I follow Phil Plait of Bad Astronomy on Twitter (@BadAstronomer), and earlier he stirred up a bit of a hornet’s nest regarding what, exactly, constitutes a decade. This is a little ironic as I had just discussed this very issue a couple days prior with a friend.
The question being asked: when does a decade begin or end? Though Phil has presented his own argument, I would like to make my own view clear.
First, we should define what a decade is. A decade is a period of 10 years, in the same way that a century is a period of 100 years and a millenia is a period of 1000 years. Some of the confusion arises from what we saw on January 1st, 2000. Many were hailing the beginning of a new millenia, and the start of the 21st century. The problem was that the 21st century didn’t actually start until January 1, 2001 because there was no year 0. Similarly, some argued that “the new millenia” didn’t start until January 1, 2001.
I will help clarify this issue of decades by using the year 2000 as an example. Since a millenia means only a period of 1000 years, it was actually reasonable to say that January 1, 2000 was the beginning of a new millenia, because there is nothing in the definition of a millenia that says when our starting point must begin. A person may justly say that a new millenia had begun, so long as they acknowledged that the beginning of the last millenia in their context was January 1, 1000.
The question of whether it was the 21st century, though, is a more rigidly defined problem. When we talk of whether it is the 21st century, the 11th century, or the 5th century, we are referring to how many centuries have occurred since the start of the common era (or, as the religious might call it, Anno Domini). So given that there was no year 0, it could not possibly have been the 21st century since the start of the common era unless it was January 1, 2001 or later.
How does this relate to decades? First, we have the more general view. January 1, 2010 is indeed the beginning of a new decade, since nothing in the definition of decade necessarily implies that a decade begin on a year ending in 1, 2, 3, etc. To make it more clear, we might say that it is the end of the 00’s decade, as we have colloquially referred to decades in terms of the number in the tens slot. So December 31, 2009 was the end of a decade in the same way that December 31, 1969 was the end of a decade.
However, on the more particular view, one could not argue that January 1, 2010 is the beginning of the 201st decade; only January 1, 2011 could be, given that there was no year 0.
Personally, I think the short-hand of referring to periods as the 60’s, 70’s, 80’s, 90’s, or “Aughties” is not very fruitful. It doesn’t save us any appreciable time, but does add ambiguity (since there have been 20 different decades of the 60’s thus far). For clarity’s sake, I think one should be explicit in stating their starting point. For example, there is more clarity in saying that December 31, 1999 was the end of the 90’s, since we have specifically stated what our starting date is. I choose to scrap such nomenclature altogether and stick only to counting decades since the common era began (I would say, for example, that 2010 is the last year of the 200th decade), but alas I suppose I’m in the minority there. I find referring to decades by the number in the tens slot to be a tad short-sighted, but I leave it to the reader to determine whether this is necessary short-hand given our extraordinarily short lifetime when compared against the scale of the universe.