Lunar and Solar Calendars

Calendars are typically constructed around two astronomical phenomena: the phases of the moon and the seasons created by the movement of the sun. The former is approximately 30 days in length, the latter 365.25 days. Since the number of lunar cycles do not easily fit in the number of solar days in a year, various attempts have been made over the centuries to create a calendar that brings them into some systematic form of harmony.

The Gregorian Calendar

The Gregorian calendar used today is a revision of the original Julian calendar. The Julian calander was a huge improvement over the earlier Roman calendar. In the 1500s most European countries adopted the improved Gregorian calendar except for Russia, which held out until the 20th century. The counting of centuries used in the western hemisphere follows the Julian and especially Gregorian calendars.

Other Calendars

Other calendars completely ignore this numbering system, and do not refer to this time as the 21st century at all. For Jews, this is the 5772nd century, for Muslims, it is the 15th century, and for the Chinese, it is the seventy-eighth 60-year cycle.