Quick, what year will it be after December 31? If you said 2015, you'd be right — but only according to the Gregorian calendar. While the Gregorian might be the most commonly used calendar internationally, it is only one of many currently used by different cultures around the world.
Systems of measuring and naming periods of time have been used since ancient times. Calendars are useful because they can help people anticipate recurring events in nature: knowing when the tides come in and out, how long until winter, or when certain animals will migrate, for example, can give a society a leg-up in survival. Creating a way to reference a certain occurrence can also create cultural or religious significance. The earliest calendars were often built around celestial bodies like the planet Venus or the Moon, whose regular movements in the night sky were easily recognizable.
The Gregorian calendar, currently the international standard, is a refinement of the Julian calendar, which was created by Julius Caesar in 46 BCE. The Gregorian calendar is a solar calendar, which means it is based on movements of the Sun: a Gregorian year is approximately 365.25 days long, which is how long it takes the Earth to complete one full trip around the Sun. Leap years of 366 days were added in every four years in the Julian calendar to offset the extra quarter-day in each year, while the Gregorian removed three leap years every century to further calibrate for accuracy. Because the Gregorian calendar aligns with the movement of the Earth around the Sun, it also corresponds to changes in seasons at different latitudes.
Other calendars, like the Islamic calendar, are based on the phases of the Moon and are called lunar calendars. Islamic years have 354 days instead of 365, and the calendar began in 622 CE on the Gregorian calendar. The Islamic calendar maintains strong significance in Muslim countries and communities, where it is used to determine when religious events and traditions fall.
Yet other calendars like the Hebrew and Hindu calendars are lunisolar, which means they give the date in reference to movements of both the Sun and the Moon. Other cultures and countries, like China and Japan, use the Gregorian calendar for practical purposes but maintain other calendars for religious or cultural reasons. The Chinese lunisolar calendar is used to find auspicious days on which to hold weddings or funerals, while years in Japan are often referred to by the year of the reign of the current emperor.
To learn more about different systems of calendars and timekeeping, take a look at the Lunar Cycle and Fossils and Geologic Time lessons. The A Walk Through Time tool links to a resource about the history of timekeeping, while the Backs to the Future Science Update discusses the indigenous Aymara people of South America and their perception of the past and future.
Image credit: clipart.com
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