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12312012-52012’s final hours arrive on Monday, December 31. People around the world will mark the end of the old year and celebrate the promise of a new year with parties and fireworks. They will revel in the possibilities for self-improvement and make resolutions to be more organized, healthy, well-read, or diligent.

Despite the fun, calendars also serve practical purposes. Calendars mark the passing of time. They chart the moon for holy men, and track the sun for farmers. Calendars calculate the timing of religious holidays.

Most of the world uses the Gregorian calendar, also known as the Western or Christian calendar, which tracks the sun’s movement. However, this has not always been the dominate calendar.  Some countries, including China and Israel, follow different calendars and do not mark the beginning of a new year on January 1.


What is the history of calendars? When and why was the Gregorian calendar introduced? What other calendars are used and how are they different? This week, commemorate the New Year by discovering more about calendars.
The Calendar Through the Ages—

Calendars Through the Ages aims to investigate current calendars, ancient calendars, and how calendars help people to organize the world. Begin with an overview. Then, open the interactive node view. This allows users to explore a series of questions and themes. As you open a cloud, new ones will appear. Visit each layer of clouds to learn about our calendar: the months, days, and years; anecdotes and anomalies; how to write the date correctly; and the roles of equinoxes and solstices. Investigate the six other calendars used today. Read about calendars of the past, including the Mayan, French, early Roman, and ancient calendars of the world.


The dominant international calendar has been revised several times. The calendar most of the world uses today, the Gregorian calendar, was revised from the Julian calendar. Why would a calendar be revised? Delve deeper into calendar revisions and the challenges of tying historical events to unreliable or differing calendars.

For a little fun, engage your imagination and revisit the role of solar and lunar calendars in Star Wars. View a month of moons and gaze at tonight’s moon.

Calendar Questions—

What are your fierce wonderings, your burning questions, about calendars? FAQ Calendars tries to answer all of your calendar questions. Wondering about the astronomical basis for calendars or the role of the equinoxes and solstices? What would today’s date be on the previous Julian calendar? What was wrong with the Julian calendar?  When was the Gregorian calendar adopted? How is Sweden’s calendar history curious? How are years countedChristian-based calendars need to know Jesus’ birthday. How is this calculated? 

Easter is very important in the Gregorian calendar. Easter, the celebration of the death and resurrection of Jesus, is historically tied to the Jewish holiday of Passover. However, the Jewish calendar is lunar and the Gregorian calendar is solar. So, when is Easter? Easter is a perfect example of the complexity and balance required to create a calendar; discover how Easter is calculated.

12312012-2Perhaps you have other miscellaneous questions. Or, questions about other calendars currently in use: Persian, Hebrew, Islamic, and Chinese. Is the Gregorian calendar ever used in these countries?

Do you wonder about the week: the origins of the 7-day week, or the names of the days. When you read history, it is important to remember that some previous historical times and cultures did not use the Gregorian calendar. This includes the French Revolution and the Maya.  

Digging Deeper—

12312012-9 More advanced students may be interested in learning more about the history of the Gregorian calendar. The BBC’s Melvyn Bragg talks with three experts about its origins and how calendars have changed over time. Transport yourself back in history and follow the development of the contemporary calendar. Imagine there was no calendar—no named months of the year or days of the week. How would you chart the passing of time? What natural observations would you track? Unearth how early man tracked time (00:12 to 4:00).

12312012-8 Ancient civilizations—the Babylonians, Egyptians, and Mayan—used calendars. Listen to more about the Babylonian calendar and what they contributed to our understanding of time (3:59 to 6:02). The Babylonian calendar was adopted and revised by the Romans. Hear about the early Roman calendar (8:00 to 9:55). Julius Cesar initiated a calendar reform called the Julian reform. Discover why this was necessary and what Cesar added to the calendar that we continue to use (9:55 to 11:21).

The Roman calendar is based on the sun. Easter is based on the lunar calendar. The tension between these two systems made it difficult to fix Easter’s date. Yet, dates were really important to early Christians who wanted to mark Easter (14:50 to 16:50). Examine the Christian calendar and the significance of Easter to the calendar (20:09 to 21:50).