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It's an old question that got a lot of nitpicking attention 24 years ago. And astronomy-nerds frequently assert that they have the authority to answer this definitively! And with astronomical knowledge! Ya see (the argument goes), the calendar had no year zero. The years CE or AD started with the number 1. Therefore, (they say, implying it's some sort of mathematical proof) each century must also begin with the year 1 after the final hundred. They assert that the nineteenth century thus began on January 1 of the year 1801, the twentieth on January 1 of the year 1901, and the twenty-first began on January 1, 2001. Never mind that there never was a year "1" since our calendar did not exist at that time, so the argument is founded on a fantasy day. And of course, as well all know, all that really counts are the dates on the party invitations, and Jan. 1, 2000 was the big celebration for the dawn of the 21st century. And I say "rightly so".

I had not noticed until this week that there's another argument in favor of the "zero" year rollover of the centuries that's been staring us in the face all these years, and it arises from one of the oldest and most rigorous ideas in astronomy: the epoch of astronomical catalogs of stars. A few days ago, I was working some precession calculations, and I was testing out the intended epoch of the Piazzi star catalog, also known as the Palermo catalog, created primarily by the astronomers G. Piazzi and N. Cacciatore at the observatory in Palermo, Sicily. The epoch date of the catalog is January 1, 1800, also known as 1800.0. And star catalogs a century later used 1900.0 and recent catalogs used 2000.0. It makes the math a small bit easier, and in fact that's one of the best reasons in favor of the "zero" or "rollover" year as the starting date of a new century. Astronomical "epochs" have preferentially used the zero year as the start of each century for... well, for centuries! Note that this didn't stop some commentators from suggesting, when Piazzi discovered the minor planet/dwarf planet Ceres on January 1, 1801, that he had discovered it "on the first night of the new century" (opinions differed even then), but no one would have considered changing the epoch date of the catalog. The astronomers' century, the mathematical century, began on January 1, 1800. The 21st century began on January 1, 2000.

Just throwing this out there for posterity. If any of you are still around on December 31, 2099, presumably with your consciousness transferred into android bodies, you'll remember this little story and throw it out there you to your astronomy-nerd android-friends. :)

Frank Reed