Perplexed About Absolute Zero

Okay, this is well outside my bailiwick, but I've been thinking about it recently after catching part of an episode of Nova covering absolute zero. So, if I understand correctly absolute zero is the theoretical temperature where everything basically stops moving and there would truly be zero heat energy. And, that temperature is at 0 kelvin, which based on all the searching I could do is -273.15° Celsius.

Now, maybe I'm a bit confused about how the Celsius scale came about, but it seems to be based on the properties of water (0° freezing point and 100° boiling point). Here's why I'm really confused and maybe I am missing something obvious here. If we chose an arbitrary compound (water, in the case of Celsius) to define some temperature scale, how is it that we got so lucky that the theoretical temperature of absolute zero fits so neatly, only requiring 2 decimal places?

I would have guessed that the chances of this would be astronomically against us. Everything I've looked at for more information never shows an absolute zero value that expands past the hundredths place. And, if the real value isn't exactly -273.15° Celsius, then how is it that there have been all these scientific breakthroughs that help get us closer and closer to absolute zero (to the billionths of a kelvin!).

There are answers out there, and I am fairly sure that some of you reading this have them. So, what am I missing? Did we really just get ridiculously lucky in our choice of Celsius that Kelvin translates cleanly to it? What's the real deal?

*** Edit ***

Well, that was fast... J.T. figured it was based on the redefinition of Celsius, and upon further research he is correct. So, the Celsius of 1900 is not the same as the Celsius of 2008.

See the little bit in the orange box on: A Brief History of Temperature Measurement.

In 1967, the Thirteenth General Conference on Weights and Measures changed the name of the thermodynamic temperature unit degree Kelvin (symbol °K) to merely kelvin (symbol K). The conference redefined Celsius temperature as the thermodynamic temperature minus 273.15 kelvin.