Millennium and the Year 0

Date: 01/03/99 at 22:39:33
From: Katie
Subject: Millennium

I am doing a science project on when the new millennium really begins.  
I already know that it truly begins in the year 2001 because there was 
no zero year between B.C. and A.D. I was wondering if you knew why or 
could give me some sites about why the early mathematicians didn't let 
a whole year pass before naming it one. I would also appreciate any 
other information you could give me about my project. I have already 
looked it up under many different subjects in Encarta 98 and tried 
tons of different ways to get information on the Web, but I'm having 
trouble finding information. 

Thanks,
Katie

Date: 01/04/99 at 08:47:26
From: Doctor Rick
Subject: Re: Millennium

Hi, Katie, welcome to Ask Dr. Math!

Thanks for asking this question, it's a good one. To answer it, you 
will need to look at the history of the calendar, and the history of 
math.

BC dates were not used in the BC era. BC means "before Christ"; that 
is, before the birth of Christ. Before he was born, people (even the 
relatively few people who were expecting his birth) did not know just 
when it would be, so they couldn't date their calendars that way! As a 
matter of fact, AD and BC were not invented until around AD 525, by 
Dionysius Exiguus. 

In AD 525, people in Europe didn't have a clear idea about negative 
numbers. In fact, it wasn't until 1657 that a mathematician (John 
Hudde) used a single variable to represent either a positive or a 
negative number. For all those years until 1657, positive and negative 
numbers were handled separately. Nobody drew a number line with 
positive numbers on one side, negative numbers on the other, and 0 in 
between.

Let's get back to Dionysius. He identified the year 1 AD ("1 Anno 
Domini," the first year of the Lord) as the year that Christ was born 
(he was probably off by 4 to 6 years). The previous year was 1 BC, the 
first year "Before Christ." (I have not figured out why one 
abbreviation is in Latin while the other is in English.) 

Dionysius would not have thought of 1 BC as the year -1. He would not 
have thought of putting BC and AD together on a number line (timeline). 
BC and AD were two separate cases. To find the time between two AD 
dates, you would take their difference. To find the time between an 
AD date and a BC date, you would add them and subtract 1. This would 
not be seen as a problem. It's just the way they solved other kinds of 
problems (like quadratic equations, if you've seen them) - as a set of 
special cases.

I have found some Web sites that can give you more information on the 
things I have said.

Here is an interesting Web page about the origin of the calendar (see 
the section "Century and Millennium"):

  Larry Freeman's Calendar Origin Page
  http://ourworld.compuserve.com/homepages/Larry_Freeman/calendar.htm   

Here is another (see the section "The start of Anno Domini Dating"):

  Historical Ecclesiastical Calendar: 
  Easter and its Relevance for Chronology
  http://ourworld.compuserve.com/homepages/khagen/Easter.html#AD   

Here is a Web page about the origin of negative numbers:

  Negative Numbers - Kevin Brown
  http://www.mathpages.com/home/kmath298.htm   

One of Jeff Miller's pages is my reference for John Hudde:

  Earliest Uses of Symbols for Variables
  http://jeff560.tripod.com/variables.html   

- Doctor Rick, The Math Forum
  http://mathforum.org/dr.math/