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* Particle Physics and Astronomy Research Council*

** Royal Greenwich Observatory**

** Information Leaflet No. 52:`The Year AD 2000 '**

** THE YEAR AD 2000.**

The year AD 2000 will be a notable one. Many people are asking the questions, `Will 2000 be a leap-year?' and `Will it be the start of the new millennium?'

** Leap-Years.**

Leap-years were introduced into the calendar by Julius Caesar. They are necessary as the length of the year is not an integral number of days. The Julian calendar uses the fact that the length of the year is close to 365 and a quarter days. So a basic year with 365 days with an additional extra day every fourth year will give a good approximation.

This calendar was used until the 16th century when the small discrepancy between the approximate length of the year, 365.25 days, and the true length, 365.24219 days, added up to several days. Pope Gregory realised that this meant that the date of Easter would eventually not fall in the spring but would become closer and closer to Dec 25, Christmas.

In 1582 the Gregorian calendar was instituted. It changed the rule for determining whether a year should be a leap-year by stating that century years should only be leap years if they were divisible by 400. The effect of this is to make the adopted average year-length 365.2425 days, an approximation that will only amount to one day's error after 4000 years.

Britain adopted the Gregorian calendar in 1752 when september 2nd was followed by September 14th. We still use the Gregorian calendar and so the year 2000, which is divisible by 400, will be a leap-year.

** Millennia.**

A millennium is a period of 1000 years. The question of which year is the first year of the millennium hinges on the date of the first year AD.

Unfortunately the sequence of years going from BC to AD does not include a Year 0. The sequence of years runs 3 BC, 2 BC, 1 BC, AD 1, AD 2, AD 3 etc. This means that the first year of the first millennium was 1 AD. The one thousandth year was AD 1000 and the first day of the second millennium was AD 1001.

It is thus clear that the start of the new millennium will be 1 Jan 2001.

** Celebrations.**

The year AD 2000 will certainly be celebrated, as is natural for a year with such a round number but, accurately speaking, we will be celebrating the 2000th year or the last year of the millennium, not the start of the new millennium. Whether this will be an excuse for more celebrations in the following year will have to be seen!

Produced by the Information Services Department of the Royal Greenwich Observatory.

PJA Wed May 23 09:42:10 GMT 1996