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### First Day of the 21st Century

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Date: 12/01/1999 at 12:04:15
From: Rob Prejean
Subject: 1st day of 21st century??

What is the true date of the first day of the 21st century? From what
day are we counting 2000 years?
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Date: 12/01/1999 at 12:28:38
From: Doctor Rick
Subject: Re: 1st day of 21st century??

Hi, Rob.

A millennium is by definition "a period of 1000 years." The question,
therefore, is, What span of years was covered by the first millennium?
... the second millennium? etc.

If the first millennium began with the year 0 AD, then you could
count: 0, 1, 2, ..., 98, 99 would be 100 years. Those 100 years would
be the first century ("period of 100 years"). The second century would
begin on January 1, AD 100. Similarly, the second millennium would
begin on January 1, 1000, and the third millennium on January 1, 2000.

However, it's an historical fact that THERE WAS NO YEAR AD 0. The
first century began on January 1, AD 1. You can follow the same logic
as above to see that the first century encompassed the years 1, 2,
..., 100, and the second century began on Jan. 1, AD 101. Likewise,
the next millennium will begin on Jan. 1, 2001.

Here is where the history of math comes into the picture. If it were
not for what follows, I would not care a bit when people want to say
the millennium begins - it's just an arbitrary day with no deep
significance. But I am fascinated by the history of math, and here we
have one occasion when math history has a significant impact on things
that are happening now. I have told others that I would like to see
the year 2000 celebrated in schools as the Year of the Zero.

Here's why. The calendar was developed around AD 525, by Dionysius
Exiguus. At that time in Europe, numbers were written in Roman
numerals. Our Hindu-Arabic number system had not been invented. There
was no such thing as zero; the earliest known use of a zero was in
India 350 years later.

Using Roman numerals, Dionysius had no choice: numbers started with I.
The year after I BC was AD I in his reckoning. There was no number to
give to a year in between.

This seems very strange to us, who are accustomed to a number line of
integers: -4, -3, -2, -1, 0, 1, 2, 3, 4, ... But Dionysius and his
contemporaries had no concept of negative numbers either, at least not
as a continuous progression from negative through zero to positive.
"Negative" quantities were treated separately from positive
quantities. It did not seem strange to them that the number of years
from AD 10 to AD 40 is 30 - 10 = 20, but the number of years from 10
BC to AD 10 is 10 + 10 - 1 = 19. The problems are different, and you
solve them differently; that's the way it works.

If we had it to do over again, we would probably have used the number
0 for the year in which Jesus Christ was born. Then his first birthday
would have been in the year 1.

As it is, it would be a lot of work to renumber the years so that
there is a year 0. Either we would have to rename AD 1 to AD 0, and
likewise with all the years after - so that this is 1998, and the
millennium is still a year away - or we would rename 1 BC to 0 AD,
and every BC date would change, so all the ancient history books would
need to be altered.

(By the way, if we were going to renumber the years, we'd probably
want to correct Dionysius' other problem: his date for the birth of
Jesus Christ was most likely 5 or 6 years late. This year really
should be AD 2004; we missed the millennium.)

The best way is to keep things as they are, and make the year 2000
"the Year of the Zero," an opportunity to remind ourselves what it
used to be like before people had the brilliant idea of zero.
"Nothing" has had quite an impact on the world!

- Doctor Rick, The Math Forum
http://mathforum.org/dr.math/
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Associated Topics:
High School History/Biography
Middle School History/Biography

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