early October.	New year: The first day of the Muslim year is Muharram 1.	New year: Falls sometime between late January and mid-February. This year, the Year of the Dragon, started February 5.
Current year: Begins its count from Creation, 3761 B.C.; current year is 5760.	Current year: Begins its count from Muhammed's pilgrimage from Mecca to Medina in 622 A.D.; current year is 1420.	Current year: Uses a complex 60-year cycle composed of 10 "celestial stemms" and 12 terrestial signs that each correspond to an animal. The current 60-year cycle began in 1984; current year is 4698.
Use: Synagogues around the world follow the Hebrew calendar for all religious (e.g., determining the dates of the holidays) matters. In Israel, the Hebrew calendar sets all religious and life cycle (e.g., weddings) events, but the country follows the Gregorian calendar for civil matters. This site offers an automatic Gregorian to Hebrew date converter and has a full explanation about the Hebrew calendar.	Use: Saudi Arabia, Yemen, and a few other Muslim countries use the calendar to determine the official era, while countries such as Egypt, Jordan, and Morocco use both the Muslim and Gregorian calendars. Learn more about moon sightings and see a graphical Muslim calendar in the article "Islamic Calendar based on predicted lunar visibility."	Use: The traditional calendar sets the spring and mid-autumn festivals and other agricultural festivals, but the Gregorian calendar is used for administrative and civil matters. For a look at some of the traditions surrounding the Chinese New Year, read the Riverdeep Today article, "Long Life and Prosperity."

Learn More

• Calendars through the Ages is a Web exhibit that contains explanations about the calendar and examines other calendars, including the Hebrew, Islamic, and Chinese.
• The Worldwide Leap Year Festival is celebrated from February 26-29 in the twin cities of Anthony, New Mexico/Texas, the official "leap year capital of the world." Congress bestowed the honor on these small communities in 1988. Since then, the towns organize a large festival that includes a parade, carnival, and many other activities.
• Check out Calendar Zone, a site filled with links to cultural, religious, historical, reference, and many other types of calendars.

Related Resources

• Mapping Time: The Calendar by E. G. Richards traces the history of the calendar, examines the science of calendar-making, compares calendars from some of the world's major religions, and explores problems related to the Gregorian calendar.
• A new book, The Sun in the Church, by J. L. Heilbron focuses on the Roman Catholic Church's support of the study of astronomy. The author also examines how some great cathedrals were built as functioning observatories in order to set the exact date of Easter.