It's also 100 years since the first International Women's Day, 100 years since Gustav Mahler died and 200 years since Franz Liszt was born (great composers, both), and, apparently, 2500 years since Pheidippides ran the first marathon in 490 BCE (this was seemingly celebrated incorrectly last year, with people not realising that, in the "Western" method of recording time, there was no year zero).
The year 2011 is also the 200th anniversary of the appearance of a paper with the snappy title Essai d'une manière de déterminer les masses relatives des molécules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons by one Amedeo Avogadro.
Although nobody realised it at the time, this paper was to fundamentally change chemistry. In order to understand why, we have to traverse some fairly hardcore ideas, for which I apologise in advance.
In 1811, chemists were just starting to get to grips with the concept of atoms. Although atoms had been proposed more than 2000 years previously by Greek philosophers, experimental evidence for their existence had only been obtained (and crucially, correctly interpreted) in the preceding decade.
More importantly, the ways in which atoms combined to form molecules were not understood with any certainty. In the above mentioned missive, Avogadro made the perceptive suggestion that "equal volumes of different gases at the same temperature and pressure contain the same number of molecules".
For example, if you take two balloons, one containing 22.4 litres of nitrogen gas (we'll come to why we chose such an unusual volume soon) and the other containing 22.4 litres of oxygen gas, both at atmospheric pressure and room temperature, you can be sure that they both contain the same number of molecules.
So far, so boring. The interesting bit comes when we weigh the gas in the two balloons. Even though each balloon contains the same volume of gas, the mass of each gas is different; the nitrogen will weigh about 28 grams, while the oxygen will weigh about 32g.
As each balloon, according to Avogadro, contains the same number of molecules, this must mean that an individual molecule of nitrogen gas weighs less than an individual molecule of oxygen gas (it's lighter by a factor of 28/32, or 0.875, for those of you who are mathematically inclined).
Now this might not seem to be of earth-shattering importance, but in actual fact, it is. For one thing, it gives us a method by which to measure the relative masses of individual atoms.
But, more importantly, as molecules tend to contain different atoms in simple, whole number ratios (for example, N2O (nitrous oxide), NO (nitric oxide) or NO2 (nitrogen dioxide)), if we want to prepare these molecules, we can now figure out what mass of reactants we will require in order to obtain these ratios.
The number of molecules of any gas in 22.4 litres at atmospheric pressure and room temperature is about 600,000,000,000,000,000,000,000 (or 6x1023 in abbreviated form).
This number is almost beyond comprehension - for example, 6x1023 grains of sand would cover New Zealand and Australia to a depth of about 1.3m, but yet, when we hold 12g of carbon, or 63.5g of copper, or 197g of gold in our hands, we are holding this number of atoms - this gives you some idea of just how tiny atoms are.
As those of you who have studied chemistry at school or university may recognise, 6x1023 is the basis of the Avogadro constant, which is, in turn, the basis of a unit called the mole (no, not the small furry animal), a measure of the amount of substance.
One mole of anything contains 6x1023 units of that thing, be they atoms, molecules, or grains of sand.
The aforementioned 22.4 litre balloons each contain 1 mole of nitrogen gas and oxygen gas, respectively.
The mole is one of the seven fundamental units of science. And we chemists, who should be celebrating this anniversary with enormous gusto, wouldn't be able to do without it.
• Dr Blackman is an associate professor in the chemistry department at the University of Otago.
