Millennia

A millennium is a period of time consisting of one thousand years. It is a larger unit than a century (one hundred years) and a decade (ten years). The English term "millennium" derives from the Latin words mille (thousand) and annus (year).

Millennia are commonly used to divide history and to measure long periods of time, for example when discussing the development of agriculture or ancient cities. They also feature in cultural and social discourse, particularly at the beginning of a new millennium.

Counting methods

The Gregorian calendar, the most widely used in the world, has no year zero. The year before AD 1 is known as 1 BC. Therefore, by strict reckoning, the first millennium began on 1 January AD 1 and ended on 31 December AD 1000. The second millennium ran from 1001 to 2000, while the third millennium began on 1 January 2001.

The third millennium and the Year 2000 celebrations

Global millennium celebrations largely focused on the night of 31 December 1999, because the year number reached 2000. Historians and astronomical bodies noted that the third millennium technically began in 2001. The substance of the debate concerned how to count, not a new scientific fact.

Other calendars

Other calendars, such as the Islamic Hijri calendar based on lunar months, have shorter years compared to the solar calendar. Consequently their millennia do not coincide with the Gregorian ones. Historians therefore use alternative methods to place different chronologies on a common scale.

Scientific use

In geology and climate science, a millennium is a small unit compared to geological timescales, which are measured in millions of years. However, it is used to assess short-term climate change, for example when examining ice records or tree rings spanning recent millennia.