Caltech Question of the Month: Is January 1, 2000, the first day of the last year of the 20th century, or the first day of the 21st century?

Submitted by Eileen Wise, Pasadena California, and answered by Dr. Kevin C. Knox, Ahmanson Postdoctoral Instructor in History at Caltech.

According to such august authorities as the U.S. Naval Observatory, the final day of the 20th century is December 31, 2000. Those who argue that January 1, 2001, must be the beginning of the third millenium do so on the grounds that there was no such thing as A.D. 0.The astronomer Dionysius Exiguus, who devised the Christian calendar in the sixth century A.D. (Anno Domini), went directly from 1 B.C. to A.D. 1.The probable reason that Dionysius did so is that the number zero had yet to be introduced into the Western world from India: at the time, astronomers and the like suffered through calculations using Roman numerals.

For this reason, advocates of "2001" contend that since the calendar began at A.D. 1, and since a millenium is 1000 years, all millennia begin with a year one.

Yet this declaration can be challenged. Some maintain that the true millenium has already come to pass, arguing that we now know that early Christian mathematicians miscalculated the birth of Jesus. Since Christ was most likely born around 4 BC, the second millenium should have ended in 1997.

The decision of when to celebrate the new millenium is perhaps best described as an aesthetic choice. The length of one year-that is, the time that it takes the earth to complete its orbit around the sun-is subject to extremely precise astronomical measurements. But deciding from when to count these years is, ultimately, arbitrary.

It seems most people will celebrate the advent of the new millenium on December 31, 1999. If you insist on adhering to the guidelines of the U.S. Naval Observatory you will probably be in the minority. However, given the predicted shortage of champagne for the end of this year, if you do wait until 2001 you will probably find it easier to secure sufficient quantities of bubbly to make it a festive affair.