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| | Millennium -- from Eric Weisstein's World of Astronomy |
 | | Since there was no year 0, the years 1-1000 AD correspond to the first millennium, 1001-2000 to the second, etc. The third millennium therefore began on January 1, 2001. |
 | | However, rather than wasting time arguing about whether 2000 or 2001 is the start of the "new" millennium, we can simply note that the year 2000 corresponds to an odometer-like "rolling over" of the digits of the year, a very rare event which is clearly a calendrically significant! |
 | | Because 2000, 4000, 6000, etc. are leap years and 1000, 3000, 5000, etc. are not, the number of leap days in each millennium alternates between 242 and 243, with the first, third, etc. millennia (i.e., 1-1000, 2001-3000, etc.) having 242 leap days, and the second, fourth, etc. (i.e., 1001-2000, 3001-4000, etc.) having 243 leap days. |
| scienceworld.wolfram.com /astronomy/Millennium.html (225 words) |
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